Chapter 12 Meiosis and Sexual Reproduction (Sections 12.4 - 12.6) 1

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

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

Homologous Chromosomes Align

Crossing Over

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 12.6 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

Crossing Over Figure 12.6 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

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 12.6 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

Crossing Over Figure 12.6 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

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

Crossing Over Figure 12.6 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

Crossing Over C Crossing over mixes up paternal and maternal alleles on homologous chromosomes. Figure 12.6 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

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 12.6 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

ANIMATION: Crossing over To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

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

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.

Segregation of Chromosomes

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 12.7 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

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 12.7 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

ANIMATION: Random alignment To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

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

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

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

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

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

Animal Life Cycle

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

ANIMATION: Sperm formation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

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)

Sperm Formation in Animals

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

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

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)

Egg Formation in Animals

Egg Formation in Animals 5 6 7 egg female germ cell Figure 12.10 General mechanism of egg formation in animals. Left, an illustration of human sperm surrounding an egg during fertilization. Fig. 12.10, p. 183

Egg Formation in Animals female germ cell 5 6 egg 7 Figure 12.10 General mechanism of egg formation in animals. Left, an illustration of human sperm surrounding an egg during fertilization. Stepped Art Fig. 12.10, p. 183

ANIMATION: Egg formation To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

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

Fertilization Two gametes fuse at fertilization, resulting in a diploid zygote Human sperm surrounding an egg during fertilization Figure 12.10 General mechanism of egg formation in animals. Left, an illustration of human sperm surrounding an egg during fertilization.

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

Comparing Mitosis and Meiosis

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 12.11 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. 12.11, p. 184

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 12.11 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. 12.11, p. 184

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