1. Chapter 12 Meiosis and Sexual Reproduction

2. 12.1 Why Sex?
In sexual reproduction, offspring arise from two parents and inherit genes from both
Asexual reproduction produces genetically identical copies of a parent (clones)

3. Advantages of Sexual Reproduction
An adaptive trait can spread more quickly through a sexually reproducing population than through an asexually reproducing one
Collectively, offspring of sexual reproducers have a better chance of surviving the effects of a harmful mutation that arises in the population

4. Sexual Reproduction in Animals and Plants
Figure 12.1 Moments in the stages
of sexual reproduction of humans
(left) and plants (right). Sexual
reproduction mixes up the genetic
material of two organisms.
In flowering plants, pollen grains
(orange) germinate on flower carpels
(yellow). Pollen tubes with
male gametes inside grow from
the grains down into tissues of the
ovary, which house the flower’s
female gametes.

5. Meiosis and Sexual Reproduction
Sexual reproduction generates new combinations of traits in fewer generations than does asexual reproduction
The process inherent to sexual reproduction that gives rise to this variation is meiosis, a nuclear division mechanism that halves the chromosome number

6. 12.2 Meiosis Halves the Chromosome Number
Sexual reproduction mixes up alleles from two parents
Meiosis, the basis of sexual reproduction, is a nuclear division mechanism that occurs in immature reproductive cells of eukaryotes

7. Homologous Chromosomes
Somatic (body) cells of humans and other sexually reproducing organisms are diploid
One chromosome of each pair is maternal, and the other is paternal
Except for a pairing of nonidentical sex chromosomes, homologous chromosomes carry the same set of genes

8. Introducing Alleles
Genes are regions in an organism’s DNA that encode information about heritable traits
In sexual reproduction, pairs of genes are inherited on pairs of chromosomes, one maternal and one paternal
Alleles are different forms of the same gene
Offspring of sexual reproducers inherit new combinations of alleles, the basis of traits

9. A Corresponding colored patches in this fluorescence micrograph indicate corresponding DNA sequences in a homologous chromosome pair. These chromosomes carry the same set of genes.
Figure 12.2 Animated Genes on chromosomes.
Different forms of a gene are called alleles.
Figure 12-2a p190

10. Genes occur in pairs on homologous chromosomes.
The members of each pair of genes may be identical, or they may differ slightly, as alleles.
B Homologous chromosomes carry the same series of genes, but the DNA sequence of any one of those genes might differ just a bit from that of its partner on the homologous chromosome.
Figure 12.2 Animated Genes on chromosomes.
Different forms of a gene are called alleles.
Figure 12-2b p190

11. ANIMATED FIGURE: Genetic terms
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12. Gametes and Germ Cells
Sexual reproduction involves the fusion of reproductive cells (gametes) from two parents
In plants and animals, gametes form inside special reproductive structures or organs
Division of immature reproductive cells (germ cells) gives rise to gametes

13. Reproductive organs of a human male
Figure 12.3 Animated Examples of reproductive organs. In human ovaries or testes, meiosis in
germ cells produces gametes called eggs and sperm. In the anthers or ovaries of flowering plants,
meiosis produces haploid spores that give rise to gametes by mitosis.
testis (where sperm originate)
Figure 12-3a p190

14. Reproductive organs of a human female
Figure 12.3 Animated Examples of reproductive organs. In human ovaries or testes, meiosis in
germ cells produces gametes called eggs and sperm. In the anthers or ovaries of flowering plants,
meiosis produces haploid spores that give rise to gametes by mitosis.
ovary (where eggs develop)
Figure 12-3b p190 14

15. Reproductive organs of a flowering plant
anther (where sexual spores that give rise to sperm cells form)
ovary (where sexual spores that give rise to egg cells form)
Figure 12.3 Animated Examples of reproductive organs. In human ovaries or testes, meiosis in
germ cells produces gametes called eggs and sperm. In the anthers or ovaries of flowering plants,
meiosis produces haploid spores that give rise to gametes by mitosis.
Figure 12-3c p190

16. ANIMATED FIGURE: Reproductive organs
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17. What Meiosis Does
Meiosis in animal germ cells gives rise to eggs (female gametes) or sperm (male gametes)
Gametes have a single set of chromosomes, so they are haploid (n): Their chromosome number is half of the diploid (2n) number
Meiosis of a human germ cell (2n) produces gametes with 23 chromosomes: one of each pair (n)

