Presentation is loading. Please wait.

Presentation is loading. Please wait.

Ch. 8 Mitosis and Meiosis.

Similar presentations


Presentation on theme: "Ch. 8 Mitosis and Meiosis."— Presentation transcript:

1 Ch. 8 Mitosis and Meiosis

2 Introduction: Cell Division
Cell division allows for growth, the replacement of damaged cells, and development from an embryo into an adult. In sexually reproducing organisms, eggs and sperm result from mitosis and meiosis © 2012 Pearson Education, Inc. 2

3 CELL DIVISION AND REPRODUCTION
© 2012 Pearson Education, Inc. 3

4 8.1 Cell division plays many important roles in the lives of organisms
Organisms reproduce more individuals of their own species (this was a key characteristic of life) Cell division is reproduction at the cellular level, requires the duplication of chromosomes, and sorts new sets of chromosomes into the resulting pair of daughter cells. Student Misconceptions and Concerns 1. As the authors note in Module 8.1, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. 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 is variously restricted and regulated. Teaching Tips 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 will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? © 2012 Pearson Education, Inc. 4

5 8.1 Cell division plays many important roles in the lives of organisms
Cell division is used for reproduction of single-celled organisms growth of multicellular organisms from a fertilized egg into an adult repair and replacement of cells sperm and egg production Student Misconceptions and Concerns 1. As the authors note in Module 8.1, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. 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 is variously restricted and regulated. Teaching Tips 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 will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? © 2012 Pearson Education, Inc. 5

6 8.1 Cell division plays many important roles in the lives of organisms
Living organisms reproduce by two methods: Asexual reproduction produces offspring that are identical to the original cell or organism involves inheritance of all genes from one parent. Sexual reproduction produces offspring that are similar to the parents, but show variations in traits involves inheritance of unique sets of genes from two parents Student Misconceptions and Concerns 1. As the authors note in Module 8.1, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. 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 is variously restricted and regulated. Teaching Tips 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 will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like? © 2012 Pearson Education, Inc. 6

7 Yeast cell dividing - ASEXUAL
Figure 8.1A A yeast cell producing a genetically identical daughter cell by asexual reproduction Yeast cell dividing - ASEXUAL 7

8 Barnacles mating - SEXUAL
Figure 8.1B A sea star reproducing asexually Barnacles mating - SEXUAL 8

9 Leaf cutting in African Violet - Asexual
Figure 8.1C An African violet reproducing asexually from a cutting (the large leaf on the left) Leaf cutting in African Violet - Asexual 9

10 Figure 8.1D Sexual reproduction produces offspring with unique combinations of genes.
10

11 Early cell division in human embryo
Figure 8.1E Dividing cells in an early human embryo Early cell division in human embryo 11

12 Human Kidney cell in cell division
Figure 8.1F A human kidney cell dividing Human Kidney cell in cell division 12

13 8.2 Prokaryotes reproduce by binary fission
Prokaryotes (bacteria and archaea) reproduce by binary fission (“dividing in half”). The chromosome of a prokaryote is a singular circular DNA molecule associated with proteins much smaller than those of eukaryotes Student Misconceptions and Concerns 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 is variously restricted and regulated. Teaching Tips 1. The principle that “every cell comes from another cell” is worth thinking through with your class. Students might expect that, like automobiles, computers, and cell phones, parts are constructed and cells are assembled. In our society, few nonliving 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 is a common expectation in biology. Further, students who think through this principle might ask how the first cells formed. They might wonder further whether the same environments that produced these cells are still in existence. The conditions on Earth when life first formed were very different from those we know today. Chapter 15 addresses the origin and early evolution of life on Earth. 2. Consider contrasting the timing of DNA replication and cytokinesis in prokaryotes and eukaryotes. In prokaryotes, addressed in Module 8.2, these processes are overlapping. However, as revealed in the next few modules, these events are separate in eukaryotes. © 2012 Pearson Education, Inc. 13

