1. Chapter 12 Meiosis and Sexual Reproduction
(Sections ) 1

2. How Meiosis Introduces Variations in Traits
Two events in meiosis introduce novel (new) combinations of alleles into gametes:
Crossing over in prophase I
Random Segregation of chromosomes during meiosis I
Along with fertilization, these events contribute to the variation in combinations of traits among the offspring of sexually reproducing species

3. Crossing Over in Prophase I
In prophase I, chromatids of homologous chromosomes align along their length and exchange segments (crossing over)
crossing over
Process in which homologous chromosomes exchange corresponding segments during prophase I of meiosis
Introduces novel combinations of traits among offspring

4. Homologous Chromosomes Align

5. Crossing Over

6. Crossing Over
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 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.
Fig. 12.6, p. 180

7. Crossing Over
Figure 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.
Fig. 12.6a, p. 180

8. Crossing Over
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.
Figure 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.
Fig. 12.6a, p. 180

9. Crossing Over
Figure 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.
Fig. 12.6b, p. 180

10. Crossing Over
B Close contact between homologous chromosomes promotes crossing over between nonsister chromatids. Paternal and maternal chromatids exchange corresponding pieces.
Figure 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.
Fig. 12.6b, p. 180

11. Crossing Over
Figure 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.
Fig. 12.6c, p. 180

12. Crossing Over C Crossing
over mixes up paternal and maternal alleles on homologous chromosomes.
Figure 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.
Fig. 12.6c, p. 180

13. 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.
Crossing Over
B Close contact between homologous chromosomes promotes crossing over between nonsister chromatids. Paternal and maternal chromatids exchange corresponding pieces.
Figure 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
Fig. 12.6, p. 180

14. ANIMATION: Crossing over
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15. Segregation of Chromosomes Into Gametes
When homologous chromosomes separate in meiosis I, one of each chromosome pair goes to each of the two new nuclei
For each chromosome pair, the maternal (from mom) or paternal version (from dad) is equally likely to end up in either nucleus

16. Each time a human germ cell undergoes meiosis, the four gametes that form end up with one of 8,388,608 (or 223) possible combinations of homologous chromosomes
because there are 2 of each type of chromosome (maternal and paternal) and 23 types of chromosomes in humans.

17. Segregation of Chromosomes

18. Segregation of Chromosomes
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 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.
Fig. 12.7, p. 181

19. Segregation of Chromosomes
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 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.
Stepped Art
Fig. 12.7, p. 181

20. ANIMATION: Random alignment
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21. Key Concepts Stages of Meiosis
Meiosis is a nuclear division process that occurs only in cells set aside for sexual reproduction
Meiosis reduces the chromosome number by sorting a reproductive cell’s chromosomes into four new nuclei

22. Key Concepts Recombinations and Shufflings
During meiosis, homologous chromosomes come together and swap segments (crossing over after tetrad formation)
Then they are randomly sorted into separate nuclei (they could end up in any of 4 sperm or eggs)
Both processes lead to novel (new) combinations of alleles among offspring

23. 12.5 From Gametes to Offspring
Gametes (eggs and sperm) are specialized cells that are the basis of sexual reproduction
All gametes are haploid, but they differ in other details
Gamete formation differs among plants and animals

24. Gamete Formation in Animals
In animals, a zygote matures into a multicelled body that produces gametes of its own by meiosis of diploid germ cells
In male animals, the germ cell develops into a primary spermatocyte, which undergoes meiosis to form four sperm
In female animals, a germ cell becomes a primary oocyte, which undergoes meiosis to form one egg and three polar bodies, which degenerate

25. Key Terms sperm Mature male gamete Haploid product of meiosis egg
Mature female gamete, or ovum

26. Animal Life Cycle

27. mitosis meiosis fertilization
Animal Life Cycle
mitosis
zygote (2n)
multicelled body (2n)
diploid
meiosis
fertilization
haploid
Figure 12.8 Comparing the life cycles of animals and plants.
gametes (n)
Fig. 12.8b, p. 182

