1. Meiosis and Sexual Reproduction
Chapter 9

2. Why Sex?

3. 9.1 Genes and Alleles Genes Alleles
Sequences of DNA that encode heritable traits
Alleles
Slightly different forms of the same gene
Each specifies a different version of gene product

4. Sexual and Asexual Reproduction
Asexual reproduction (1 parent)
Offspring inherit parent’s genes
Clones (identical copies of parent)
Sexual reproduction (2 parents)
Offspring differ from parents and each another
Different combinations of alleles
Different details of shared traits

5. Sexual Reproduction
Meiosis, gamete formation, and fertilization occur in sexual reproduction
Meiosis and fertilization shuffle parental alleles
Offspring inherit new combinations of alleles

6. Where Gametes Form

7. ovules inside an anther (where ovary (where sexual sexual spores
that give rise to
sperm form)
ovules inside an
ovary (where sexual
spores that give
to eggs form)
Flowering plant
Fig. 9.3a, p.140

8. (where sperm originate) ovary (where eggs develop)
testis
(where sperm originate)
ovary
(where eggs
develop)
Human male
Human female
Fig. 9.3b-c, p.140

9. Animation: Reproductive organs
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10. Key Concepts: SEXUAL VS. ASEXUAL REPRODUCTION
By asexual reproduction, one parent alone transmits its genetic information to offspring
By sexual reproduction, offspring typically inherit information from two parents that differ in their alleles
Alleles are different forms of the same gene; they specify different versions of a trait

11. 9.2 What Meiosis Does Meiosis Fertilization
Nuclear division mechanism that precedes gamete formation in eukaryotic cells
Halves parental chromosome number
Fertilization
Fusion of two gamete nuclei
Restores parental chromosome number
Forms zygote (first cell of new individual)

12. Meiosis and Fertilization

13. germ cell germ cell 2n each chromosome duplicated during interphase n
MEIOSIS I
separation of
homologues
MEIOSIS II
separation of
sister chromatids
gametes
gametes 2n
diploid number
restored at
fertilization
zygote
Fig. 9.12, p.150

14. Animation: Meiosis I and II
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15. Homologues Sexual reproducers inherit pairs of chromosomes
1 from maternal parent, 1 from paternal parent
The pairs are homologous (“the same”)
Except nonidentical sex chromosomes (X and Y)
Same length, shape, genes
All pairs interact at meiosis
One chromosome of each type sorts into gametes

16. Homologous Chromosomes

17. 9.3 Tour of Meiosis
All chromosomes are duplicated during interphase, before meiosis
Two divisions, meiosis I and II, divide the parental chromosome number by two
Each forthcoming gamete is haploid (n)

18. Meiosis I The first nuclear division
Each duplicated chromosome lines up with its homologous partner
The two homologous chromosomes move apart, toward opposite spindle poles

19. Prophase I
Chromosomes condense and align tightly with their homologues
Each homologous pair undergoes crossing over
Microtubules form the bipolar spindle
One pair of centrioles moves to the other side of the nucleus

20. Prophase I (cont.) Nuclear envelope breaks up
Microtubules growing from each spindle pole penetrate the nuclear region
Microtubules tether one or the other chromosome of each homologous pair

21. Prophase I

22. Metaphase I
Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator

23. Metaphase I

24. Anaphase I
Microtubules separate each chromosome from its homologue, moving to opposite spindle poles
Other microtubules overlap midway between spindle poles, slide past each other to push poles farther apart
As anaphase I ends, one set of duplicated chromosomes nears each spindle pole

25. Anaphase I

26. Telophase I Two nuclei form All chromosomes are still duplicated
Typically, the cytoplasm divides
All chromosomes are still duplicated
Each still consists of two sister chromatids

27. Telophase I

28. Meiosis II The second nuclear division
Sister chromatids of each chromosome are pulled away from each other
Each is now an individual chromosome

29. Prophase II

30. Metaphase II

31. Anaphase II and Telophase II
In anaphase II, one chromosome of each type is moved toward opposite spindle poles
Occurs in both nuclei formed in meiosis I
By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes

32. Anaphase II

33. Telophase II

34. Prophase I Metaphase I Anaphase I Telophase I
Meiosis I
newly forming
microtubules of
the spindle
one pair of homologous chromosomes
spindle equator (midway between the two poles)
plasma membrane
breakup
of nuclear
envelope
centrosome with
a pair of centrioles,
moving to opposite
sides of nucleus
Prophase I
Metaphase I
Anaphase I
Telophase I
Chromosomes were duplicated earlier, in
interphase.
Prior to metaphase I, one
set of microtubules had
tethered one chromosome
of each type to one spindle
pole and another set tethered
its homologue to the other spindle pole.
One of each duplicated
chromosome, maternal
or paternal, moves to a
spindle pole; its homologue
moves to the opposite pole.
One of each type
of chromosome has
arrived at a spindle
pole. In most species,
the cytoplasm divides
at this time.
Fig. 9.5a, p.142

