Chapter 12 Meiosis and Sexual Reproduction

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)

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

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.

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

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

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

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

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

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

ANIMATED FIGURE: Genetic terms 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

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

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

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

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

ANIMATED FIGURE: Reproductive organs 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

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)

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

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

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

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

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

Summary of Steps in Meiosis

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

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

3D ANIMATION: Meiosis

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

ANIMATION: Meiosis I and II 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

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

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

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

ANIMATED FIGURE: 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

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

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

ANIMATED FIGURE: 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

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

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

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

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

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

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

ANIMATED FIGURE: Generalized life cycles 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

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

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

ANIMATED FIGURE: 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

Egg Formation in Animals female germ cell 5 6 egg 7 Figure 12.10 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

ANIMATED FIGURE: 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

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

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

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

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

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 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

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 12.11 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

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 12.11 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

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

Mouse Cell Nuclei During Meiosis Figure 12.12 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.

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