14 Division into two daughter cells
Plasma membrane Cell wall Duplication of the chromosome and separation of the copies Continued elongation of the cell and movement of the copies Prokaryotic chromosome 1 2 3 Division into two daughter cells Figure 8.2A_s3 Binary fission of a prokaryotic cell (step 3) 14

15 THE EUKARYOTIC CELL CYCLE AND MITOSIS
© 2012 Pearson Education, Inc. 15

16 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division
Eukaryotic cells are more complex and larger than prokaryotic cells have more genes store most of their genes on multiple chromosomes within the nucleus Eukaryotic chromosomes are composed of chromatin consisting of one long DNA molecule proteins that help maintain the chromosome structure and control the activity of its genes Student Misconceptions and Concerns 1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. 2. Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.3B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many why (a) chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips 1. Figure 8.3B is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids. 2. The authors make an analogy between the precise packaging of DNA into chromosomes and packing a home for a move to another home. Tap into the intuitive advantages of packaging DNA using this or any other analogy of highly packaged materials (perhaps a boxed “desk” that requires some assembly). 3. The concepts of 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. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. © 2012 Pearson Education, Inc. 16

17 Figure 8.3A A plant cell (from an African blood lily) just before cell division
Chromatin condensing into chromosomes just before cell division in Lilium 17

18 Chromosomes DNA molecules Sister chromatids Chromosome duplication
Figure 8.3B Chromosomes DNA molecules Sister chromatids Chromosome duplication Sister chromatids Centromere Figure 8.3B Chromosome duplication and distribution Chromosome distribution to the daughter cells 18

19 Sister chromatids Centromere
Figure 8.3B_2 Sister chromatids Centromere Figure 8.3B_2 Chromosome duplication and distribution (part 2) 19

20 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division
Before a eukaryotic cell begins to divide, it duplicates all of its chromosomes, resulting in two copies called sister chromatids joined together by a narrowed “waist” called the centromere. When a cell divides, the sister chromatids separate from each other, now called chromosomes, and sort into separate daughter cells. Student Misconceptions and Concerns 1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. 2. Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.3B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many why (a) chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips 1. Figure 8.3B is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids. 2. The authors make an analogy between the precise packaging of DNA into chromosomes and packing a home for a move to another home. Tap into the intuitive advantages of packaging DNA using this or any other analogy of highly packaged materials (perhaps a boxed “desk” that requires some assembly). 3. The concepts of 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. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. © 2012 Pearson Education, Inc. 20

21 Chromosome duplication
Chromosomes DNA molecules Chromosome duplication Sister chromatids Centromere Figure 8.3B_1 Chromosome duplication and distribution (part 1) Chromosome distribution to the daughter cells 21

22 8.4 The cell cycle multiplies cells
The cell cycle is an ordered sequence of events that extends from the time a cell is first formed from a dividing parent cell until its own division The cell cycle consists of two stages, characterized as follows: Interphase: duplication of cell contents G1—growth, increase in cytoplasm S—duplication of chromosomes G2—growth, preparation for division Mitotic phase: division Mitosis—division of the nucleus Cytokinesis—division of cytoplasm Teaching Tips 1. The authors note in Module 8.4 that each of your students consists of about 10 trillion cells. It is likely that this number is beyond comprehension for most of your students. Consider sharing several simple examples of the enormity of that number to try to make it more meaningful. For example, the U.S. population in 2011 is about 312 million people. To give every one of those people about $32,000, we will need a total of 10 trillion dollars. Here is another example. If we gave you $32,000 every second, it would take 10 years to give you 10 trillion dollars. The US Debt Clock helps relate these large numbers to the US national debt at 2. In G1, the chromosomes have not duplicated. 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. © 2012 Pearson Education, Inc. 22

23 I N T E R P H A S G1 (first gap) S (DNA synthesis) M G2 (second gap)
Cytokinesis G2 (second gap) Mitosis Figure 8.4 The eukaryotic cell cycle T MI O IC PH A SE 23

24 8.5 Cell division is a continuum of dynamic changes
Mitosis progresses through a series of stages: prophase prometaphase metaphase anaphase telophase Cytokinesis often overlaps telophase Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 24

