1. Cellular Reproduction: Cells from Cells
Cellular Reproduction:
Cells from Cells

2. WHAT CELL REPRODUCTION ACCOMPLISHES
May result in the birth of new organisms
More commonly involves the production of new cells
© 2010 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word “clone” in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. You might wish to point out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed and cells are assembled. In our society, few non-living products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology.

3. Cell division
When a cell undergoes reproduction two daughter cells are produced that are genetically identical to each other and to the parent cell
Before a parent cell splits into two, it duplicates its chromosomes, the structures that contain most of the organism’s DNA.
During cell division, each daughter cell receives one set of chromosomes.
© 2010 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word “clone” in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. You might wish to point out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed and cells are assembled. In our society, few non-living products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology.

4. Chromosome duplication Sister chromatids Chromosome distribution to
daughter cells
Figure 8.5
Figure 8.5 Duplication and distribution of a single chromosome.

5. Mitosis/ Meiosis Cytokinesis Mitosis Uses for mitosis
© 2010 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word “clone” in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. You might wish to point out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed and cells are assembled. In our society, few non-living products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology.

6. FUNCTIONS OF CELL DIVISION
Cell Replacement
Growth via Cell Division
Colorized TEM LM
Human kidney cell
Early human embryo
Figure 8.1a
Figure 8.1a Three functions of cell division. (Part 1)

7. Single-celled organisms reproduce by simple cell division
Asexual reproduction
Single-celled organisms reproduce by simple cell division
There is no fertilization of an egg by a sperm
Uses mitosis
Clone
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word “clone” in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. You might wish to point out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed and cells are assembled. In our society, few non-living products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology.

8. FUNCTIONS OF CELL DIVISION Asexual ReproductionLM
Amoeba
Sea stars
African Violet
Figure 8.1b
Figure 8.1b Three functions of cell division. (Part 2)

9. Meiosis Uses for meiosis Sexual reproduction
Through sexual reproduction offspring end up with new combinations of alleles which lead to variations in physical and behavioral traits.
© 2010 Pearson Education, Inc.
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word “clone” in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. You might wish to point out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed and cells are assembled. In our society, few non-living products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology.

10. Sexually reproducing organisms use: Meiosis for reproduction
Mitosis for growth and maintenance
Student Misconceptions and Concerns
1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling can be prevented if each gamete has only half the genetic material of the adult cells.
2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells holds great potential but is variously restricted and regulated.
3. As the authors note, biologists use the term “daughter” to indicate offspring and not gender. Students with little experience in this terminology can easily become confused.
Teaching Tips
1. The authors do not use the word “clone” in this chapter. You might wish to point out to your students that asexual reproduction produces clones.
2. You might wish to point out that asexual reproduction is common in prokaryotes and single-celled organisms.
3. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus, eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and support growth.)
4. Virchow’s principle of “every cell from a cell” (not specifically addressed in this chapter) is worth thinking through with your class. Students might expect that like automobiles, computers, and cell phones, cell parts are constructed and cells are assembled. In our society, few non-living products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet, this common expectation exists in biology.

11. Table 8-1, p. 137

12. Fig. 8-2, p. 137
Figure 8.2
A multicelled animal develops by repeated divisions of a fertilized egg. This photo shows early frog embryos, each a product of three divisions of one fertilized egg.
Figure It Out: Each of these embryos consists of how many cells?
Answer: Eight.

13. Are made of chromatin, a combination of DNA and protein molecules
Chromosomes
Are made of chromatin, a combination of DNA and protein molecules
Are not visible in a cell until cell division occurs
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

14. LM
Chromosomes
Figure 8.3
Figure 8.3 A plant cell just before division (colored by stains).

15. Eukaryotic Chromosomes
Each eukaryotic chromosome contains one very long DNA molecule, typically bearing thousands of genes.
Most genes are located on chromosomes in the cell nucleus.
The number of chromosomes in a eukaryotic cell depends on the species.
Different individuals of a single species have the same number and types of chromosomes.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

16. Number of chromosomes in body cells Species Indian muntjac deer 6
Koala 16
Opossum 22
Giraffe 30
Mouse 40
Human 46
Duck-billed platypus 54
Buffalo 60
Dog 78
Red viscacha rat
102
Figure 8.2
Figure 8.2 The number of chromosomes in the cells of selected mammals.

