1. Meiosis and Sexual Life Cycles
Chapter 13
Meiosis and Sexual Life Cycles

2. Overview: Hereditary Similarity and Variation Living organisms
Are distinguished by their ability to reproduce their own kind

3. Heredity
Is the transmission of traits from one generation to the next
Variation
Shows that offspring differ somewhat in appearance from parents and siblings
Figure 13.1

4. Genetics
Is the scientific study of heredity and hereditary variation

5. Concept 13.1: Offspring acquire genes from parents by inheriting chromosomes

6. Inheritance of Genes Genes Are the units of heredity
Are segments of DNA

7. Each gene in an organism’s DNA
Has a specific locus on a certain chromosome
We inherit
One set of chromosomes from our mother and one set from our father

8. Comparison of Asexual and Sexual Reproduction
In asexual reproduction
One parent produces genetically identical offspring by mitosis
Figure 13.2
Parent
Bud
0.5 mm

9. In sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents

10. Concept 13.2: Fertilization and meiosis alternate in sexual life cycles
A life cycle
Is the generation-to-generation sequence of stages in the reproductive history of an organism

11. Sets of Chromosomes in Human Cells
In humans
Each somatic cell has 46 chromosomes, made up of two sets
One set of chromosomes comes from each parent

12. A karyotype
Is an ordered, visual representation of the chromosomes in a cell
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids
Figure 13.3

13. Figure 13.3 Preparing a Karyotype
APPLICATION A karyotype is a display of
condensed chromosomes arranged in pairs.
Karyotyping can be used to screen for
abnormal numbers of chromosomes or
defective chromosomes associated with
certain congenital disorders, such as Down
syndrome.
TECHNIQUE Karyotypes are prepared from
isolated somatic cells, which are treated with
a drug to stimulate mitosis and then grown in
culture for several days. A slide of cells
arrested in metaphase is stained and then viewed
with a microscope equipped with a digital camera.
A digital photograph of the chromosomes is entered
into a computer, and the chromosomes are
electronically rearranged into pairs according to
size and shape.
RESULTS This karyotype shows the chromosomes
from a normal human male. The patterns of stained
bands help identify specific chromosomes and parts
of chromosomes. Although difficult to discern in the
karyotype, each metaphase chromosome consists of
two, closely attached sister chromatids (see diagram).
5 µm
Pair of homologous
chromosomes
Centromere
Sister
chromatids

14. Figure 13.3 Preparation of a human karyotype (Layer 2)

15. Figure 13.3 Preparation of a human karyotype (Layer 3)

16. Figure 13.3 Preparation of a human karyotype (Layer 4)

17. Figure 13.x2 Human female chromosomes shown by bright field G-banding

18. Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

19. Figure 13.x4 Human male chromosomes shown by bright field G-banding

20. Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

21. Homologous chromosomes
Are the two chromosomes composing a pair
Have the same characteristics
May also be called autosomes

22. Sex chromosomes Are distinct from each other in their characteristics
Are represented as X and Y
Determine the sex of the individual, XX being female, XY being male

23. A diploid cell Has two sets of each of its chromosomes
In a human has 46 chromosomes (2n = 46)

24. In a cell in which DNA synthesis has occurred
All the chromosomes are duplicated and thus each consists of two identical sister chromatids
Figure 13.4
Key
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

25. Unlike somatic cells
Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes

26. Behavior of Chromosome Sets in the Human Life Cycle
At sexual maturity
The ovaries and testes produce haploid gametes by meiosis

27. During fertilization The zygote
These gametes, sperm and ovum, fuse, forming a diploid zygote
The zygote
Develops into an adult organism

28. The human life cycle Key Figure 13.5 Haploid gametes (n = 23)
Haploid (n)
Diploid (2n)
Haploid gametes (n = 23)
Ovum (n)
Sperm
Cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

29. The Variety of Sexual Life Cycles
The three main types of sexual life cycles
Differ in the timing of meiosis and fertilization

30. In animals Meiosis occurs during gamete formation
Gametes are the only haploid cells
Gametes
Figure 13.6 A
Diploid
multicellular
organism
Key
MEIOSIS
FERTILIZATION n 2n
Zygote
Haploid
Mitosis
(a) Animals

31. (b) Plants and some algae
Exhibit an alternation of generations
The life cycle includes both diploid and haploid multicellular stages
MEIOSIS
FERTILIZATION n 2n
Haploid multicellular
organism (gametophyte)
Mitosis
Spores
Gametes
Zygote
Diploid
multicellular
organism
(sporophyte)
(b) Plants and some algae
Figure 13.6 B