25 8.5 Cell division is a continuum of dynamic changes
A mitotic spindle is required to divide the chromosomes, composed of microtubules, and produced by centrosomes, structures in the cytoplasm that organize microtubule arrangement and contain a pair of centrioles in animal cells Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 25

26 (with centriole pairs)
INTERPHASE MITOSIS Prophase Prometaphase Centrosome Early mitotic spindle Chromatin Fragments of the nuclear envelope Kinetochore Centrosomes (with centriole pairs) Centrioles Nuclear Plasma membrane Chromosome, consisting of two sister chromatids Centromere Spindle microtubules Figure 8.5_1 The stages of cell division by mitosis: Interphase through Prometaphase 26

27 8.5 Cell division is a continuum of dynamic changes
Interphase the cytoplasmic contents double two centrosomes form chromosomes duplicate in the nucleus during the S phase nucleoli, sites of ribosome assembly, are visible Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 27

28 8.5 Cell division is a continuum of dynamic changes
Prophase In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle In the nucleus chromosomes coil and become compact nucleoli disappear Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 28

29 8.5 Cell division is a continuum of dynamic changes
Prometaphase Spindle microtubules reach chromosomes, where they attach at kinetochores on the centromeres of sister chromatids and move chromosomes to the center of the cell through associated protein “motors.” Other microtubules meet those from the opposite poles. The nuclear envelope disappears Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 29

30 Telophase and Cytokinesis
MITOSIS Anaphase Metaphase Telophase and Cytokinesis plate Cleavage furrow Nuclear envelope forming Daughter chromosomes Mitotic spindle Figure 8.5_5 The stages of cell division by mitosis: Metaphase through Cytokenesis 30

31 8.5 Cell division is a continuum of dynamic changes
Metaphase The mitotic spindle is fully formed Chromosomes align at the cell equator Kinetochores of sister chromatids are facing the opposite poles of the spindle Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 31

32 8.5 Cell division is a continuum of dynamic changes
Anaphase Sister chromatids separate at the centromeres Daughter chromosomes are moved to opposite poles of the cell as motor proteins move the chromosomes along the spindle microtubules kinetochore microtubules shorten The cell elongates due to lengthening of non-kinetochore microtubules Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 32

33 8.5 Cell division is a continuum of dynamic changes
Telophase The cell continues to elongate The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei Chromatin uncoils and nucleoli reappear The spindle disappears Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 33

34 8.5 Cell division is a continuum of dynamic changes
During cytokinesis, the cytoplasm is divided into separate cells. The process of cytokinesis differs in animal and plant cells. Teaching Tips Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. © 2012 Pearson Education, Inc. 34

35 8.6 Cytokinesis differs for plant and animal cells
In animal cells, cytokinesis occurs as a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin, and the cleavage furrow deepens to separate the contents into two cells. Teaching Tips 1. 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). This occurs in human development during the formation of the placenta. 2. The authors make an analogy between a drawstring on a hooded sweatshirt and the mechanism of cytokinesis in animal cells. Students seem to appreciate this association. 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. © 2012 Pearson Education, Inc. 35

36 Cleavage Cytokinesis Cleavage furrow Contracting ring of
microfilaments Daughter cells Figure 8.6A Cleavage of an animal cell 36

37 8.6 Cytokinesis differs for plant and animal cells
In plant cells, cytokinesis occurs as a cell plate forms in the middle, from vesicles containing cell wall material the cell plate grows outward to reach the edges, dividing the contents into two cells each cell now possesses a plasma membrane and cell wall Teaching Tips 1. 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). This occurs in human development during the formation of the placenta. 2. The authors make an analogy between a drawstring on a hooded sweatshirt and the mechanism of cytokinesis in animal cells. Students seem to appreciate this association. 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 sweat pants, the microfilaments are just beneath the surface of the cell’s plasma membrane. © 2012 Pearson Education, Inc. 37

38 New Cytokinesis Cell wall of the parent cell Plasma membrane Daughter
nucleus Plasma membrane Vesicles containing cell wall material Cell plate forming New cells Figure 8.6B Cell plate formation in a plant cell 38