17. Gene
Each gene has a specific location on a chromosome
Alleles
Located at the same place on a chromosome
example: in plants the allele for tall is short

18. Histones are proteins used to package DNA in eukaryotes.
The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding.
Histones are proteins used to package DNA in eukaryotes.
Nucleosomes consist of DNA wound around histone molecules.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

19. DNA double helix Histones “Beads on a string” Nucleosome
TEM
Nucleosome
Tight helical fiber
Looped domains
Duplicated chromosomes
(sister chromatids)
TEM
Centromere
Figure 8.4
Figure 8.4 DNA packing in a eukaryotic chromosome.

20. Duplicated chromosome
Chromosome (one
long piece of DNA)
Centromere
Sister
chromatids
Duplicated chromosome
Figure 8.UN2
Figure UN 8.2 Summary: Duplicated chromosome

21. When the cell divides, the sister chromatids separate from each other.
Once separated, each chromatid is:
Considered a full-fledged chromosome
Identical to the original chromosome
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

22. A An unduplicated pair of chromosomes in a cell in G1.
C Mitosis and cytoplasmic division package one copy of each chromosome into each of two new cells.
A An unduplicated pair of chromosomes in a cell in G1.
B By G2, each chromosome has been duplicated.
Stepped Art
Fig. 8-4, p. 139
Figure 8.4
How mitosis maintains the chromosome number.

23. The Cell Cycle
A cell cycle is the orderly sequence of events that extend from the time a cell is first formed from a dividing parent cell to its own division into two cells.
Most of the cell cycle is spent in interphase.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

24. Mitosis 1. Interphase 3 phases Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

25. S phase (DNA synthesis; chromosome duplication)
Interphase: metabolism and
growth (90% of time) G1 G2
Mitotic (M) phase:
cell division
(10% of time)
Cytokinesis
(division of
cytoplasm)
Mitosis
(division of nucleus)
Figure 8.6
Figure 8.6 The eukaryotic cell cycle.

26. Fig. 8-5a, p. 140
Figure 8.5: Animated!
Mitosis. Micrographs here and opposite show plant cells (onion root, left), and animal cells (whitefish embryo, right). This page, interphase cells are shown for comparison, but interphase is not part of mitosis. Opposite page, the stages of mitosis. The drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but nearly all eukaryotic cells have more than two. The two chromosomes of the pair inherited from one parent are pink; the two chromosomes from the other parent are blue.

27. Mitosis and Cytokinesis
During mitosis the mitotic spindle, a football-shaped structure of microtubules, guides the separation of two sets of daughter chromosomes.
Spindle microtubules grow from two centrosomes, material that in animal cells contain centrioles.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

28. 2. Prophase Interphase Mitosis Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

29. The chromosomes become visible as distinct structures 2
Prophase
The chromosomes become
visible as distinct structures
as they condense further.
Microtubules assemble
and move one of the two
centrosomes to the opposite
side of the nucleus, and the
nuclear envelope breaks up.
Fig. 8-5b (2), p. 141
Figure 8.5: Animated!
Mitosis. Micrographs here and opposite show plant cells (onion root, left), and animal cells (whitefish embryo, right). This page, interphase cells are shown for comparison, but interphase is not part of mitosis. Opposite page, the stages of mitosis. The drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but nearly all eukaryotic cells have more than two. The two chromosomes of the pair inherited from one parent are pink; the two chromosomes from the other parent are blue.

30. 3. Metaphase 1. Interphase 2. Prophase Mitosis
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

31. 3
Metaphase
All of the chromosomes are aligned midway between the spindle poles. Microtubules attach each chromatid to one of the spindle poles, and its sister to the opposite pole.
Fig. 8-5b (4), p. 141
Figure 8.5: Animated!
Mitosis. Micrographs here and opposite show plant cells (onion root, left), and animal cells (whitefish embryo, right). This page, interphase cells are shown for comparison, but interphase is not part of mitosis. Opposite page, the stages of mitosis. The drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but nearly all eukaryotic cells have more than two. The two chromosomes of the pair inherited from one parent are pink; the two chromosomes from the other parent are blue.