32. (c) Most fungi and some protists
In most fungi and some protists
Meiosis produces haploid cells that give rise to a haploid multicellular adult organism
The haploid adult carries out mitosis, producing cells that will become gametes
MEIOSIS
FERTILIZATION n 2n
Haploid multicellular
organism
Mitosis
Gametes
Zygote
(c) Most fungi and some protists
Figure 13.6 C

33. Concept 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid
Takes place in two sets of divisions, meiosis I and meiosis II

34. The Stages of Meiosis An overview of meiosis Figure 13.7 Interphase
Homologous pair
of chromosomes
in diploid parent cell
Chromosomes
replicate
Homologous pair of replicated chromosomes
Sister
chromatids
Diploid cell with
replicated
chromosomes 1 2
Homologous
separate
Haploid cells with
replicated chromosomes
Sister chromatids
Haploid cells with unreplicated chromosomes
Meiosis I
Meiosis II

35. Meiosis I
Reduces the number of chromosomes from diploid to haploid
Meiosis II
Produces four haploid daughter cells

36. Interphase and meiosis I
Centrosomes
(with centriole pairs)
Sister
chromatids
Chiasmata
Spindle
Tetrad
Nuclear
envelope
Chromatin
Centromere
(with kinetochore)
Microtubule
attached to
kinetochore
Tertads line up
Metaphase
plate
Homologous
chromosomes
separate
Sister chromatids
remain attached
Pairs of homologous
chromosomes split up
Chromosomes duplicate
Homologous chromosomes
(red and blue) pair and exchange
segments; 2n = 6 in this example
INTERPHASE
MEIOSIS I: Separates homologous chromosomes
PROPHASE I
METAPHASE I
ANAPHASE I
Figure 13.8

37. MEIOSIS II: Separates sister chromatids
Telophase I, cytokinesis, and meiosis II
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II AND
MEIOSIS II: Separates sister chromatids
Cleavage
furrow
Sister chromatids
separate
Haploid daughter cells
forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing single chromosomes
Two haploid cells
form; chromosomes
are still double
Figure 13.8

38. A Comparison of Mitosis and Meiosis
Meiosis and mitosis can be distinguished from mitosis
By three events in Meiosis l

39. Synapsis and crossing over
Homologous chromosomes physically connect and exchange genetic information

40. Tetrads on the metaphase plate
At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates

41. Separation of homologues
At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell
In anaphase II of meiosis, the sister chromatids separate

42. (before chromosome replication)
A comparison of mitosis and meiosis
Figure 13.9
MITOSIS
MEIOSIS
Prophase
Duplicated chromosome
(two sister chromatids)
Chromosome
replication
Parent cell
(before chromosome replication)
Chiasma (site of
crossing over)
MEIOSIS I
Prophase I
Tetrad formed by
synapsis of homologous
chromosomes
Metaphase
Chromosomes
positioned at the
metaphase plate
Tetrads
Metaphase I
Anaphase I
Telophase I
Haploid
n = 3
MEIOSIS II
Daughter
cells of
meiosis I
Homologues
separate
during
anaphase I;
sister
chromatids
remain together
Daughter cells of meiosis II n
Sister chromatids separate during anaphase II
Anaphase
Telophase
Sister chromatids
separate during
anaphase 2n
Daughter cells
of mitosis
2n = 6

43. Reshuffling of genetic material in meiosis
Concept 13.4: Genetic variation produced in sexual life cycles contributes to evolution
Reshuffling of genetic material in meiosis
Produces genetic variation

44. Origins of Genetic Variation Among Offspring
In species that produce sexually
The behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation

45. Independent Assortment of Chromosomes
Homologous pairs of chromosomes
Orient randomly at metaphase I of meiosis

46. In independent assortment
Each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs
Key
Maternal set of
chromosomes
Paternal set of
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Possibility 2
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4
Figure 13.10

47. Crossing Over Crossing over
Produces recombinant chromosomes that carry genes derived from two different parents
Figure 13.11
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

48. Random Fertilization The fusion of gametes
Will produce a zygote with any of about 64 trillion diploid combinations

49. Evolutionary Significance of Genetic Variation Within Populations
Is the raw material for evolution by natural selection

50. Mutations Sexual reproduction
Are the original source of genetic variation
Sexual reproduction
Produces new combinations of variant genes, adding more genetic diversity

51. How do cells at the completion of meiosis compare with cells that have replicated their DNA and are just about to begin meiosis?
They have twice the amount of cytoplasm and half the amount of DNA.
They have half the number of chromosomes and half the amount of DNA.
They have the same number of chromosomes and half the amount of DNA.
They have half the number of chromosomes and one-fourth the amount of DNA.
They have half the amount of cytoplasm and twice the amount of DNA.
Answer: 4
Source: Barstow - Test Bank for Biology, Seventh Edition, Question #18

