1. Chapter 8: Cell Reproduction
Chapter 8: Cell Reproduction
Original slide set from:

2. STANDARDS
SPI Describe the relationship between the cell cycle and reproduction.
- I can determine the movement of chromosomes during cell reproduction.

3. With your partner
Find two different definitions of “chromosome”

4. Chromosome definition:
Chromosome Structure
Chromosome definition:
in a eukaryotic cell, one of the structures in the nucleus that are made up of DNA and protein; in a prokaryotic cell, the main ring of DNA

5. Chromosomes What do they look like? How many do humans have?
They look like an “X” just before the cell divides and an “I” after they divide.
46 chromosomes (23 from mom, 23 from dad)

6. With your partner
Explain the difference in the two shapes of chromosomes

7. Chromosome Vocabulary
Histones:
Chromatid:
Centromere:
a type of protein found in the chromosomes of eukaryotic cells but not prokaryotic cells.
one side of a chromosome that become visible during meiosis or mitosis
The region of the chromosome that holds the two sister chromatids together during mitosis

8. Chromosome or Chromatin?
Tightly coiled DNA & proteins during cell division; DNA cannot be “read” while in this form.
Loosely coiled DNA & proteins during the cell’s life other than cell division. DNA can be “read” to direct activities of the cell.

10. WITH YOUR PARTNER:
Do more complex organisms have more chromosomes?

11. What is cell reproduction?

12. WHAT CELL REPRODUCTION ACCOMPLISHES
May result in the birth of new organisms
More commonly involves the production of new cells

13. Cell Division
Cell division plays important roles in the lives of organisms.
Replaces damaged or lost cells
Permits growth
Allows for reproduction

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

15. Asexual Reproduction
Single-celled organisms reproduce by simple cell division
There is no fertilization of an egg by a sperm
The parent and its offspring have identical genes.

16. Asexual Reproduction Binary Fission
Prokaryotic cells divide through a simple form of division called Binary Fission
3 step process
Single “naked” strand splits and forms a duplicate of itself.
The two copies move to opposite sides of the cell
Cell “pinches” into two new and identical cells called "daughter cells". (Cell wall then forms if applicable) 16

17. Asexual Reproduction
Mitosis is the type of cell division responsible for:
Asexual reproduction
Growth and maintenance of multicellular organisms
Some multicellular organisms, such as sea stars, can grow new individuals from fragmented pieces.
Growing a new plant from a clipping

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

19. Sexual Reproduction
Sexual reproduction requires fertilization of an egg by a sperm using a special type of cell division called meiosis.
Thus, sexually reproducing organisms use:
Meiosis for reproduction
Mitosis for growth and maintenance

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

21. Chromosomes Chromosomes:
Are made of chromatin, a combination of DNA and protein molecules
Are not visible in a cell until cell division occurs
Before a parent cell splits into two, it duplicates its chromosomes

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

23. Eukaryotic Cell Genetic Information
Most genes on chromosomes in cell nucleus
A few genes found in mitochondrial and chloroplast DNA
Each chromosome: one very long DNA molecule, typically with thousands of genes.
Histones are proteins used to package DNA.
Nucleosomes consist of DNA wound around histone molecules.

24. Laura Coronado Bio 10 Chapter 8
DNA double helix
Histones
“Beads on
a string”
TEM
Nucleosome
Figure 8.4 DNA packing in a eukaryotic chromosome.
Tight helical fiber
Looped domains
Duplicated chromosomes
(sister chromatids)
TEM
Centromere
Figure 8.4
Laura Coronado Bio Chapter 8

25. Laura Coronado Bio 10 Chapter 8
Chromosome (one
long piece of DNA)
Centromere
Sister
chromatids
Figure UN 8.2 Summary: Duplicated chromosome
Duplicated chromosome
Laura Coronado Bio Chapter 8
Figure 8.UN2

26. Chromosomes
Before a cell divides, it duplicates all of its chromosomes, resulting in two copies called sister chromatids.
Sister chromatids are joined together at a narrow “waist” called the centromere.
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

27. Laura Coronado Bio 10 Chapter 8
Chromosome
duplication
Sister
chromatids
Figure 8.5 Duplication and distribution of a single chromosome.
Chromosome
distribution to
daughter cells
Laura Coronado Bio Chapter 8
Figure 8.5

28. 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.
The cell cycle consists of two distinct phases:
Interphase
The mitotic phase