39 8.7 Anchorage, cell density, and chemical growth factors affect cell division
The cells within an organism’s body divide and develop at different rates. Cell division is controlled by the presence of essential nutrients, growth factors, proteins that stimulate division, density-dependent inhibition, in which crowded cells stop dividing, and anchorage dependence, the need for cells to be in contact with a solid surface to divide. Teaching Tips Students who closely examine a small abrasion on their skin might notice that the wound tends to heal from the outer edges inward. This space-filling mechanism is a natural example of density-dependent inhibition, which is also seen when cells in a cell culture dish stop dividing when they have formed a complete layer. © 2012 Pearson Education, Inc. 39

40 Effect of Growth Factors
Cultured cells suspended in liquid The addition of growth factor Effect of Growth Factors Figure 8.7A An experiment demonstrating the effect of growth factors on the division of cultured animal cells 40

41 Density Dependent Inhibition
Anchorage Single layer of cells Removal Restoration of single layer by cell division Density Dependent Inhibition Figure 8.7B An experiment demonstrating density-dependent inhibition, using animal cells grown in culture 41

42 8.8 Growth factors signal the cell cycle control system
The cell cycle control system is a cycling set of molecules in the cell that Trigger and coordinate key events in the cell cycle Checkpoints in the cell cycle can stop an event or signal an event to proceed Teaching Tips The cell cycle control system depicted in Figure 8.8A is like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, unlike a washing machine, the components of the control system of a cell cycle are not all located in one place. © 2012 Pearson Education, Inc. 42

43 8.8 Growth factors signal the cell cycle control system
There are three major checkpoints in the cell cycle. G1 checkpoint allows entry into the S phase or causes the cell to leave the cycle, entering a non-dividing G0 phase. G2 checkpoint M checkpoint Research on the control of the cell cycle is one of the hottest areas in biology today Teaching Tips The cell cycle control system depicted in Figure 8.8A is like the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, unlike a washing machine, the components of the control system of a cell cycle are not all located in one place. © 2012 Pearson Education, Inc. 43

44 G1 checkpoint G0 G1 S Control system M G2 M checkpoint G2 checkpoint
Figure 8.8A A schematic model for the cell cycle control system 44

45 8.10 Review: Mitosis provides for growth, cell replacement, and asexual reproduction
When the cell cycle operates normally, mitosis produces genetically identical cells for growth replacement of damaged and lost cells asexual reproduction Teaching Tips Figure 8.10 visually summarizes key functions of mitosis. It is an important image to introduce mitosis or summarize mitosis after addressing its details. © 2012 Pearson Education, Inc. 45

46 Growth in onion root tip
Figure 8.10A Growth (in an onion root) 46

47 Cell Replacement in Bone Marrow
Figure 8.10B Cell replacement (in bone marrow) 47

48 Asexual Reproduction: Hydra
Figure 8.10C Asexual reproduction (of a hydra) 48

49 MEIOSIS © 2012 Pearson Education, Inc. 49

50 8.11 Chromosomes are matched in homologous pairs
In humans, somatic cells have 23 pairs of homologous chromosomes one member of each pair from each parent The human sex chromosomes X and Y differ in size and genetic composition The other 22 pairs of chromosomes are autosomes with the same size and genetic composition Student Misconceptions and Concerns Some students might conclude that sex chromosomes function only in determining the sex of the individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait noted in Module 8.11. © 2012 Pearson Education, Inc. 50

51 8.11 Chromosomes are matched in homologous pairs
Homologous chromosomes are matched in length centromere position gene locations A locus (plural, loci) is the name for the position of a specific gene Different versions of a gene may be found at the same locus on maternal and paternal chromosomes Student Misconceptions and Concerns Some students might conclude that sex chromosomes function only in determining the sex of the individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait noted in Module 8.11. © 2012 Pearson Education, Inc. 51

52 Pair of homologous chromosomes One duplicated chromosome
Locus Centromere Sister chromatids One duplicated chromosome Figure 8.11 A pair of homologous chromosomes 52

