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Chapter 12 Meiosis and Sexual Reproduction

Chapter 12 Meiosis and Sexual Reproduction. Advantages of Sexual Reproduction. An adaptive trait can spread more quickly through a sexually reproducing population than through an asexually reproducing one

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Chapter 12 Meiosis and Sexual Reproduction

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  1. Chapter 12Meiosis and Sexual Reproduction

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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 12-2a p190

  7. 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 12-2b p190

  8. 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

  9. Reproductive organs of a human male testis (where sperm originate) Figure 12-3a p190

  10. Reproductive organs of a human female ovary (where eggs develop) Figure 12-3b p190

  11. 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 12-3c p190

  12. 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

  13. 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)

  14. 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

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

  16. 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

  17. Summary of Steps in Meiosis

  18. 1 2 3 4 Prophase I. Homologous chromosomes condense, pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up. Metaphase I. The homologous chromosome pairs are aligned midway between spindle poles. Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles. Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form. plasma membrane spindle nuclear envelope breaking up centrosome one pair of homologous chromosomes Meiosis Meiosis IOne diploid nucleus to two haploid nuclei Stepped Art

  19. 5 6 7 8 Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up. Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of the spindle. Anaphase II. All sister chromatids separate. The now unduplicated chromo -somes head to the spindle poles. Telophase II. A cluster of chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form. No DNA replication Meiosis Meiosis IITwo haploid nuclei to four haploid nuclei Stepped Art

  20. 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

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

  22. A Here, we focus on only two of the many genes on a chromosome. In this example, one gene has alleles AA 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. C Crossing over mixes up paternal and maternal alleles on homologous chromosomes. Stepped Art Figure 12-6 p194

  23. 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

  24. 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 Stepped Art Figure 12-7 p195

  25. 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

  26. 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

  27. mitosis multicelled sporophyte (2n) zygote (2n) Diploid Fertilization Meiosis Haploid gametes (n) spores (n) multicelled gametophyte (n) Figure 12-8a p196

  28. 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

  29. 2 3 4 1 male germ cell sperm Sperm Formation in Animals Stepped Art

  30. 5 6 7 egg female germ cell Egg Formation in Animals Stepped Art

  31. Fertilization

  32. 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

  33. 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. Metaphase I • Chromosomes align midway between spindle poles. Telophase I • Chromosome clusters arrive opposite spindle poles. • New nuclear envelopes form. • Chromosomes decondense. Comparing Mitosis and Meiosis Meiosis IOne diploid nucleus to two haploid nuclei Stepped Art

  34. Prophase • Chromosomes condense. • Spindle forms and attaches chromosomes to spindle poles. • Nuclear envelope breaks up. Metaphase • Chromosomes align midway between spindle poles. Anaphase • Sister chromatids separate and move toward at spindle poles. Telophase • Chromosome clusters arrive opposite spindle poles. • New nuclear envelopes form. • Chromosomes decondense Comparing Mitosis and Meiosis MitosisOne diploid nucleus to two diploid nuclei Stepped Art

  35. 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. Telophase II • Chromosome clusters arrive at spindle poles. • New nuclear envelopes form. • Chromosomes decondense. Comparing Mitosis and Meiosis Meiosis IITwo haploid nuclei to four haploid nuclei Stepped Art

  36. 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

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