Meiosis

1. Meiosis Chapter 10

2. Asexual Reproduction Single parent produces offspring All offspring are genetically identical to one another and to parent

3. Sexual Reproduction Two Parents produce offspring Involves: Meiosis Gamete production Fertilization Produces genetic variation among offspring

5. Homologous Chromosomes Cell has two of each chromosome One chromosome in each pair from mother, other from father Homologous chromosome have similar structure They carry the same traits (e.g. hair color), but may carry different alleles (different versions of the same trait; e.g. blond vs. brown hair)

6. Why do organisms do Meiosis? To preserve the chromosome number of cells in organism undergoing sexual reproduction Meiosis results in haploid cells called gametes

7. Where do organisms perform Meiosis? In animals, Meiosis only occurs in organs called gonads (sex organs) Testes Male gonads; make sperm (male gametes) Ovaries Female gonads; make eggs (ova, female gametes)

8. Gamete Formation

9. Sexual Reproduction Chromosomes are duplicated in germ cells (diploid) Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes (haploid) Gametes meet at fertilization to form zygotes (diploid)

10. Sexual Reproduction

11. Meiosis: Two Divisions Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions Results in a reduction of the chromosome number from diploid to haploid (cuts the chromosome number in half

12. Meiosis I Homologous chromosomes bind to each other and are later separated on from another

13. Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other, resulting in four haploid cells

14. Meiosis � Overview

15. Meiosis I - Stages

16. Prophase I Each duplicated chromosome pairs with homologue Homologues swap segments Each chromosome becomes attached to spindle

17. Metaphase I Chromosomes are pushed and pulled into the middle of cell The spindle is fully formed

18. Anaphase I Homologous chromosomes segregate The sister chromatids remain attached

19. Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division

20. Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes

21. Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles

22. Anaphase II Sister chromatids separate to become independent chromosomes

23. Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells

24. Crossing Over: Homologous Recombination Unique to meiosis does NOT happen during mitosis Swapping of DNA between two homologous chromosomes A piece of DNA from a chromosome from dad swaps places with a piece of DNA from the chromosome from mom This can occur when chromosomes are aligned during prophase I, takes place between nonsister chromatids of homologous chromosomes This leads to genetic recombination which in turn leads to variation in the traits of offspring

26. Crossing Over

27. Effect of Crossing Over After crossing over, each chromosome contains both maternal and paternal segments Creates new allele combinations in offspring

28. Random Alignment During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes Initial contacts between microtubules and chromosomes are random

29. Random Alignment (assortment) Either the maternal or paternal member of a homologous pair can end up at either pole The chromosomes in a gamete are a mix of chromosomes from the two parents

30. Possible Chromosome Combinations As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2n (n is number of chromosome types)

31. Possible ChromosomeCombinations

32. Animal Life Cycle

33. Oogenesis

34. Spermatogenesis

35. Fertilization Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Which two gametes unite is random Adds to variation among offspring

36. Factors Contributing to Variation among Offspring Crossing over during prophase I Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization

37. Variation: The numbers Germ Cell (?) Germ Cell (?) Sperm X Egg (223 combinations) (223 combinations) Zygote ~64 trillion combinations

39. Mitosis Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones Mitosis & Meiosis Compared Meiosis Function Sexual reproduction Occurs in germ cells Produces variable offspring

40. Prophase vs. Prophase I Prophase (Mitosis) Homologous pairs do not interact with each other Prophase I (Meiosis) Homologous pairs become zippered together and crossing over occurs

41. Anaphase, Anaphase I, and Anaphase II Anaphase I (Meiosis) Homologous chromosomes separate from each other Anaphase/Anaphase II (Mitosis/Meiosis) Sister chromatids of a chromosome separate from each other

42. Results of Mitosis and Meiosis Mitosis Two diploid cells produced Each identical to parent Meiosis Four haploid cells produced Differ from parent and one another

43. Then what: Early Development

44. Early Development - Starfish