28. ANIMATION: Sperm formation
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29. Sperm Formation in Animals
1. A diploid male germ cell (spermatogonium) develops into a diploid primary spermatocyte as it replicates its DNA
2. Meiosis I in the primary spermatocyte results in two haploid secondary spermatocytes
3. Four haploid spermatids form when secondary spermatocytes undergo meiosis II
4. Spermatids mature as sperm (haploid male gametes)

30. Sperm Formation in Animals

31. Sperm Formation in Animals2 3 4 1
male germ cell
Figure General mechanism of sperm formation in animals.
sperm
Fig. 12.9, p. 183

32. Sperm Formation in Animals
male germ cell 1 2 3
sperm 4
Figure General mechanism of sperm formation in animals.
Stepped Art
Fig. 12.9, p. 183

33. Egg Formation in Animals
5. A diploid female germ cell (oogonium) develops into a diploid primary oocyte as it replicates its DNA
6. Meiosis I in the primary oocyte results in a haploid secondary oocyte and a haploid polar body
Unequal cytoplasmic division makes the polar body much smaller than the oocyte
7. Meiosis II, followed by unequal cytoplasmic division in the secondary oocyte, results in a polar body and ovum (egg)

34. Egg Formation in Animals

35. Egg Formation in Animals5 6 7
egg
female germ cell
Figure General mechanism of egg formation in animals. Left, an illustration of human sperm surrounding an egg during fertilization.
Fig , p. 183

36. Egg Formation in Animals
female germ cell 5 6
egg 7
Figure General mechanism of egg formation in animals. Left, an illustration of human sperm surrounding an egg during fertilization.
Stepped Art
Fig , p. 183

37. ANIMATION: Egg formation
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38. Key Concepts Sexual Reproduction in the Context of Life Cycles
Gametes form by different mechanisms in males and females, but meiosis is part of both processes

39. Fertilization
Two gametes fuse at fertilization, resulting in a diploid zygote
Human sperm surrounding an egg during fertilization
Figure General mechanism of egg formation in animals. Left, an illustration of human sperm surrounding an egg during fertilization.

40. 12.6 Mitosis and Meiosis: An Ancestral Connection?
Mitosis makes exact copies of chromosomes; meiosis mixes genes up and halves the chromosome number
Though their end results differ, the four stages of mitosis and meiosis II are similar
Sexual reproduction probably originated by mutations that affected processes of mitosis

41. Comparing Mitosis and Meiosis

42. Comparing Mitosis and Meiosis
Meiosis I One diploid nucleus to two haploid nuclei
Prophase I
• Chromosomes condense.
• Homologous chromosomes pair.
• Crossovers occur (not shown).
• Spindle forms and attaches chromosomes to spindle poles.
• Nuclear envelope breaks up.
Anaphase I
• Homologous chromosomes separate and move toward at spindle poles.
Figure Comparative summary of key features of
mitosis and meiosis, starting with a diploid cell. Only two paternal and two maternal chromosomes are shown. Both were duplicated in interphase, prior to nuclear division. A spindle of microtubules moves the chromosomes in mitosis as well as meiosis.
Mitosis maintains the parental chromosome number. Meiosis halves it, to the haploid number.
Mitotic cell division is the basis of asexual reproduction among eukaryotes. It also is the basis of growth and tissue repair of multicelled eukaryotic species.
Metaphase I
• Chromosomes align midway between spindle poles.
Telophase I
• Chromosome clusters arrive opposite spindle poles.
• New nuclear envelopes form.
• Chromosomes decondense.
Fig , p. 184

43. 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 Comparative summary of key features of
mitosis and meiosis, starting with a diploid cell. Only two paternal and two maternal chromosomes are shown. Both were duplicated in interphase, prior to nuclear division. A spindle of microtubules moves the chromosomes in mitosis as well as meiosis.
Mitosis maintains the parental chromosome number. Meiosis halves it, to the haploid number.
Mitotic cell division is the basis of asexual reproduction among eukaryotes. It also is the basis of growth and tissue repair of multicelled eukaryotic species.
Stepped Art
Fig , p. 184

44. Key Concepts Mitosis and Meiosis Compared
Similarities between mitosis and meiosis suggest that meiosis may have originated by evolutionary remodeling of mechanisms that already existed for mitosis and, before that, for repairing damaged DNA