52. Which number represents G2? *
Which number represents G2? * I II
III IV V
Answer: 2
Source: Barstow - Test Bank for Biology, Seventh Edition, Question #46
Discussion Notes for the Instructor
Before showing this figure you might want to remind students that this represents a cell/s undergoing meiosis. There are several questions which can be asked to guide the discussion of this question, including:
During which stage of the meiotic cell cycle does a cell contain the greatest amount of DNA?
During which stage of the meiotic cell cycle does the cell contain the least amount of DNA?
Using these questions as an outline, discussion of the choices might look like this:
Choice A, during G2 the cell contains the greatest amount of DNA, so this can not be correct.
Choice B, correct
Choice C, similar to choice A
Choice D, similar to choice A
Choice E, similar to choice A

53. Which number represents the DNA content of a sperm cell?
Which number represents the DNA content of a sperm cell? I II
III IV V
Answer: 5
Source: Barstow - Test Bank for Biology, Seventh Edition, Question #47

54. The DNA content of a diploid cell in the G1 phase of the cell cycle is measured. If the DNA content is x, then the DNA content of the same cell at metaphase of meiosis I would be
0.25x
0.5x x.
2x.
4x.
Answer: 4
Source: Campbell/Reece - Biology, Seventh Edition, EOC Self-Quiz Question #4

55. The DNA content of a diploid cell in the G1 phase of the cell cycle is measured. If the DNA content is x, then the DNA content at metaphase of meiosis II would be
0.25x.
0.5x. x.
2x.
4x.
Answer: 3
Source: Campbell/Reece - Biology, Seventh Edition, EOC Self-Quiz Question #5

56. The DNA content of a cell is measured in the G2 phase. After meiosis I, the DNA content of one of the two cells produced would be
equal to that of the G2 cell.
twice that of the G2 cell.
one-half that of the G2 cell.
one-fourth that of the G2 cell.
impossible to estimate due to independent assortment of homologous chromosomes.
Answer: 3
Source: Taylor - Student Study Guide for Biology, Seventh Edition, Test Your Knowledge Question #7

57. Which of the following would not be considered a haploid cell?
daughter cell after meiosis II
gamete
daughter cell after mitosis in gametophyte generation of a plant
cell in prophase I
cell in prophase II
Answer: 4
Source: Taylor - Student Study Guide for Biology, Seventh Edition, Test Your Knowledge Question #17

58. A cell in G2 before meiosis compared with one of the four cells produced by that meiotic division has
twice as much DNA and twice as many chromosomes.
four times as much DNA and twice as many chromosomes.
four times as much DNA and four times as many chromosomes.
half as much DNA but the same number of chromosomes.
half as much DNA and half as many chromosomes.
Answer: 2
Source: Taylor - Student Study Guide for Biology, Seventh Edition, Test Your Knowledge Question #20

59. Children with Down Syndrome

60. Down Syndrome—As a function of mother’s age
1 in 1000 births in U.S.
1 in 12 births at age 50
Most frequent genetic cause of mental retardation
I.Q. =

61. Down Syndrome—As a function of mother’s age

62. Human Somatic Cells have 46 Chromosomes
How many chromosomes in human gametes?
How do you know?

63. Down Syndrome How many chromosomes are in the egg? Sperm?
How do you know?
D.S. is due to an error in Meiosis

64. Down Syndrome Karyotype
Down syndrome due to an error in meiosis
What’s meiosis?
What is wrong with the Karyotype?
Why are most trisomies fatal?
Trisomies involving the sex chromo’s sex chromosomes are not fatal.

65. Adult Human (Somatic cells: 46 C)
Human Life Cycle
Role of mitosis?
Meiosis
produces gametes
A reductive division (46  23 chromosomes)
Don’t confuse meiosis with mitosis
What if gametes were made by mitosis?
Sperm (23 C)
Fertilization
Egg (23 C)
Fertilization
Zygote (46 C)
Mitosis
Meiosis
Meiosis
Adult Human (Somatic cells: 46 C)

66. Normal Meiosis followed by Fertilization
Meiosis I
Meiosis II
Fertilization
Normal sperm
Chromosomes
Normal diploid
zygote
Both daughter cells
have one copy of
each chromosome
Product of Meiosis:
Haploid Egg Cell

67. Genetic Basis of Down Syndrome Diploid plus one extra copy
Nondisjunction of chromosome pair #21 during meiosis leads to Down Syndrome
Diploid minus one copy
of chromosome 21:
Zygote dies
No copy of
chromosome 21
Normal
sperm
Chromosome
pair #21
Are misaligned
Two copies of
chromosome 21
Diploid plus one extra copy
of chromosome #21:
Down syndrome

68. Down Syndrome—a function of mom’s age
Why is the incidence of Down Syndrome a function of mom’s age and not that of the dad?