18. Two Divisions in Meiosis
Meiosis partitions the chromosomes of one diploid nucleus (2n) into four haploid (n) nuclei
In meiosis I, each duplicated homologous chromosome is separated from its partner
In meiosis II, sister chromatids are separated

19. Meiosis Halves the Chromosome Number
A In meiosis I, each duplicated chromosome in the nucleus pairs with its homologous partner.
B Homologous partners separate. The still-duplicated chromosomes are packaged into two new nuclei.
C Sister chromatids separate in meiosis II. The now unduplicated chromosomes are packaged into four new nuclei.
Figure 12.4 How meiosis halves the chromosome number
Stepped Art

20. Fertilization
The diploid chromosome number is restored at fertilization, when two haploid gametes (one egg and one sperm) fuse to form a zygote, the first cell of a new individual

21. Take-Home Message: Sexual Reproduction Increases Variation in Heritable Traits
Corresponding genes on homologous chromosomes vary in sequence as alleles
Alleles are the basis of traits; sexual reproduction mixes up alleles from two parents
Meiosis is the basis of sexual reproduction in eukaryotes; it precedes the formation of gametes or spores
Meiosis halves the diploid (2n) chromosome number, to the haploid number (n); when two gametes fuse at fertilization, the chromosome number is restored; the zygote has one set of chromosomes from each parent

22. 12.3 Visual Tour of Meiosis Meiosis halves the chromosome number
During meiosis, chromosomes of a diploid nucleus become distributed into four haploid nuclei

23. Summary of Steps in Meiosis

24. Meiosis Meiosis I One diploid nucleus to two haploid nuclei
Prophase I. Homologous chromosomes condense,
pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up.
plasma membrane
spindle
nuclear envelope breaking up
centrosome 1
Metaphase I. The homologous chromosome pairs
are aligned midway between spindle poles.
one pair of
homologous chromosomes 2
Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles. 3
Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form. 4
Figure 12.5 Animated Meiosis. Two pairs of chromosomes are illustrated
in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue
and pink. Micrographs show meiosis in a lily plant cell (Lilium regale).
Figure It Out: During which phase of meiosis does the chromosome
number become reduced?
Stepped Art

25. Meiosis Meiosis II Two haploid nuclei to four haploid nuclei
Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up.
No DNA replication 5
Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of
the spindle. 6
Anaphase II. All sister chromatids separate. The now unduplicated chromo -somes head to the spindle poles. 7
Telophase II. A cluster of chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. 8
Figure 12.5 Animated Meiosis. Two pairs of chromosomes are illustrated
in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue
and pink. Micrographs show meiosis in a lily plant cell (Lilium regale).
Figure It Out: During which phase of meiosis does the chromosome
number become reduced?
Stepped Art

26. 3D ANIMATION: Meiosis

27. Take-Home Message: What happens to a cell during meiosis?
During meiosis, the nucleus of a diploid (2n) cell divides twice.
Four haploid (n) nuclei form, each with a full set of chromosomes—one of each type

28. ANIMATION: Meiosis I and II
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29. 12.4 How Meiosis Introduces Variation in Traits
Crossovers and the random sorting of chromosomes into gametes result in new combinations of traits among offspring
Along with fertilization, these events contribute to the variation among the offspring of sexually reproducing species

30. Crossing Over in Prophase I
Crossing over is the process by which a chromosome and its homologous partner exchange heritable information in corresponding segments
Crossing over occurs during condensation in prophase I

31. A Here, we focus on only two of the many genes on a chromosome
A Here, we focus on only two of the many genes on a chromosome. In this example, one gene has alleles A A and a; the other has alleles B and b.
B Close contact between homologous chromosomes promotes crossing over between nonsister chromatids. Paternal and maternal chromatids exchange corresponding pieces.
Figure 12.6 Animated Crossing over. Blue signifies a paternal
chromosome, and pink, its maternal homologue. For clarity, we show
only one pair of homologous chromosomes and one crossover, but
more than one crossover may occur in each chromosome pair.
C Crossing over mixes up paternal and maternal alleles on homologous chromosomes.
Stepped Art
Figure 12-6 p194

