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Transcript
Meiosis and Sexual Reproduction Chapter 10 Impacts, Issues: Why Sex? An adaptive trait tends to spread more quickly through a sexually reproducing population than through an asexually reproducing one Fig. 10-1b, p. 154 10.1 Introducing Alleles Asexual reproduction produces genetically identical copies of a parent (clones) Sexual reproduction introduces variation in the combinations of traits among offspring Genes and 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 Chromosome Pair: Maternal and Paternal 10.1 Key Concepts Sexual vs Asexual Reproduction In asexual reproduction, one parent transmits its genetic information to offspring In sexual reproduction, offspring typically inherit information from two parents who differ in their alleles Alleles are different forms of the same gene; they specify different versions of a trait 10.2 What Meiosis Does Meiosis • A nuclear division mechanism that precedes cytoplasmic division of immature reproductive cells in sexually-reproducing eukaryotic species Halving the Diploid Number A diploid cell has two nonidentical copies of every chromosome (except XY sex chromosomes) • Humans have 23 pairs of homologous chromosomes Meiosis in germ cells halves the diploid number of chromosomes (2n) to the haploid number (n), producing haploid gametes • Eggs and sperm have 23 unpaired chromosomes Gamete Production Gametes are produced in specialized reproductive structures or organs Fig. 10-3a, p. 156 anther (where sexual spores that give rise to sperm form) ovules inside an ovary (where sexual spores that give rise to eggs form) a Flowering plant Fig. 10-3a, p. 156 Fig. 10-3b, p. 156 testis (where sperm originate) b Human male Fig. 10-3b, p. 156 Fig. 10-3c, p. 156 ovary (where eggs develop) c Human female Fig. 10-3c, p. 156 Restoring the Diploid Number Human gametes (eggs and sperm) have 23 chromosomes – one of each homologous pair The diploid number (23 pairs) is restored at fertilization, when two haploid gametes fuse and form a diploid zygote, the first cell of a new individual Human Chromosomes: Homologous Pairs Two Divisions, Not One In meiosis, DNA is replicated once and divided twice (meiosis I and meiosis II), forming four haploid nuclei In meiosis I, each duplicated homologous chromosome is separated from its partner In meiosis II, sister chromatids are separated Two Divisions, Not One Meiosis I Meiosis II each chromosome in the cell pairs with its homologous partner then the partners separate p. 157 two chromosomes (unduplicated) one chromosome (duplicated) p. 157 10.3 Visual Tour of Meiosis 10.3 Visual Tour of Meiosis Meiosis I newly forming microtubules plasma membrane of the spindle breakup of nuclear envelope A Prophase I one pair of homologous chromosomes centrosome with a pair of centrioles, moving to opposite sides of nucleus B Metaphase I C Anaphase I D Telophase I Fig. 10-5a, p. 158 Fig. 10-5a (1), p. 158 Fig. 10-5a (1), p. 158 Fig. 10-5a (2), p. 158 Fig. 10-5a (2), p. 158 Fig. 10-5a (3), p. 158 Fig. 10-5a (3), p. 158 Fig. 10-5a (4), p. 158 Fig. 10-5a (4), p. 158 Fig. 10-5b (1), p. 159 Fig. 10-5b (2), p. 159 Fig. 10-5b (3), p. 159 Fig. 10-5b (4), p. 159 Meiosis I newly forming microtubules plasma membrane of the spindle breakup of nuclear envelope A Prophase I one pair of homologous chromosomes centrosome with a pair of centrioles, moving to opposite sides of nucleus B Metaphase I C Anaphase I D Telophase I Stepped Art Fig. 10-5a, p. 158 Meiosis II There is no DNA replication between the two nuclear divisions. E Prophase II F Metaphase II G Anaphase II H Telophase II Stepped Art Fig. 10-5b, p. 159 Animation: Meiosis step-by-step 10.2-10.3 Key Concepts Stages of Meiosis Meiosis reduces the chromosome number Meiosis occurs only in cells set aside for sexual reproduction Meiosis sorts a reproductive cell’s chromosomes into four haploid nuclei, which are distributed to descendent cells by cytoplasmic division 10.4 How Meiosis Introduces Variation in Traits Crossovers and the random sorting of chromosomes in meiosis introduce novel combinations of alleles into gametes, resulting in new combinations of traits among offspring Crossing Over in Prophase I Crossing over • The process by which a chromosome and its homologous partner exchange heritable information in corresponding segments • Occurs during condensation in prophase I Crossing Over Between Homologous Chromosomes Fig. 10-6a, p. 160 Fig. 10-6b, p. 160 A A a a B B b b B Here, we focus on only two genes. One gene has alleles A and a; the other has alleles B and b. Fig. 10-6b, p. 160 Fig. 10-6c, p. 160 crossover C Close contact between the homologous chromosomes promotes crossing over between nonsister chromatids, so paternal and maternal chromatids exchange segments. Fig. 10-6c, p. 160 Fig. 10-6d, p. 160 D Crossing over mixes up paternal