29. S phase (DNA synthesis; chromosome duplication)
Interphase: metabolism and
growth (90% of time) G1 G2
Mitotic (M) phase:
cell division
(10% of time)
Figure 8.6 The eukaryotic cell cycle.
Cytokinesis
(division of
cytoplasm)
Mitosis
(division of nucleus)
Laura Coronado Bio Chapter 8
Figure 8.6

30. Interphase

31. Laura Coronado Bio 10 Chapter 8
Interphase
Most of a cell cycle is spent in interphase.
During interphase, a cell:
Performs its normal functions
Doubles everything in its cytoplasm
Grows in size
Laura Coronado Bio Chapter 8

32. Interphase Interphase 3 Stages
G1 (Gap 1) Phase - Cell performs its normal function (cells which do not divide stay in this stage for their entire life span)
-cells grow and mature
S (Synthesis) Phase - Here the cell actively duplicates its DNA in preparation for division
G2 (Gap 2) Phase - Amount of cytoplasm (including organelles) increases in preparation for division.
Another possibility
G0 Phase cells do not prepare for cell division
Generally straight from G1 phase
Example: fully developed cells in Central Nervous System never divide again

33. Mitosis

34. Mitosis The mitotic (M) phase includes two overlapping processes:
Mitosis, in which the nucleus and its contents divide evenly into two daughter nuclei
Cytokinesis, in which the cytoplasm is divided in two

35. Mitosis and Cytokinesis
Mitosis consists of four distinct phases:
(A) Prophase
(B) Metaphase
(C) Anaphase
(D) Telophase
Cytokinesis typically:
Occurs during telophase
Divides the cytoplasm
Is different in plant and animal cells

36. (with centriole pairs) Early mitotic spindle Fragments of
INTERPHASE
PROPHASE
Centrosomes
(with centriole pairs)
Early mitotic
spindle
Fragments of
nuclear envelope
Centrosome
Chromatin
Centromere
Nuclear
envelope
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Spindle
microtubules
Figure 8.7a Cell reproduction: A dance of the chromosomes. (Part 1) LM
Figure 8.7.a

37. Prophase Chromosomes condense Nuclear membrane breaks down
Centrioles migrate to opposite poles (in animal cells)
Microtubules attach to chromosomes and centrioles

38. Prophase

39. Nuclear envelope forming Cleavage furrow Spindle Daughter chromosomes
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Cleavage
furrow
Spindle
Daughter
chromosomes
Figure 8.7b Cell reproduction: A dance of the chromosomes. (Part 2)
Figure 8.7b

40. Metaphase
Chromosomes line up along the center of the cell

41. Metaphase

42. Anaphase Microtubules shorten
Chromatids separate and pull to opposite sides

43. Anaphase

44. Telophase Nuclear membrane forms around each set of chromosomes
Chromosomes unwind

45. Cell membrane separates the two daughter cells
Cytokinesis
Cytoplasm split in two
Cell membrane separates the two daughter cells

46. Telophase and Cytokinesis

47. Animal cell mitosis

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

49. Plant Cell Mitosis
Plant cell mitosis is similar to animal cell mitosis BUT cytokinesis is different
In plant, fungi and algae cell, a cell plate forms in the middle of the cell to divide the two cells.

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

51. Result of Mitosis
2 daughter cells that are identical to each other and identical to the parent cell

52. 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-ahead” 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. 52

53. What Is Cancer? Cancer is a disease of the cell cycle.
Cancer cells do not respond normally to the cell cycle control system.
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 53

54. Cancer Treatment Cancer treatment can involve:
Radiation therapy, which damages DNA and disrupts cell division
Chemotherapy, which uses drugs that disrupt cell division 55

55. Cancer Prevention and Survival
Certain behaviors can decrease the risk of cancer:
Not smoking
Exercising adequately
Avoiding exposure to the sun
Eating a high-fiber, low-fat diet
Performing self-exams
Regularly visiting a doctor to identify tumors early 56

56. Meiosis

57. Homologous Chromosomes
Different individuals of a single species have the same number and types of chromosomes.
A human somatic cell:
Is a typical body cell
Has 46 chromosomes
A karyotype is an image that reveals an orderly arrangement of chromosomes.
Homologous chromosomes are matching pairs of chromosomes that can possess different versions of the same genes.