32. ANIMATED FIGURE: Crossing over
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33. Chromosome Segregation
Homologous chromosomes can be attached to either spindle pole in prophase I, so each homologue can be packaged into either one of the two new nuclei
Random assortment produces 1023 (8,388,608) possible combinations of homologous chromosomes

34. The four possible alignments of three pairs of chromosomes in a nucleus at metaphase I.1
Resulting combinations
of maternal and paternal chromosomes in the two nuclei that form at telophase I. 2
Resulting combinations of maternal and paternal chromosomes in the four nuclei that form
at telophase II. Eight different combinations are possible. 3
Figure 12.7 Animated Hypothetical segregation of three pairs of chromosomes in meiosis I.
Maternal chromosomes are pink; paternal, blue. Which chromosome of each pair gets packaged
into which of the two new nuclei that form at telophase I is random. For simplicity, no crossing
over occurs in this example, so all sister chromatids are identical.
1 The four possible alignments of three pairs of chromosomes in a nucleus at metaphase I.
2 Resulting combina- tions of maternal and paternal chromosomes in the two nuclei that form at telophase I.
3 Resulting combinations of maternal and paternal chromosomes in the four nuclei that form at telo- phase II. Eight different combinations are possible.
Stepped Art
Figure 12-7 p195

35. ANIMATED FIGURE: Random alignment
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36. Take-Home Message: How does meiosis introduce variation in combinations of traits?
Crossing over is recombination between nonsister chromatids of homologous chromosomes during prophase I; it makes new combinations of parental alleles
Homologous chromosomes can be attached to either spindle pole in prophase I, so each homologue can end up in either one of the two new nuclei

37. 12.5 From Gametes to Offspring
Aside from meiosis, the details of gamete formation and fertilization differ among plants and animals

38. Gamete Formation in Plants
Sporophytes
Diploid bodies with specialized structures that form spores (haploid cells) that give rise to gametophytes through mitosis
Gametophytes
A multicelled haploid body inside which one or more gametes form

39. multicelled sporophyte (2n) multicelled gametophyte (n)
mitosis
multicelled sporophyte (2n)
zygote (2n)
Diploid
Fertilization
Meiosis
Haploid
Figure 12.8 Animated Comparing
the life cycles of animals and plants.
gametes (n)
spores (n)
multicelled gametophyte (n)
Figure 12-8a p196 39

40. multicelled body (2n) Meiosis gametophyte (n)
mitosis
multicelled body (2n)
zygote (2n)
Diploid
Fertilization
Meiosis
Haploid
Figure 12.8 Animated Comparing
the life cycles of animals and plants.
gametophyte (n)
Figure 12-8b p196 40

41. multicelled sporophyte (2n)
Figure 12.8 Animated Comparing
the life cycles of animals and plants.
multicelled body (2n)
Figure 12-8c p196

42. ANIMATED FIGURE: Generalized life cycles
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43. Gamete Formation in Animals
Males
Meiosis of primary spermatocytes produces four haploid spermatids, which mature into sperm
Females
Meiosis of a primary oocyte forms cells of different sizes; the secondary oocyte gets most of the cytoplasm and matures into an ovum (egg); other cells (polar bodies) get little cytoplasm and degenerate

44. Sperm Formation in Animals
male germ cell 1 2 3
sperm 4
Figure 12.9 Animated General mechanism of sperm formation in animals.
1 A male germ cell develops into a primary spermatocyte as it replicates its DNA. Both types of cell are diploid.
2 Meiosis I in the primary spermatocyte results in two secondary spermatocytes, which are haploid.
3 Four haploid sper- matids form when the secondary spermatocytes undergo meiosis II.
4 Spermatids mature as sperm (haploid male gametes).
Stepped Art

45. ANIMATED FIGURE: Sperm formation
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46. Egg Formation in Animals
female germ cell 5 6
egg 7
Figure Animated General mechanism of egg formation in animals. Below, human sperm surround an egg.
5 A female germ cell (an oogonium) develops into a primary oocyte as it replicates its DNA. Both types of cell are diploid.
6 Meiosis I in the primary oocyte results in a secondary oocyte and a first polar body. Unequal cytoplasmic division makes the polar body much smaller than the oocyte. Both cells are haploid. Polar bodies typically degenerate.
7 Meiosis II followed by unequal cytoplasmic division in the secondary oocyte results in a polar body and an ovum, or egg. Both cells are haploid.
Stepped Art

47. ANIMATED FIGURE: Egg formation
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48. Fertilization
Chance combinations of maternal and paternal chromosomes through fertilization produce a unique combination of genetic information
Fertilization
The fusion of two haploid gametes (sperm and egg) resulting in a diploid zygote

49. Fertilization Figure 12.10 Animated General mechanism of egg formation
in animals. Below, human sperm surround an egg.