53 8.12 Gametes have a single set of chromosomes
An organism’s life cycle is the sequence of stages leading from the adults of one generation to the adults of the next Humans and many animals and plants are diploid, with body cells that have two sets of chromosomes - one from each parent Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips 1. 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. 2. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair maybe a sandal and a sneaker! 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 growth.) © 2012 Pearson Education, Inc. 53

54 8.12 Gametes have a single set of chromosomes
Meiosis is a process that converts diploid nuclei to haploid nuclei. Diploid cells have two homologous sets of chromosomes. Haploid cells have one set of chromosomes. Meiosis occurs in the sex organs, producing gametes—sperm and eggs. Fertilization is the union of sperm and egg. The zygote has a diploid chromosome number, one set from each parent. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips 1. 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. 2. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair maybe a sandal and a sneaker! 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 growth.) © 2012 Pearson Education, Inc. 54

55 Multicellular diploid adults (2n  46)
Haploid gametes (n  23) Egg cell Sperm cell Fertilization n Meiosis Ovary Testis Diploid zygote (2n  46) 2n Mitosis Key Haploid stage (n) Diploid stage (2n) Multicellular diploid adults (2n  46) Figure 8.12A The human life cycle 55

56 8.12 Gametes have a single set of chromosomes
All sexual life cycles include an alternation between a diploid stage a haploid stage Producing haploid gametes prevents the chromosome number from doubling in every generation Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips 1. 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. 2. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair maybe a sandal and a sneaker! 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 growth.) © 2012 Pearson Education, Inc. 56

57 A pair of homologous chromosomes in a diploid parent cell duplicated
Sister chromatids 1 2 3 INTERPHASE MEIOSIS I MEIOSIS II Figure 8.12B How meiosis halves chromosome number 57

58 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis is a type of cell division that produces haploid gametes in diploid organisms Two haploid gametes combine in fertilization to restore the diploid state in the zygote Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 58

59 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis and mitosis are preceded by the duplication of chromosomes meiosis is followed by two consecutive cell divisions and mitosis is followed by only one cell division Because in meiosis, one duplication of chromosomes is followed by two divisions, each of the four daughter cells produced has a haploid set of chromosomes Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 59

60 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis I – Prophase I – events occurring in the nucleus. Chromosomes coil and become compact. Homologous chromosomes come together as pairs by synapsis. Each pair, with four chromatids, is called a tetrad. Nonsister chromatids exchange genetic material by crossing over. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 60

61 Chromosomes duplicate Prophase I Metaphase I Anaphase I
Centrosomes (with centriole pairs) Centrioles Sites of crossing over Spindle Tetrad Nuclear envelope Chromatin Sister chromatids Fragments of the nuclear Centromere (with a kinetochore) Spindle microtubules attached to a kinetochore Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Chromosomes duplicate Prophase I Metaphase I Anaphase I INTERPHASE: MEIOSIS I: Homologous chromosomes separate Figure 8.13_left The stages of meiosis: Interphase and Meiosis I 61

62 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis I – Metaphase I – Tetrads align at the cell equator. Meiosis I – Anaphase I – Homologous pairs separate and move toward opposite poles of the cell. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 62

63 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis I – Telophase I Duplicated chromosomes have reached the poles. A nuclear envelope re-forms around chromosomes in some species. Each nucleus has the haploid number of chromosomes. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 63

64 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis II follows meiosis I without chromosome duplication. Each of the two haploid products enters meiosis II. Meiosis II – Prophase II Chromosomes coil and become compact (if uncoiled after telophase I). Nuclear envelope, if re-formed, breaks up again. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 64

65 Cleavage furrow Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II MEIOSIS II: Sister chromatids separate Sister chromatids separate Haploid daughter cells forming Telophase II and Cytokinesis Figure 8.13_right The stages of meiosis: Telophase I and Meiosis II 65

66 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator. Meiosis II – Anaphase II Sister chromatids separate and chromosomes move toward opposite poles. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 66