and maternal alleles on homologous chromosomes. A A a a B B b b Fig. 10-6d, p. 160 Animation: Crossing over Segregation of Chromosomes into Gametes 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 Random Assortment A Alignment in nucleus at metaphase I B Alignments C Nuclei of the in two nuclei four resulting at metaphase II gametes Possible lineup #1 Possible lineup #2 Possible lineup #3 Possible lineup #4 Fig. 10-7, p. 161 A Alignment in nucleus at metaphase I B Alignments C Nuclei of the in two nuclei four resulting at metaphase II gametes Possible lineup #1 Possible lineup #2 Possible lineup #3 Possible lineup #4 Stepped Art Fig. 10-7, p. 161 Animation: Random alignment 10.4 Key Concepts: Chromosome Recombinations and Shufflings During meiosis, each pair of maternal and paternal chromosomes swaps segments Then, each chromosome is randomly segregated into one of the new nuclei Both processes lead to novel combinations of alleles – and traits – among offspring 10.5 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 Life Cycles of Plants and Animals Fig. 10-8a, p. 162 mitosis multicelled sporophyte (2n) zygote (2n) fertilization DIPLOID meiosis HAPLOID spores (n) gametes (n) mitosis multicelled gametophyte (n) mitosis a Plant life cycle Fig. 10-8a, p. 162 Fig. 10-8b, p. 162 mitosis multicelled body (2n) zygote (2n) fertilization DIPLOID meiosis HAPLOID gametes (n) b Animal life cycle Fig. 10-8b, p. 162 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 sperm (mature, haploid male gametes) secondary spermatocytes (haploid) diploid male germ cell primary spermatocyte (diploid) spermatids (haploid) A Growth B Meiosis I and cytoplasmic division C Meiosis II and cytoplasmic division Fig. 10-9, p. 163 sperm (mature, haploid male gametes) secondary spermatocytes (haploid) diploid male germ cell A Growth primary spermatocyte (diploid) B Meiosis I and cytoplasmic division spermatids (haploid) C Meiosis II and cytoplasmic division Stepped Art Fig. 10-9, p. 163 Animation: Sperm formation Egg Formation in Animals Fig. 10-10 (left), p. 163 first polar body (haploid) oogonium (diploid female germ cell) primary oocyte (diploid) secondary oocyte (haploid) A Growth three polar bodies (haploid) B Meiosis I and cytoplasmic division ovum (haploid) C Meiosis II and cytoplasmic division Fig. 10-10 (left), p. 163 three polar bodies first polar (haploid) body (haploid) oogonium primary oocyte (diploid) (diploid female germ cell) A Growth B Meiosis I and cytoplasmic division secondary oocyte (haploid) ovum (haploid) C Meiosis II and cytoplasmic division Stepped Art Fig. 10-10, p. 163 Animation: Egg formation More Shufflings at 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 10.5 Key Concepts: Sexual Reproduction in Context of Life Cycles Gametes form by different mechanisms in males and females In most plants, spore formation and other events intervene between meiosis and gamete formation 10.6 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 Fig. 10-11a (1), p. 165 Fig. 10-11a (2), p. 165 Fig. 10-11a (3), p. 165 Fig. 10-11b, p. 165 one diploid nucleus Prophase • Chromosomes condense. • Nuclear envelope breaks up. • Bipolar spindle forms; it attaches chromosomes to spindle poles. two diploid nuclei Metaphase • Chromosomes align midway between spindle poles. Anaphase • Sister chromatids separate as they are pulled toward spindle poles. Telophase • Chromosome clusters arrive at spindle poles. • Chromosomes decondense. • New nuclear envelopes form. Fig. 10-11b, p. 165 Fig. 10-11c, p. 164 one diploid nucleus Prophase I • Chromosomes condense. • Homologous chromosomes pair. • Bipolar spindle forms; it attaches chromosomes to spindle poles. • Crossovers occur. • Nuclear envelope breaks up. two haploid nuclei Metaphase I • Chromosomes align midway between spindle poles. Anaphase I • Homologous chromosomes separate as they are pulled toward spindle poles. Telophase I • Chromosome clusters arrive at spindle poles. • New nuclear envelopes form. • Chromosomes decondense. Fig. 10-11c, p. 164 Fig. 10-11d, p. 165 two haploid nuclei Prophase II • Chromosomes condense. • Bipolar spindle forms; it attaches chromosomes to spindle poles. • Nuclear envelope breaks up. four haploid nuclei Metaphase II • Chromosomes align midway between spindle poles. Anaphase II • Sister chromatids separate as they are pulled toward spindle poles. Telophase II • Chromosome clusters arrive at spindle poles. • New nuclear envelopes form. • Chromosomes decondense. Fig. 10-11d, p. 165 10.6 Key Concepts Mitosis and Meiosis Compared Meiosis may have originated by evolutionary remodeling of mechanisms that already existed for mitosis, and before that, for repairing damaged DNA Animation: Comparing mitosis and meiosis Animation: Generalized life cycles Animation: Meiosis Animation: Meiosis I and II Animation: Reproductive organs Video: Why sex?