50. Take-Home Message: How does meiosis fit into the life cycle of plants and animals?
Meiosis and cytoplasmic division precede the development of haploid gametes in animals and spores in plants
The union of two haploid gametes at fertilization results in a diploid zygote

51. 12.6 Mitosis and Meiosis – An Ancestral Connection?
Though they have different results, mitosis and meiosis are fundamentally similar processes
Meiosis may have evolved by the remodeling of existing mechanisms of mitosis

52. Comparing Mitosis and Meiosis
Meiosis I One diploid nucleus to two haploid nuclei
Telophase I
• Chromosome clusters arrive opposite spindle poles.
• New nuclear envelopes form.
• Chromosomes decondense.
Anaphase I
• Homologous chromosomes separate and move toward at spindle poles.
Prophase I
• Chromosomes condense.
• Homologous chromosomes pair.
• Crossovers occur (not shown).
• Spindle forms and attaches chromosomes to spindle poles.
• Nuclear envelope breaks up.
Metaphase I
• Chromosomes align midway between spindle poles.
Figure Animated Comparative summary of key features of
mitosis and meiosis, starting with a diploid cell. Only two paternal
and two maternal chromosomes are shown, for clarity. Mitosis maintains
the parental chromosome number. Meiosis halves it, to the
haploid number.
Stepped Art

53. Comparing Mitosis and Meiosis
Mitosis One diploid nucleus to two diploid nuclei
Telophase
• Chromosome clusters arrive opposite spindle poles.
• New nuclear envelopes form.
• Chromosomes decondense
Metaphase
• Chromosomes align midway between spindle poles.
Prophase
• Chromosomes condense.
• Spindle forms and attaches chromosomes to spindle poles.
• Nuclear envelope breaks up.
Anaphase
• Sister chromatids separate and move toward at spindle poles.
Figure Animated Comparative summary of key features of
mitosis and meiosis, starting with a diploid cell. Only two paternal
and two maternal chromosomes are shown, for clarity. Mitosis maintains
the parental chromosome number. Meiosis halves it, to the
haploid number.
Stepped Art

54. Comparing Mitosis and Meiosis
Meiosis II Two haploid nuclei to four haploid nuclei
Telophase II
• Chromosome clusters arrive at spindle poles.
• New nuclear envelopes form.
• Chromosomes decondense.
Prophase II
• Chromosomes condense.
• Spindle forms and attaches chromosomes to spindle poles.
• Nuclear envelope breaks up.
Metaphase II
• Chromosomes align midway between spindle poles.
Anaphase II
• Sister chromatids separate and move toward opposite spindle poles.
Figure Animated Comparative summary of key features of
mitosis and meiosis, starting with a diploid cell. Only two paternal
and two maternal chromosomes are shown, for clarity. Mitosis maintains
the parental chromosome number. Meiosis halves it, to the
haploid number.
Stepped Art

55. Evidence in BRCA Genes
Molecular products of BRCA genes monitor and repair breaks in DNA, for example during DNA replication prior to mitosis
These same molecules monitor and fix breaks in homologous chromosomes during crossing over in prophase I of meiosis
Mutations that affect these molecules can affect the outcomes of both mitosis and meiosis
Sexual reproduction probably originated by mutations that affected mitosis

56. Mouse Cell Nuclei During Meiosis
Figure Fluorescence micrograph of mouse cell nuclei during
meiosis. Blue stain: DNA; green: paired-up chromosomes
before crossing over; red: BRCA1, which is clustered around
telomeres and the sex chromosomes. This checkpoint protein
is involved in DNA repair in mitosis (DNA damage) and meiosis
(sealing crossover breaks), and in X chromosome inactivation.

57. Take-Home Message: Are the processes of mitosis and meiosis related?
Meiosis may have evolved by the remodeling of existing mechanisms of mitosis