67 8.13 Meiosis reduces the chromosome number from diploid to haploid
Meiosis II – Telophase II Chromosomes have reached the poles of the cell. A nuclear envelope forms around each set of chromosomes. With cytokinesis, four haploid cells are produced. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips Challenge students to identify which stage of meiosis is most like mitosis. Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students to identify essential differences. © 2012 Pearson Education, Inc. 67

68 8.14 Mitosis and meiosis have important similarities and differences
Mitosis and meiosis both begin with diploid parent cells that have chromosomes duplicated during the previous interphase. However the end products differ. Mitosis produces two genetically identical diploid somatic daughter cells. Meiosis produces four genetically unique haploid gametes. Student Misconceptions and Concerns 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 is prevented if each gamete has only half the genetic material of the adult cells. Teaching Tips 1. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. 2. 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 pairs of chromosomes separate. Consider sketching a comparison of the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I. Figure 8.14 helps to make this important distinction. You might create a test question in which you ask students to draw several pairs of homologous chromosomes lined up at metaphase in mitosis versus meiosis I. © 2012 Pearson Education, Inc. 68

69 (before chromosome duplication)
Prophase Metaphase Duplicated chromosome (two sister chromatids) MITOSIS Parent cell (before chromosome duplication) Chromosome duplication Site of crossing over 2n  4 Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the Tetrad formed by synapsis of homologous chromosomes Metaphase I Prophase I MEIOSIS I Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis No further chromosomal duplication; sister chromatids anaphase II n Daughter cells of meiosis II Daughter cells of meiosis I Haploid n  2 Anaphase I Telophase I Homologous anaphase I; remain together MEIOSIS II Figure 8.14 Comparison of mitosis and meiosis 69

70 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Genetic variation in gametes results from independent orientation at metaphase I random fertilization Teaching Tips 1. The possible number of combinations produced by independent orientation of human chromosomes at meiosis metaphase I is 223 or 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! 2. Another way to represent the various combinations produced by independent orientation of chromosomes at meiosis metaphase I continues the shoe analogy. Imagine that you have two pairs of shoes. One pair is black, the other is white. You want to make a new pair drawing one shoe from each original pair. Four possible pairs can be made. You can have (1) the left black and left white, (2) the right black and right white, (3) the left black and right white, or (4) the right black and left white. Actually using two pairs of shoes from your students can inject humor and create a concrete example that reduces confusion. For an additional bit of humor, ask the class if 46 students want to contribute their shoes as you try to demonstrate all 8,388,608 combinations! © 2012 Pearson Education, Inc. 70

71 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Independent orientation at metaphase I Each pair of chromosomes independently aligns at the cell equator There is an equal probability of the maternal or paternal chromosome facing a given pole The number of combinations for chromosomes packaged into gametes is 2n where n = haploid number of chromosomes Teaching Tips 1. The possible number of combinations produced by independent orientation of human chromosomes at meiosis metaphase I is 223 or 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! 2. Another way to represent the various combinations produced by independent orientation of chromosomes at meiosis metaphase I continues the shoe analogy. Imagine that you have two pairs of shoes. One pair is black, the other is white. You want to make a new pair drawing one shoe from each original pair. Four possible pairs can be made. You can have (1) the left black and left white, (2) the right black and right white, (3) the left black and right white, or (4) the right black and left white. Actually using two pairs of shoes from your students can inject humor and create a concrete example that reduces confusion. For an additional bit of humor, ask the class if 46 students want to contribute their shoes as you try to demonstrate all 8,388,608 combinations! © 2012 Pearson Education, Inc. 71

72 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Random fertilization – The combination of each unique sperm with each unique egg increases genetic variability Teaching Tips 1. The possible number of combinations produced by independent orientation of human chromosomes at meiosis metaphase I is 223 or 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! 2. Another way to represent the various combinations produced by independent orientation of chromosomes at meiosis metaphase I continues the shoe analogy. Imagine that you have two pairs of shoes. One pair is black, the other is white. You want to make a new pair drawing one shoe from each original pair. Four possible pairs can be made. You can have (1) the left black and left white, (2) the right black and right white, (3) the left black and right white, or (4) the right black and left white. Actually using two pairs of shoes from your students can inject humor and create a concrete example that reduces confusion. For an additional bit of humor, ask the class if 46 students want to contribute their shoes as you try to demonstrate all 8,388,608 combinations! © 2012 Pearson Education, Inc. 72