69. Egg formation in humans
All pre-egg cells present before girls are born
Girls are born with about a 1000 pre-egg cells
At birth all Pre-egg cells are stuck in metaphase I
Homologous chromosomes are held in the middle of pre-egg cell by spindle fibers
After Reaching Puberty, each month one pre-egg cell finishes meiosis I & II
Meiosis produces one egg
The other 3 cells are called polar bodies and die

70. Mom’s Pre-egg Cells form Prenatally
At about 12 years, women start ovulating one egg each month for the next 40 years.
A 50 yr old woman has had her eggs sitting with chromosomes aligned in metaphase I for over 50 years!
Egg spindle fibers degenerate with age—Causes....
Chromosomes move to one side
Results in nondisjunction
Nondisjunction is rare with the larger chromosomes, #’s 1 – 20—Why??
Usually lethal because too much genetic imbalance.

71. Why doesn’t the age of the father influence the incidence of Down syndrome?
Sex cell formation in Males
Sperm formation starts at puberty and continues daily for life
Each pre-sperm cell divides twice to produce 4 sperm
million sperm produced per day!
Sperm forming cells do not stop in meiosis I of metaphase
Sperm cells don't get old
Therefore, no nondisjunction

72. Screening for Down Syndrome: Amniocentesis
Fetus located using ultrasound, needle inserted to remove amniotic fluid
Not performed until 16th week of pregnancy
Fluid contains fetal biochemicals and fetal cells from skin, respiratory tract, urinary tract
Culture cells for 1-2 weeks
Make Karyotype to detect abnormal chromosome numbers
Amniocentesis

73. Screening for Down Syndrome: Chorionic Villi Sampling (CVS)
Catheter inserted vaginally and chorionic tissue removed
Perform 9-11 weeks after conception
Make Karyotype
CVS—
Done earlier in pregnancy
Less chance of complications if end pregnancy
Slightly riskier for fetus
Greater chance of infection
Amniocentesis—have results...
Done later in pregnancy
Slightly safer than CVS

74. Mitosis vs. Meiosis No cross-over
Produces 2 genetically identical somatic cells
Involves only 1 division
Chromosomes align single file in the middle of the cell during metaphase
Sister chromatids separate during anaphase
Daughter cells have the same number of chromosomes as the parent cell
Cross-over during prophase I
Produces 4 genetically different gametes
Involves 2 divisions
Homologous pairs align in pairs during metaphase I
Homologous pairs separate during anaphase 1
Sister chromatids separate during anaphase 2
Daughter cells have half the number of chromosomes as the parent cell

75. Why Meiosis Causes Genetic Variation
Independent Assortment
Homologous pairs of chromosomes align independently of one another during metaphase I
Maternal and paternal chromosomes are shuffled during meiosis
223 or 8,388,608 different combinations for each parent
Fertilization gives 70 trillion possible genetic combinations

76. One Cause of Genetic Variation: Independent Assortment
Two possible arrangements of chromosomes during metaphase I
Possibility 1
Possibility 2

77. Cross-over—the 2nd Reason for Genetic Variation
Homologous chromosomes exchange parts during Prophase I
Cross over results in thousands of genetically different gametes

78. Cross-over results in genetically different gametes
2. Regions exchanged
3. Products of meiosis
Meiosis I
Meiosis II
Paternal
copy
Parental
Crossing over
Recombinant
Homologous
chromosomes
Recombinant
Maternal
copy
Parental

79. How do chromo’s align at metaphase? What separates at anaphase?
Homologous
chromosomes
Sister
chromatids
Review of Mitosis
Product of
mitosis:
Two genetically
identical diploid
daughter cells
Parent cell
DNA Replication
Metaphase: Chromosomes align single file
Anaphase Separates sister chromatids
How do chromo’s align at metaphase?
What separates at anaphase?
How do the daughter cells compare genetically?

80. How do the chromo’s align at metaphase I? What separates at anaphase I
Review of Meiosis
Crossing over
Product of
meiosis:
Four genetically
different
haploid
daughter cells
DNA Replication
Parent cell
Metaphase I
(homologous
Chromosomes are paired)
Alignment
without
replication
Anaphase I
separates homologous chromosomes
Anaphase II
separates sister chromatids
How do the chromo’s align at metaphase I?
What separates at anaphase I
How do the chromo’s align at metaphase II?
What separates at anaphase II?