35. Anaphase I Telophase I Prophase I Metaphase I
Meiosis I
Stepped Art
Fig. 9-5a, p.142

36. Fig. 9.5b, p.142

37. Prophase II Metaphase II Anaphase II Telophase II
Meiosis II
there is
no DNA replication
between the two divisions
Prophase II
Metaphase II
Anaphase II
Telophase II
In each cell, one of two centrioles moves to the opposite side of the
cell, and a new bipolar
spindle forms.
By now, microtubules from
both spindle poles have finished a tug-of-war.
The sister chromatids of each chromosome move apart and are now individual, unduplicated
A new nuclear envelope
encloses each parcel of
chromosomes, so there
are now four nuclei.
Fig. 9.5b, p.142

38. Anaphase II Telophase II Prophase II Metaphase II
Meiosis II
Stepped Art
Fig. 9-5b, p.142

39. Haploid Daughter Cells
When cytoplasm divides, four haploid cells result
One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies

40. Key Concepts: STAGES OF MEIOSIS
Diploid cells have a pair of each type of chromosome, one maternal and one paternal
Meiosis, a nuclear division mechanism, reduces the chromosome number
Meiosis occurs only in cells set aside for sexual reproduction

41. Key Concepts: STAGES OF MEIOSIS (cont.)
Meiosis sorts out a reproductive cell’s chromosomes into four haploid nuclei
Haploid nuclei are distributed to daughter cells by way of cytoplasmic division

42. Animation: Meiosis step-by-step
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43. 9.4 Meiosis Introduces Variation in Traits
Two events in meiosis cause variation in traits in sexually reproducing species
Crossing over during prophase I of meiosis
Chromosome shuffling during metaphase I of meiosis

44. Prophase I: Crossing Over
Nonsister chromatids of homologous chromosomes undergo crossing over
They exchange segments at the same place along their length
Each ends up with new combinations of alleles not present in either parental chromosome

45. Crossing Over

46. a A maternal chromosome (purple) and paternal chromosome (blue) were duplicated earlier, during
interphase. They become visible in microscopes early in prophase I, when hey star to condense to
threadlike form.
b Each chromosome
and its homologous partner zipper
together, so all four chromatids
are tightly aligned.
mom’s
allele A
dad’s
allele a
mom’s
allele A
mom’s
allele A
mom’s
allele B
dad’s
allele b
mom’s
allele B
dad’s
allele b
c Here is a simple way to
think about crossing over. (Chromosomes are still c
ondensed and threadlike,
and each is tightly aligned
with its homologous
partner.)
d Their intimate contact
promotes crossing over
at different places along the length of nonsister chromatids.
e At the crossover site, paternal and maternal chromatids exchange
corresponding segments.
f Crossing over mixes
up maternal and paternal
alleles on homologous
chromosomes.
Fig. 9.6, p.144

47. Fig. 9.6a, p.144

48. Fig. 9.6b, p.144

49. Fig. 9.6c, p.144

50. Fig. 9.6d, p.144

51. Fig. 9.6e, p.144

52. Fig. 9.6f, p.144

53. Animation: Crossing over
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54. Metaphase I: Chromosome Shuffling
Homologous chromosomes align randomly during metaphase I
Microtubules can harness either a maternal or paternal chromosome of each homologous pair to either spindle pole
Either chromosome may end up in any new nucleus (gamete)

55. Chromosome Shuffling: Random Alignment

56. combinations possible1 2 3
combinations possible or or or
Fig. 9.7, p.145

57. combinations possible1 2 3
combinations possible or or or
Stepped Art
Fig. 9-7, p.145

58. Animation: Random alignment
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59. Key Concepts: CHROMOSOME RECOMBINATION AND SHUFFLING
During meiosis, each pair of maternal and paternal chromosomes swaps segments and exchanges alleles
Pairs get randomly shuffled, so forthcoming gametes end up with different mixes of maternal and paternal chromosomes

60. 9.5 From Gametes to Offspring
Multicelled diploid and haploid bodies are typical in life cycles of plants and animals
Plants
Sporophyte: A multicelled plant body (diploid) that makes haploid spores
Spores give rise to gametophytes (multicelled plant bodies in which haploid gametes form)

61. From Gametes to Offspring
Animals
Germ cells in the reproductive organs give rise to sperm or eggs
Fusion of a sperm and egg at fertilization results in a zygote