73 Two equally probable arrangements of chromosomes at metaphase I
Possibility A Two equally probable arrangements of chromosomes at metaphase I Possibility B Metaphase II Gametes Combination 3 Combination 4 Combination 2 Combination 1 Figure 8.15_s3 Results of the independent orientation of chromosomes at metaphase I (step 3) 73

74 8.16 Homologous chromosomes may carry different versions of genes
Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes Homologous chromosomes may have different versions of a gene at the same locus One version was inherited from the maternal parent and the other came from the paternal parent Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene Teaching Tips You might have some fun with the concept of different versions of genes. Playing with the pun “jeans,” ask students if all jeans are the same. (Some are stone washed, some have buttons instead of zippers, some have more pockets than others, etc.) These versions of clothing jeans are like different versions of genetic genes, representing options and sources of diversity. © 2012 Pearson Education, Inc. 74

75 duplicated chromosomes)
Coat-color genes Eye-color Brown Black Meiosis White Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) Chromosomes of the four gametes White coat (c); pink eyes (e) Brown coat (C); black eyes (E) E C e c Figure 8.16 Differing genetic information (coat color and eye color) on homologous chromosomes 75

76 8.17 Crossing over further increases genetic variability
Genetic recombination is the production of new combinations of genes due to crossing over. Crossing over is an exchange of corresponding segments between separate (non-sister) chromatids on homologous chromosomes. Non-sister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over. Corresponding amounts of genetic material are exchanged between maternal and paternal (non-sister) chromatids. Teaching Tips 1. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes (although this analogy is quite limited). 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 (think of shoes with switched shoelaces). Critiquing this limited analogy may also help students to think through the process of crossing over. 2. In the shoe analogy, after exchanging shoelaces, we have “recombinant shoes”! 3. Challenge students to consider the number of unique humans that can be formed by the processes of the independent orientation of chromosomes, random fertilization, and crossing over. Without crossing over, we already calculated over 70 trillion possibilities. But as the text notes in Module 8.17, there are typically one to three crossover events for each human chromosome, and these can occur at many different places along the length of the chromosome. The potential number of combinations far exceeds any number that humans can comprehend, representing the truly unique nature of each human being (an important point that delights many students!) © 2012 Pearson Education, Inc. 76

77 Chiasma Tetrad Figure 8.17A Chiasmata, the sites of crossing over 77

78 Figure 8.17B How crossing over leads to genetic recombination
Tetrad (pair of homologous chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Gametes of four genetic types 1 2 3 4 C c e E Figure 8.17B How crossing over leads to genetic recombination 78

79 ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE
© 2012 Pearson Education, Inc. 79

80 8.18 A karyotype is a photographic inventory of an individual’s chromosomes
A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs Karyotypes are often produced from dividing cells arrested at metaphase of mitosis allow for the observation of homologous chromosome pairs chromosome number chromosome structure Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at © 2012 Pearson Education, Inc. 80

81 Packed red Hypotonic and white solution Fixative blood cells Blood
culture Packed red and white blood cells Centrifuge Fluid Hypotonic solution Fixative White blood cells Stain 3 2 1 Figure 8.18_s3 Preparation of a karyotype from a blood sample (step 3) 81

82 Figure 8.18_s4 Preparation of a karyotype from a blood sample (step 4)
82

83 Centromere Sister chromatids Pair of homologous chromosomes
Figure 8.18_s5 Preparation of a karyotype from a blood sample (step 5) 5 Sex chromosomes 83

84 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome
Trisomy 21 involves the inheritance of three copies of chromosome 21 and is the most common human chromosome abnormality. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. If you have several hundred students or more in your class, it is likely that at least one of your students has a sibling with Down syndrome. The authors note that, overall, about one in every 700 babies are born with Down syndrome. 3. The National Down Syndrome Society has a website at It is a wonderful resource. © 2012 Pearson Education, Inc. 84