58. Pair of homologous chromosomes Centromere Sister chromatidsLM
Centromere
Figure 8.11 Pairs of homologous chromosomes in a male karyotype.
Sister
chromatids
One duplicated
chromosome
Figure 8.11

59. Human Chromosomes Humans have: Humans are diploid organisms in which:
Two different sex chromosomes, X and Y
Twenty-two pairs of matching chromosomes, called autosomes
Humans are diploid organisms in which:
Their somatic cells contain two sets of chromosomes
Their gametes are haploid, having only one set of chromosomes

60. 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.

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

62. Meiosis Humans are diploid organisms in which:
Their somatic cells contain two sets of chromosomes
Their gametes are haploid, having only one set of chromosomes
In humans, a haploid sperm fuses with a haploid egg during fertilization to form a diploid zygote.
Sexual life cycles involve an alternation of diploid and haploid stages.
Meiosis produces haploid gametes, which keeps the chromosome number from doubling every generation.

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
Figure 8.13 How meiosis halves chromosome number. (Step 3)
INTERPHASE BEFORE MEIOSIS
MEIOSIS I
MEIOSIS II
Figure

64. The Process of Meiosis In meiosis:
Haploid daughter cells are produced in diploid organisms
Interphase is followed by two consecutive divisions, meiosis I and meiosis II
Crossing over occurs

65. INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Pairs of homologous
MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Centrosomes
(with centriole
pairs)
Sites of
crossing over
Microtubules
attached
to chromosome
Sister chromatids
remain attached
Spindle
Sister
chromatids
Nuclear
envelope
Centromere
Pair of
homologous
chromosomes
Figure 8.14a The stages of meiosis. (Part 1)
Chromatin
Homologous
chromosomes
pair up and
exchange
segments.
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
Chromosomes
duplicate.
Figure 8.14a

66. MEIOSIS II: SISTER CHROMATIDS SEPARATE
TELOPHASE I
AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND
CYTOKINESIS
Cleavage
furrow
Sister
chromatids
separate
Haploid
daughter
cells forming
Figure 8.14b The stages of meiosis. (Part 2)
Two haploid
cells form;
chromosomes
are still
doubled.
During another round of cell division, the sister
chromatids finally separate; four haploid
daughter cells result, containing single
chromosomes.
Figure 8.14b

67. Figure 8. 14bc The stages of meiosis
Figure 8.14bc The stages of meiosis. (Part 2) Metaphase II in a lily cell. LM
Figure 8.14bc

68. Review: Comparing Mitosis and Meiosis
In mitosis and meiosis, the chromosomes duplicate only once, during the preceding interphase.
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, while meiosis II is the same as mitosis since it separates sister chromatids.

69. (before chromosome duplication)
MITOSIS
MEIOSIS
Prophase
Prophase I
MEIOSIS I
Chromosome
duplication
Chromosome
duplication
Duplicated chromosome
(two sister chromatids)
Parent cell
(before chromosome duplication)
2n  4
Homologous
chromosomes come
together in pairs.
Site of crossing over
between homologous
(nonsister) chromatids
Metaphase
Metaphase I
Chromosomes
align at the
middle of the
cell.
Homologous pairs
align at the middle
of the cell.
Anaphase
Telophase
Anaphase I
Telophase I
Chromosome with two
sister chromatids
Sister
chromatids
separate
during
anaphase.
Figure 8.15 Comparing mitosis and meiosis
Homologous
chromosomes
separate during
anaphase I;
sister chromatids
remain together.
Haploid
n  2 2n 2n
Daughter
cells of meiosis I
Daughter cells
of mitosis
MEIOSIS II
Sister chromatids
separate during
anaphase II. n n n n
Daughter cells of meiosis II
Figure 8.15

70. Independent Assortment of Chromosomes
When aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chance.
Every chromosome pair orients independently of the others during meiosis.
For any species the total number of chromosome combinations that can appear in the gametes due to independent assortment is:
2n where n is the haploid number.
For a human:
n = 23
223 = 8,388,608 different chromosome combinations possible in a gamete

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

72. Random Fertilization
A human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote.
If each gamete represents one of 8,388,608 different chromosome combinations, at fertilization, humans would have 8,388,608 × 8,388,608, or more than 70 trillion, different possible chromosome combinations.

