1. Meiosis Chapter 13

2. Sexual Reproduction Chromosomes are duplicated in germ cells Germ cells undergo meiosis and cytoplasmic division Cellular descendents of germ cells become gametes Gametes meet at fertilization

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

4. Sexual Reproduction Involves –Meiosis –Gamete production –Fertilization Produces genetic variation among offspring

5. Homologous Chromosomes Carry Different Alleles Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles

6. Sexual Reproduction Shuffles Alleles Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change

7. Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number

8. Meiosis: Two Divisions Two consecutive nuclear divisions –Meiosis I –Meiosis II DNA is not duplicated between divisions Four haploid nuclei form

9. Meiosis I Each homologue in the cell pairs with its partner, then the partners separate In-text figure Page 165 Homologous chromosomes separate

10. Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other one chromosome (duplicated) two chromosomes (unduplicated) In-text figure Page 165

11. Meiosis I - Stages Prophase IMetaphase IAnaphase ITelophase I Figure 10.4 Page 167

12. Prophase I Each duplicated chromosome pairs with homologue (sister chromatids with homologous chromosomes = tetrad) Homologues swap segments (crossing over) Each chromosome becomes attached to spindle Figure 10.4 Page 167

13. Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned (tetrad) Nonsister chromosomes exchange segments The point of crossing over is called a chiasma. Figure 10.5 Page 168

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

15. Metaphase I Spindles attach to kinetochores Chromosomes (tetrads) are pushed and pulled into the middle of cell The chromsomes line up on the plate two by two instead of single file like they do in mitosis. The spindle is fully formed Figure 10.4 Page 167

16. 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

17. Random Alignment 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

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

19. Possible Chromosome Combinations or 123 Figure 10.6 Page 169

20. Anaphase I Homologous chromosomes segregate The sister chromatids remain attached Figure 10.4 Page 167

21. Telophase I The chromosomes arrive at opposite poles Usually followed by cytoplasmic division Figure 10.4 Page 167

22. Interkinesis- This is the time between meiosis I and meiosis II. The difference between interkinesis and interphase is that DNA does not replicate. Centrosomes with centrioles repli-cate. Cetrosomes are microtubules organizing centers

23. Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes Figure 10.4 Page 167

24. Metaphase II Duplicated chromosomes line up at the spindle equator, midway between the poles Figure 10.4 Page 167

25. Anaphase II Sister chromatids separate to become independent chromosomes Figure 10.4 Page 167

26. Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes Four haploid cells Figure 10.4 Page 167

27. Plant Life Cycle multicelled sporophyte multicelled gametophytes zygote gametes spores meiosisfertilization mitosis Diploid Haploid Figure 10.7 Page 170

28. Animal Life Cycle multicelled body gametes zygote meiosisfertilization mitosis Diploid Haploid Figure 10.7 Page 170

29. Cytokinesis in female gametes is different than males. Males result in four cells of equal size to become sperm cells, but in females, meiosis results in one large egg and three small polar bodies.

30. Oogenesis Growth Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division ovum (haploid) primary oocyte (diploid) oogonium (diploid) secondary oocyte (haploid) first polar body (haploid) three polar bodies (haploid) Figure 10.8 Page 171

31. A human egg with 3 polar bodies

32. Spermatogenesis Growth Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division cell differentiation, sperm formation spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) spermato- gonium (diploid ) sperm (mature, haploid male gametes) Figure 10.9 Page 171

34. 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

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

36. 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

37. 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

38. 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

39. 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