85 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome
Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include: mental retardation, characteristic facial features, short stature, heart defects, susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and shortened life span. The incidence increases with the age of the mother. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. If you have several hundred students or more in your class, it is likely that at least one of your students has a sibling with Down syndrome. The authors note that, overall, about one in every 700 babies are born with Down syndrome. 3. The National Down Syndrome Society has a website at It is a wonderful resource. © 2012 Pearson Education, Inc. 85

86 Figure 8.19A Figure 8.19A A karyotype showing trisomy 21, and an individual with Down syndrome Trisomy 21 86

87 Figure 8.19A_1 Figure 8.19A_1 A karyotype showing trisomy 21 Trisomy 21 87

88 8.20 Accidents during meiosis can alter chromosome number
Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during meiosis I, if both members of a homologous pair go to one pole or meiosis II if both sister chromatids go to one pole. Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.20 illustrates two types of nondisjunction errors in meiosis. © 2012 Pearson Education, Inc. 88

89 MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n  1 n  1 n n
Figure 8.20B_s3 Nondisjunction in meiosis II (step 3) n  1 n  1 n n Abnormal gametes Normal gametes 89

90 8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival
Sex chromosome abnormalities tend to be less severe, perhaps because of the small size of the Y chromosome or X-chromosome inactivation. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Some syndromes related to human sexuality are not the result of abnormalities in sex chromosome number. Androgen insensitivity syndrome produces sterile males who possess mostly female sex characteristics. People with this condition are genetically male, but have bodies that fail to respond to male sex hormones. The National Institute of Health web site “Genetics Home Reference” can provide additional details about this and most genetic disorders at © 2012 Pearson Education, Inc. 90

91 8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival
The following table lists the most common human sex chromosome abnormalities. In general, a single Y chromosome is enough to produce “maleness,” even in combination with several X chromosomes, and the absence of a Y chromosome yields “femaleness.” Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Some syndromes related to human sexuality are not the result of abnormalities in sex chromosome number. Androgen insensitivity syndrome produces sterile males who possess mostly female sex characteristics. People with this condition are genetically male, but have bodies that fail to respond to male sex hormones. The National Institute of Health web site “Genetics Home Reference” can provide additional details about this and most genetic disorders at © 2012 Pearson Education, Inc. 91

92 Table 8.21 Table 8.21 Abnormalities of sex chromosome number in humans 92

93 8.22 EVOLUTION CONNECTION: New species can arise from errors in cell division
Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets. The formation of polyploid species is widely observed in many plant species but less frequently found in animals. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. In general, flowering plants are more likely to form new species through polyploidy than animals, because unlike most animals, many flowering plants can fertilize themselves. 3. The gray tree frog, which is found over most of the eastern half of the United States, from Florida and Texas to Ontario and Maine, consists of two species Hylachrysoscelis, which is diploid, and Hylaversicolor, which is tetraploid. The two species cannot be distinguished except by the number of chromosomes in their cells. The tetraploid species is thought to have been formed by an error in meiosis, similar to that frequently seen in plants. © 2012 Pearson Education, Inc. 93

94 Figure 8.22 Figure 8.22 The gray tree frog (Hyla versicolor), a tetraploid organism 94

95 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer
Chromosome breakage can lead to rearrangements that can produce genetic disorders or, if changes occur in somatic cells, cancer. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Challenge students to create a simple sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome. © 2012 Pearson Education, Inc. 95

96 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer
These rearrangements may include a deletion, the loss of a chromosome segment, a duplication, the repeat of a chromosome segment, an inversion, the reversal of a chromosome segment, or a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal. Student Misconceptions and Concerns Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.3B and 8.14 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.19–8.23. Teaching Tips 1. The Human Genome Website is a tremendous asset for nearly every discussion related to human genetics. It can be accessed at 2. Challenge students to create a simple sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome. © 2012 Pearson Education, Inc. 96


Download ppt "Ch. 8 Mitosis and Meiosis."

Similar presentations


Ads by Google