62. Comparing Plant And Animal Life Cycles

63. Fig. 9.8a, p.146

64. DIPLOID HAPLOID multicelled gametophyte
meiosis
multicelled
sporophyte
(2n)
zygote
(2n)
DIPLOID
fertilization
meiosis
HAPLOID
gametes
spores
(n)
(n)
multicelled
gametophyte
(n)
meiosis
meiosis
a Plant life cycle
Fig. 9.8a, p.146

65. Fig. 9.8b, p.146

66. multicelled body DIPLOID HAPLOID
meiosis
multicelled
body
(2n)
zygote
(2n)
DIPLOID
fertilization
meiosis
HAPLOID
gametes
(n)
b Animal life cycle
Fig. 9.8b, p.146

67. Animation: Generalized life cycles
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68. Introducing Variation in Offspring
Three events cause new combinations of alleles in offspring:
Crossing over during prophase I (meiosis)
Random alignment of maternal and paternal chromosomes at metaphase I (meiosis)
Chance meeting of gametes at fertilization
All three contribute to variation in traits

69. Sperm Formation in Animals

70. secondary spermatocytes (haploid) primary spermatocyte (diploid)
sperm (mature,
haploid male
gametes)
secondary spermatocytes (haploid)
primary spermatocyte (diploid)
diploid male
germ cell
spermatids (haploid)
a Growth
b Meiosis I and
cytoplasmic division
c Meiosis II and cytoplasmic division
Fig. 9.9, p.147

71. Animation: Sperm formation
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72. Egg Formation in Animals

73. three polar bodies (haploid)
first polar body (haploid)
oogonium
(diploid
female
germ cell)
primary oocyte (diploid)
secondary oocyte (haploid)
ovum (haploid)
a Growth
b Meiosis I and cytoplasmic division
c Meiosis II and cytoplasmic division
Fig. 9.10a, p.147

74. Animation: Egg formation
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75. Key Concepts: SEXUAL REPRODUCTION IN LIFE CYCLES
In animals, gametes form by different mechanisms in males and females
In most plants, spore formation and other events intervene between meiosis and gamete formation

76. 9.6 Comparing Mitosis and Meiosis
Both mitosis and meiosis require bipolar spindle to move and sort duplicated chromosomes
Some mechanisms of meiosis resemble those of mitosis, and may have evolved from them
Example: DNA repair enzymes function in both

77. Differences in Mitosis and Meiosis
Mitosis maintains parental chromosome number
Duplicates genetic information
Occurs in body cells
Meiosis halves chromosome number
Introduces new combinations of alleles in offspring
Occurs only in cells for sexual reproduction

78. Comparing Mitosis and Meiosis

79. Comparing Mitosis and Meiosis

80. Comparing Mitosis and Meiosis

81. Prophase I Metaphase I Anaphase I Telophase I
In a diploid (2n) germ cell,
duplicated chromosomes
now condense. The bipolar
spindle forms and tethers the
chromosomes. Crossovers
occur between homologues.
Each maternal chromosome
and its paternal homologue
are randomly aligned midway
between the two spindle
poles. Either one may get
attached to either pole.
Homologous
partners
separate
and move
to opposite
poles.
There are two clusters
of chromosomes. New
nuclear envelopes may
form and the cytoplasm
may divide before
meiosis II begins.
Fig. 9.11a, p.148

82. cell, the duplicated chromosomes now condense. Bipolar spindle forms
Mitosis
Prophase
Metaphase
Anaphase
Telophase
In a diploid (2n) body
cell, the duplicated chromosomes now
condense. Bipolar
spindle forms
and tethers the chromosomes.
All chromosomes
aligned at the
spindle equator.
Sister chromatids of each chromosome
moved to opposite
spindle poles.
Two diploid (2n) nuclei form. After cytoplasmic
division, there are two diploid body cells.
Fig. 9.11b, p.149

83. no interphase and no DNA replication between the two nuclear divisions
Prophase II
Metaphase II
Anaphase II
Telophase II
All chromosomes still
duplicated. New spindle
forms in each nucleus,
tethers chromosomes
to spindle poles.
All chromosomes
aligned at the
spindle equator.
Sister chromatids of
each chromosome
moved to opposite
spindle poles.
Four haploid (n) nuclei
form. After cytoplasmic
division, haploid cells
function as gametes
or spores.
Fig. 9.11c, p.149

84. Key Concepts: MITOSIS AND MEIOSIS COMPARED
Recent molecular evidence suggests that meiosis originated through mechanisms that already existed for mitosis and, before that, for repairing damaged DNA

85. Animation: Comparing mitosis and meiosis
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86. Video: Why Sex?
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