32. 4. Anaphase 1. Interphase 2. Prophase 3. Metaphase Mitosis
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

33. 4 Anaphase Motor proteins
moving along spindle microtubules drag the chromatids toward the spindle poles, and the sister chromatids separate. Each sister chromatid is now a separate chromosome.
Fig. 8-5b (5), p. 141
Figure 8.5: Animated!
Mitosis. Micrographs here and opposite show plant cells (onion root, left), and animal cells (whitefish embryo, right). This page, interphase cells are shown for comparison, but interphase is not part of mitosis. Opposite page, the stages of mitosis. The drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but nearly all eukaryotic cells have more than two. The two chromosomes of the pair inherited from one parent are pink; the two chromosomes from the other parent are blue.

34. 5. Telophase Mitosis 1. Interphase 2. Prophase 3. Metaphase
4. Anaphase
5. Telophase
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

35. The chromosomes reach the spindle poles and 5
Telophase
The chromosomes reach the spindle poles and
decondense. A nuclear envelope forms around each cluster. Mitosis is over.
Fig. 8-5b (6), p. 141
Figure 8.5: Animated!
Mitosis. Micrographs here and opposite show plant cells (onion root, left), and animal cells (whitefish embryo, right). This page, interphase cells are shown for comparison, but interphase is not part of mitosis. Opposite page, the stages of mitosis. The drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but nearly all eukaryotic cells have more than two. The two chromosomes of the pair inherited from one parent are pink; the two chromosomes from the other parent are blue.

36. Fig. 8-5b, p. 141
Figure 8.5: Animated!
Mitosis. Micrographs here and opposite show plant cells (onion root, left), and animal cells (whitefish embryo, right). This page, interphase cells are shown for comparison, but interphase is not part of mitosis. Opposite page, the stages of mitosis. The drawings show a diploid (2n) animal cell. For clarity, only two pairs of chromosomes are illustrated, but nearly all eukaryotic cells have more than two. The two chromosomes of the pair inherited from one parent are pink; the two chromosomes from the other parent are blue.

37. Cytokinesis typically: Occurs during telophase Divides the cytoplasm
Is different in plant and animal cells
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

38. Cytokinesis
1. Cleavage furrow

39. Cleavage furrow Cleavage furrow Contracting ring of microfilaments
SEM
Cleavage
furrow
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Figure 8.8a
Figure 8.8a Cytokinesis in animal cells.

40. C This contractile ring pulls the cell surface inward as it shrinks.
Fig. 8-6c, p. 142
Figure 8.6: Animated!
Cytoplasmic division of an animal cell.

41. Cytokinesis
2. Cell plate

42. Wall of parent cell Cell plate forming Daughter nucleusLM
Vesicles containing
cell wall material
Cell wall
Cell plate
New cell wall
Daughter cells
Figure 8.8b
Figure 8.8b Cytokinesis in plant cells.

43. C The cell plate expands outward along the plane of division
C The cell plate expands outward along the plane of division. When it reaches the plasma membrane, it attaches to the membrane and partitions the cytoplasm.
Fig. 8-7c, p. 143
Figure 8.7: Animated!
Cytoplasmic division of a plant cell.

44. (a) (b) (c) (d) Figure 8.UN6LM
(a)
(b)
(c)
(d)
Figure 8.UN6
Figure UN 8.6 Question 14: Slide of onion root tip

45. Cell Cycle and Mitosis
Animation 9-1 and 9-2

46. Cancer Cells: Growing Out of Control
Normal plant and animal cells have a cell cycle control system that consists of specialized proteins, which send stop and go signals at certain key points during the cell cycle.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

47. Cancer is a disease of the cell cycle.
What Is Cancer?
Cancer is a disease of the cell cycle.
Cancer cells do not respond normally to the cell cycle control system.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

