1. Meiosis: The Cellular Basis of Sexual Reproduction Chapter 11

2. 11.1 The Mechanisms of Meiosis  Meiosis is based on the interactions and distribution of homologous chromosome pairs  Meiosis produces four genetically different cells with half the parental chromosomes

3. Sexual Reproduction  Sexual reproduction produces offspring by union of male and female gametes (sperm and egg) Meiosis produces gametes with half chromosome number Evolutionary advantage: Genetic shuffling of sex

4. Fertilization  Fertilization fuses nuclei of egg and sperm Zygote cell produced by fertilization Restores parental chromosome number

5. Homologous Chromosome Pairs  Paternal chromosomes from male parent, maternal from female parent Homologous chromosome pairs Alleles may be different within homologous pairs  Meiosis separates homologous pairs Before meiosis, diploid (2n) After meiosis, haploid (n)

6. Meiosis and Fertilization Cycle

7. Meiotic Cell Cycle  Mitosis and meiosis compared Both: Similar cell divisions, meiosis divides twice Mitosis: Two identical daughter cells, diploid Meiosis: Four genetically different cells, haploid  Premeiotic interphases similar to mitotic interphase Chromosomes copied into sister chromatids

8. Meiosis I  Meiosis I: First meiotic division Recombination exchanges segments between homologues Produces two haploid cells with chromatids attached

9. Meiosis II  Meiosis II: Second meiotic division Sister chromatids separate into separate cells Produces 4 recombined haploid cells

10. Meiotic Divisions

11. Meiotic Cell Cycle (1)  Prophase I Sister chromatids condense to chromosomes Synapsis (pairing of homologs) Tetrads (Fully paired homologs) Recombination mixes alleles across tetrads  Prometaphase I Nuclear envelope breaks down Kinetochores attach to polar spindles

12. Meiotic Divisions

13. Meiotic Cell Cycle (2)  Metaphase I and Anaphase Tetrads align on metaphase plate Homologs segregate, move to poles (sister chromatids attached) Nondisjunction creates abnormal chromosome numbers  Telophase I and Interkinesis No change in chromosomes Spindle disassembles

14. Meiotic Divisions

15. Meiotic Cell Cycle (3)  Prophase II, Prometaphase II, and Metaphase II Chromosomes condense, spindles form Nuclear envelope breaks, kinetochores form Chromosomes align on metaphase plate  Anaphase II and Telophase II Spindles separate chromatids Spindles disassemble New nuclear envelopes form

16. Meiotic Divisions

18. Animation: Meiosis I and II

19. Comparison of Meiosis and Mitosis

21. Sex Chromosomes in Meiosis  Sex chromosomes Different in males and females Human females XX, males XY XX fully homologous, XY homologous in short region

22. Sex Chromosomes in Meiosis  Meiosis and sex chromosome inheritance Gametes produced by females may receive either X chromosome Gametes produce by males may receive either X or Y chromosome

23. 11.2 Mechanisms That Generate Variability  Recombination depends on physical exchanges between homologous chromatids  Segregation of maternal and paternal chromosomes is random  Random joining of male and female gametes in fertilization adds additional variability

24. Recombination of Chromatids  Recombination (crossing over) Key genetic shuffle of prophase I  Tetrads held together at synaptonemal complex Two of four chromatids exchange alleles Chiasmata or crossovers are points of exchange

25. Crossing Over

26. Animation: Crossing over

27. Synaptonemal Complex

28. Random Segregation  Random segregation Key genetic shuffle of metaphase I  Each chromosome of a homologous pair may randomly end up at either spindle pole Any combination of maternal and paternal chromosomes can be segregated to gametes 2 X number of possible combinations

29. Random Spindle Connections

30. Random Fertilization  Random chance of male and female gamete forming zygote  Meiosis allows randomness necessary for Mendelian laws of inheritance Recombination, random segregation, and random fertilization are mechanisms of randomness

31. 11.3 The Time and Place of Meiosis in Organismal Life Cycles  In animals, diploid phase dominant and meiosis followed directly by gamete formation  In most plants and fungi, generations alternate between haploid and diploid phases  In some fungi and other organisms, haploid phase dominant and diploid phase single cell

32. Animal Life Cycles  Diploid phase dominates animal life cycles Meiosis followed directly by gamete formation Haploid phase is reduced and short, no mitosis  In males, all four nuclei from meiosis form separate sperm cells  In females, only one nucleus becomes an egg

33. Animal Life Cycles

34. Plants and Fungi Life Cycles  Alternation of generations Alternate between haploid and diploid phase  Fertilization produces sporophytes Diploid individuals  Sporophytes produce haploid spores (meiosis) Multicellular gametophytes (mitosis) Gametophytes produce gametes

35. Plants and Fungi Life Cycles

36. Other Fungi and Algae Life Cycles  In some organisms, diploid phase limited to single cell zygote  Zygote undergoes meiosis Mitosis only occurs in haploid cells Gametes usually designated + or -

37. Other Fungi and Algae Life Cycles

38. Animation: Meiosis overview PLAY ANIMATION