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

74. Crossing Over In crossing over:
Homologous chromosomes exchange genetic information
Genetic recombination, the production of gene combinations different from those carried by parental chromosomes, occurs

75. 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
Figure 8.18 The results of crossing over during meiosis for a single pair of homologous chromosomes. (Step 5)
Gametes
Recombinant
chromosomes
combine genetic
information from
different parents.
Recombinant chromosomes
Figure

76. How Accidents during Meiosis Can Alter Chromosome Number
In nondisjunction, the members of a chromosome pair fail to separate during anaphase, producing gametes with an incorrect number of chromosomes.
Nondisjunction can occur during meiosis I or II.
If nondisjunction occurs, and a normal sperm fertilizes an egg with an extra chromosome, the result is a zygote with a total of 2n + 1 chromosomes.
If the organism survives, it will have an abnormal number of genes.

77. Figure 8.20-3 NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II
Pair of homologous
chromosomes fails
to separate.
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Gametes
Figure 8.20 Two types of nondisjunction. (Step 3)
Number of
chromosomes
n  1
n  1
n – 1
n – 1
n  1
n – 1 n n
Abnormal gametes
Abnormal gametes
Normal gametes
Figure

78. Abnormal egg cell with extra chromosome n  1 Normal sperm cell
Figure 8.21 Fertilization after nondisjunction in the mother.
Normal
sperm cell
Abnormal zygote
with extra
chromosome 2n  1
n (normal)
Figure 8.21

79. Down Syndrome Down Syndrome: Is also called trisomy 21
Is a condition in which an individual has an extra chromosome 21
Affects about one out of every 700 children
The incidence of Down Syndrome increases with the age of the mother.
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.

80. LM
Figure 8.22 Trisomy 21 and Down syndrome.
Chromosome 21
Figure 8.22

81. Infants with Down syndrome90 80 70 60
Infants with Down syndrome
(per 1,000 births) 50 40 30 20
Figure 8.23 Maternal age and Down syndrome. 10 20 25 30 35 40 45 50
Age of mother
Figure 8.23

82. Abnormal Numbers of Sex Chromosomes
Nondisjunction can also affect the sex chromosomes.

83. Table 8.1 Abnormalities of Sex Chromosome Number in Humans

84. Evolution Connection: The Advantages of Sex
Asexual reproduction conveys an evolutionary advantage when plants are:
Sparsely distributed
Superbly suited to a stable environment
Sexual reproduction may convey an evolutionary advantage by:
Speeding adaptation to a changing environment
Allowing a population to more easily rid itself of harmful genes

85. Figure 8.24 Sexual and asexual reproduction.

86. Distribution via mitosis Duplication of all chromosomes Genetically
Figure UN 8.1 Summary: Cell division
Genetically
identical
daughter
cells
Figure 8.UN1

87. DNA synthesis; chromosome duplication chromosome duplication
S phase
DNA synthesis; chromosome duplication
Interphase
Cell growth and
chromosome duplication G1 G2
Mitotic
(M) phase
Figure UN 8.3 Summary: The cell cycle
Genetically
identical
“daughter”
cells
Cytokinesis
(division of
cytoplasm)
Mitosis
(division of
nucleus)
Figure 8.UN3

88. Human Life Cycle Haploid gametes (n  23) Key n Haploid (n) Egg cell
Diploid (2n) n
Sperm cell
MEIOSIS
FERTILIZATION
Figure UN 8.4 Summary: Human life cycle
Diploid
zygote
(2n  46)
Male and female
diploid adults
(2n  46) 2n
MITOSIS
and development
Figure 8.UN4

89. MITOSIS MEIOSIS Parent cell (2n) Parent cell (2n) MEIOSIS I Pairing of
Chromosome
duplication
Pairing of
homologous
chromosome
Chromosome
duplication
Crossing over
Figure UN 8.5 Summary: Comparing Mitosis and Meiosis
Daughter
cells 2n 2n
MEIOSIS II n n n n
Daughter cells
Figure 8.UN5

90. Differences between Mitosis and Meiosis
Meiosis has 2 divisions – two rounds of chromosome separation.
Crossing over in meiosis – exchange of genetic material between homologous chromosomes – occurs during synapsis(pairing of homologous chromosomes in M I)

91. Differences between Mitosis and Meiosis
Mitosis occurs in all cells, meiosis limited to certain cells
Mitosis produces 2 identical cells, Meiosis produce 4 cells which are not identical
Mitosis : daughter cells of same ploidy as parent; Meiosis: daughter cells haploid of parent

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