48. Cancer cells can form tumors, abnormally growing masses of body cells.
The spread of cancer cells beyond their original site of origin is metastasis.
Malignant tumors can:
Spread to other parts of the body
Interrupt normal body functions
A person with a malignant tumor is said to have cancer.
Student Misconceptions and Concerns
1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic in this chapter when discussing DNA replication.
2. Students are often confused by photographs of chromosomes. A chromosome is often described as a single strand, yet photographs typically show replicated chromosomes. It remains unclear to many students why (a) chromosome structure is typically different between interphase G1and stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes replicate. The Essential Biology text addresses early in the chapter the reason why interphase chromosomes are not clearly seen in a light micrograph.
3. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer.
Teaching Tips
1. Mitochondrial DNA is widely used to analyze evolutionary relationships. Students might be challenged to search the Internet for examples of its use in tracing human evolutionary history.
2. Consider this additional analogy between histones and DNA. DNA is like a very long piece of thread wrapped around a series of spools (histones). The DNA wraps one spool, then extends to another spool, repeating this many hundreds of times all with one continuous strand of thread.
3. DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids (even though this is not the actual physical relationship between sister chromatids). In the model, we have doubled the DNA, but the molecules remain attached. (You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes.)
4. In G1, the chromosomes have not replicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
5. The cell cycle control system is somewhat like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, the components of the control system of a cell cycle are not located in one place, like a washing machine.
6. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPMAT. The first letters of interphase, prophase, metaphase, anaphase, and telophase are represented in this made-up word.
7. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). One place this occurs is in human development during the formation of the placenta.
8. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this analogy. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because like the drawstring just beneath the surface of the sweatpants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
9. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection.
10. Many other approaches (such as cancer vaccines) are under consideration to fight cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.

49. Lymph vessels Tumor Blood vessel Glandular tissue A tumor grows
from a single
cancer cell.
Cancer cells invade
neighboring tissue.
Metastasis: Cancer
cells spread through
lymph and blood
vessels to other parts
of the body.
Figure 8.9
Figure 8.9 Growth and metastasis of a malignant tumor of the breast.

50. Meiosis, the Basis of Sexual Reproduction Sexual reproduction:
Uses meiosis
Uses fertilization
Produces offspring that contain a unique combination of genes from the parents
© 2010 Pearson Education, Inc.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

51. Figure 8.10
Figure 8.10 The varied products of sexual reproduction.

52. Homologous chromosomes Diploid (2n) - contain two sets of chromosomes
Haploid (n)
- have only one set of chromosomes
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

53. Fig. 8-8, p. 144
Figure 8.8
A chromosome pair. The two appear identical in this micrograph, but any gene might differ in DNA sequence from its partner on the other chromosome.

54. Chromosomes n= chromosome # of each type Humans n= 23 Gorilla n= 24
Pea plants n= 7
Humans have 46 chromosomes or 23 pairs (23 from mother and 23 from father)
Gorillas have 48 chromosomes
Pea plants have 14 chromosomes

55. Humans somatic cells (2n) germ cells (2n) gametes (n) Humans have:
2 different sex chromosomes: X & Y
22 pairs of matching chromosomes: autosomes

56. A karyotype is an image that reveals an orderly arrangement of chromosomes.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

57. A karyotype is an image that reveals an orderly arrangement of chromosomes.

58. Stages of Meiosis 1. Meiosis I Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

59. each chromosome in the cell pairs with its homologous partner
then the partners separate
p. 145

60. Figure 8.13-2 Homologous chromosomes separate. Chromosomes duplicate.
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair of
homologous
chromosomes
Sister
chromatids
INTERPHASE BEFORE MEIOSIS
MEIOSIS I
Figure
Figure 8.13 How meiosis halves chromosome number. (Step 2)

61. Stages of Meiosis 1. Meiosis I 2. Meiosis II
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

62. two chromosomes (unduplicated) one chromosome (duplicated)

63. Figure 8.13-3 Homologous chromosomes separate. Chromosomes duplicate.
Sister
chromatids
separate.
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair of
homologous
chromosomes
Sister
chromatids
INTERPHASE BEFORE MEIOSIS
MEIOSIS I
MEIOSIS II
Figure
Figure 8.13 How meiosis halves chromosome number. (Step 3)

64. Meiosis
interphase

65. Figure 8.13-1 Chromosomes duplicate. Pair of homologous chromosomes in
diploid parent cell
Duplicated pair of
homologous
chromosomes
Sister
chromatids
INTERPHASE BEFORE MEIOSIS
Figure
Figure 8.13 How meiosis halves chromosome number. (Step 1)

