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Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction. Chapter 10. 10.1 Introducing Alleles. Asexual reproduction produces genetically identical copies of a parent ( mitosis) Sexual reproduction introduces variation in the combinations of traits among offspring ( meiosis). Meiosis and Homologous Chromosomes.

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Meiosis and Sexual Reproduction

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  1. Meiosis and Sexual Reproduction Chapter 10

  2. 10.1 Introducing Alleles • Asexual reproduction produces genetically identical copies of a parent (mitosis) • Sexual reproduction introduces variation in the combinations of traits among offspring (meiosis)

  3. Meiosis and Homologous Chromosomes • Meiosis • A nuclear division mechanism that precedes cytoplasmic division of immature reproductive cells in sexually-reproducing eukaryotic species • If meiosis did not occur, the chromosome number would continue to double with each new generation. • Homologous Chromosomes • Same length, shape, centromere in the same location, are in pairs (one from the mother and one from the father), separate from each other during meiosis I, have alleles for the same characteristics even though the gene expression may not be the same.

  4. 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 • Alleles are different or alternate forms of the same gene • Offspring of sexual reproducers inherit new combinations of alleles (ex. Eye color = hazel eyes (mother), blue eyes (father) • Study Figure 10.2

  5. 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 aka sex cells (egg, sperm, spores, gametophytes)

  6. Restoring the Diploid Number • Human gametes (eggs and sperm) have 23 chromosomes – one of each type of homologous chromosomes. • The diploid number (23 pairs or 46 chromosomes) is restored at fertilization, when two haploid gametes fuse and form a diploid zygote

  7. Two Divisions, Not One • In meiosis, DNA is replicated once and divided twice (meiosis I and meiosis II), forming four haploid cells (four daughter cells) • In meiosis I, each duplicated homologous chromosome is separated from its partner • In meiosis II, sister chromatids are separated

  8. plasma membrane newly forming microtubules of the spindle one pair of homologous chromosomes breakup of nuclear envelope centrosome with a pair of centrioles, moving to opposite sides of nucleus A Prophase I = crossing over B Metaphase I C Anaphase I D Telophase I Meiosis I Stepped Art Fig. 10-5a, p. 158

  9. There is no DNA replication between the two nuclear divisions. E Prophase II F Metaphase II G Anaphase II H Telophase II Meiosis II Stepped Art Fig. 10-5b, p. 159

  10. Animation: Meiosis step-by-step

  11. 10.4 How Meiosis Introduces Variation in Traits • Crossovers • the random sorting or alignment of chromosomes • genetic recombination of alleles • fertilization in meiosis introduce novel combinations of alleles into gametes (egg and sperm), resulting in new combinations of traits among offspring

  12. Crossing Over in Prophase I • Crossing over • The process by which a chromosome and its homologous partner (two of the same chromosomes; one from the mother and one from the father) exchange heritable information in corresponding segments • Homo = same

  13. Animation: Crossing over

  14. Segregation of Chromosomes into Gametes • Random assortment (independent assortment) produces (8,388,608) possible combinations of homologous chromosomes • Gametes (egg and sperm; 223 ) • Fertilization (fusion or combining of the egg and sperm; (223)2 = Over 70 trillion different zygotes – the first cell that is formed after fertilization) are possible.

  15. Animation: Random alignment

  16. Fig. 10-8b, p. 162

  17. Gamete Formation in Animals • Males • Meiosis of primary spermatocytes produces four haploid spermatids • Females • Meiosis of a primary oocyte forms cells of different sizes; the secondary oocyte gets most of the cytoplasm and matures into an ovum ; other three cells (polar bodies – have no function, are dumping places for excess genetic material, are produced only during meiosis.) • Study figure 10.10

  18. Sperm Formation in Animals

  19. Egg Formation in Animals

  20. 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 mature ova - egg) resulting in a diploid zygote (the first cell of a new individual with 46 chromosomes.

  21. Fig. 10-11c, p. 164

  22. Fig. 10-11d, p. 165

  23. Animation: Comparing mitosis and meiosis

  24. Animation: Generalized life cycles

  25. Animation: Meiosis

  26. Video: Why sex?

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