66. Meiosis I In crossing over:
1. Prophase I
In crossing over:
Homologous chromosomes exchange genetic information

67. A) Here, we focus on only two genes.
One gene has alleles A and a; the other
has alleles B and b.
crossover
B) Close contact between the homologous chromosomes promotes crossing over between nonsister chromatids, so paternal and maternal chromatids exchange segments.
C) Crossing over mixes up
paternal and maternal alleles on
homologous chromosomes.
Stepped Art
Fig. 8-11c, p. 148
Figure 8.11: Animated!
Crossing over. Blue signifies a paternal chromosome, and pink, its maternal homologue. For clarity, we show only one pair of homologous chromosomes and one crossover, but more than one crossover may occur in each chromosome pair.

68. Figure 8.18-5 Prophase I Duplicated pair of of meiosis homologous
chromosomes
Homologous chromatids
exchange corresponding
segments.
Chiasma, site of
crossing over
Metaphase I
Spindle
microtubule
Sister chromatids
remain joined at their
centromeres.
Metaphase II
Gametes
Recombinant
chromosomes
combine genetic
information from
different parents.
Recombinant chromosomes
Figure
Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 5)

69. Meiosis I 1. Prophase I 2. Metaphase I
Independent Assortment of Chromosomes
Every chromosome pair orients independently of the others during meiosis.

70. Metaphase of meiosis I Metaphase of meiosis II
POSSIBILITY 1
POSSIBILITY 2
Metaphase of
meiosis I
Metaphase
of meiosis II
Gametes
Combination a
Combination b
Combination c
Combination d
Figure
Figure 8.16 Results of alternative arrangements of chromosomes at metaphase of meiosis I. (Step 3)

71. Meiosis I
1. Prophase I 2. Metaphase I 3. Anaphase I

72. Meiosis I
1. Prophase I 2. Metaphase I 3. Anaphase I 4. Telophase I

73. Fig. 8-10a, p. 146
Figure 8.10: Animated!
Meiosis. Two pairs of chromosomes are illustrated in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue and pink. Micrographs show meiosis in a lily plant cell (Lilium regale). 1 Prophase I. The (duplicated) chromosomes condense, and spindle microtubules attach to them as the nuclear envelope breaks up. 2 Metaphase I. The (duplicated) chromosomes are aligned midway between spindle poles. 3 Anaphase I. Homologous chromosomes separate. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope encloses each cluster, so two haploid (n) nuclei form. 5 Prophase II. The (still duplicated) chromosomes condense, and spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The chromosomes are aligned midway between the spindle poles. 7 Anaphase II. Sister chromatids separate and become individual chromosomes (unduplicated). 8 Telophase II. A cluster of (unduplicated) chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form.

74. Meiosis II
1. Prophase II- spindle fibers reappear

75. Meiosis II
2. Metaphase II- duplicated chromosomes align at the equator

76. Meiosis II
3. Anaphase II- sister chromatids pulled toward opposite poles

77. Meiosis II
4. Telophase II a. 4 daughter nuclei form b. cytoplasm divides c. each cell has n # unduplicated chromosomes

78. There is no DNA replication between the two nuclear divisions.5
Prophase II 6
Metaphase II 7
Anaphase II 8
Telophase II
There is no DNA replication between the two nuclear divisions.
Fig. 8-10b, p. 147
Figure 8.10: Animated!
Meiosis. Two pairs of chromosomes are illustrated in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue and pink. Micrographs show meiosis in a lily plant cell (Lilium regale). 1 Prophase I. The (duplicated) chromosomes condense, and spindle microtubules attach to them as the nuclear envelope breaks up. 2 Metaphase I. The (duplicated) chromosomes are aligned midway between spindle poles. 3 Anaphase I. Homologous chromosomes separate. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope encloses each cluster, so two haploid (n) nuclei form. 5 Prophase II. The (still duplicated) chromosomes condense, and spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The chromosomes are aligned midway between the spindle poles. 7 Anaphase II. Sister chromatids separate and become individual chromosomes (unduplicated). 8 Telophase II. A cluster of (unduplicated) chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form.

79. Fig. 8-10, pp
Figure 8.10: Animated!
Meiosis. Two pairs of chromosomes are illustrated in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue and pink. Micrographs show meiosis in a lily plant cell (Lilium regale). 1 Prophase I. The (duplicated) chromosomes condense, and spindle microtubules attach to them as the nuclear envelope breaks up. 2 Metaphase I. The (duplicated) chromosomes are aligned midway between spindle poles. 3 Anaphase I. Homologous chromosomes separate. 4 Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope encloses each cluster, so two haploid (n) nuclei form. 5 Prophase II. The (still duplicated) chromosomes condense, and spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. 6 Metaphase II. The chromosomes are aligned midway between the spindle poles. 7 Anaphase II. Sister chromatids separate and become individual chromosomes (unduplicated). 8 Telophase II. A cluster of (unduplicated) chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form.

80. Meiosis
Animation 10-1

81. Review: Comparing Mitosis and Meiosis
In mitosis and meiosis, the chromosomes duplicate only once, during the interphase.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

82. The number of cell divisions varies:
Mitosis uses one division and produces two diploid cells
Meiosis uses two divisions and produces four haploid cells
All the events unique to meiosis occur during meiosis I.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

83. Prophase II Anaphase II Prophase I Anaphase I Metaphase I Telophase I
Metaphase II
Telophase II
Metaphase
Prophase
Anaphase
Telophase
Fig. 8-16, p. 152
Figure 8.16
Comparison of mitosis and meiosis. We begin both processes with a diploid cell that has two pairs of chromosomes. The chromosomes are duplicated before nuclear division begins. Mitosis maintains the chromosome number. Meiosis halves it, to the haploid number.

84. Spermatogenesis

85. Oogenesis

86. Sex determination
Animation 10-2

87. Figure 8.17
Figure 8.17 The process of fertilization: a close up view.

88. Gametes and the Life Cycle of a Sexual Organism
The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next.
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

89. Restores the chromosome number back to the diploid
Fertilization
Restores the chromosome number back to the diploid
Contributes to variation among offspring
Student Misconceptions and Concerns
1. How meiosis results in four haploid cells yet mitosis yields two diploid cells is often memorized but not understood. It can be explained like this. In mitosis and meiosis, the processes begin with replicated pairs of chromosomes. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. All of the details of these two processes, although eventually addressed, can get in the way of seeing the overall process.
2. Most people have difficulty comprehending large numbers. See teaching tips 8-10 below to help relate these large numbers to aspects of students’ lives.
Teaching Tips
1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs produce dogs, cats produce only more cats, and chickens only produce chickens. Why does “like produce like”?
2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes.
3. In the shoe analogy, females have 23 pairs of matching shoes, males have 22 matching pairs and one odd pair maybe a sandal and a sneaker!
4. You may wish to ask the class why meiosis is necessary. Why not have a male diploid cell fertilize a diploid female cell? In short, the answer is that, if this were true, at every fertilization, we would have genetic doubling.
5. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes. A point to make is that the shoes (chromosomes) before crossing over are what you inherited either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (or shoes with switched shoelaces). In this shoe analogy, after exchanging shoelaces, we have recombinant shoes!
6. You might consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous chromosomes separate. After discussing mitosis and meiosis in class, consider asking your students to sketch the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I.
7. The number 223 is 8,388,608. This number squared is more than 70 trillion. The authors rounded down to 8 million for 223 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization, is over 70 trillion!
8. There are currently about 310 million humans living in the United States. If every person in the United States received $225,806, it would equal $70 trillion. Here is another way to think of it. If you lived to be 100 years old, and spent $22, every second of your life, you would spend about $70 trillion dollars.
9. The impressive nature of such large numbers is lost on most of us who cannot comprehend such quantities. There are about 64 trillion seconds in 2 million years (actually, 2,028,000 years).
10. Depending upon the size of your class, it is likely that at least one of your students is related to a person with Down syndrome. A student in your class may even enjoy the chance to talk about their Down syndrome friend or relative.

90. Haploid gametes (n  23) Egg cell n n Sperm cell MEIOSIS FERTILIZATION
Multicellular
diploid adults
(2n  46)
Diploid
zygote
(2n  46) 2n
MITOSIS
and development
Key
Haploid (n)
Diploid (2n)
Figure 8.12
Figure 8.12 The human life cycle