Chapter 10: Meiosis & Sexual Reproduction

1. Chapter 10: Meiosis & Sexual Reproduction

2. Sexual Reproduction • Involves • Meiosis • Gamete production • Fertilization • Produces genetic variation among offspring

3. Asexual Reproduction • Single parent produces offspring • All offspring are genetically identical to one another and to parent • Ex: binary fission in bacteria, budding in some protists and fungi

4. Homologous Chromosomes Fig. 10-2, p.156

5. Sexual Reproduction Shuffles Alleles • Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits • Alleles are parts of the chromosome that carry the code for a specific trait • This variation in traits is the basis for evolutionary change

6. Gamete Formation • Gametes are sex cells (sperm, eggs) • Arise from germ cells ovaries anther ovary testes Figure 10-3Page 156

7. Fig. 10-3, p.156

8. FLOWERING PLANT anther (where cells that give rise to male gametes originate) ovules, inside an ovary (where cells that give rise to female gametes originate) Fig. 10-3a, p.156

9. HUMAN MALE testis (where sperm originate) Fig. 10-3b, p.156

10. HUMAN FEMALE ovary (where eggs develop) Fig. 10-3c, p.156

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

12. Human Karyotype 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY) Fig. 10-4, p.157

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

14. Meiosis I Each homologue in the cell pairs with its partner, then the partners separate p. 158

15. Meiosis II • The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated) p. 158

16. Prophase I Metaphase I Anaphase I Telophase I Meiosis I - Stages

17. Prophase I • Each duplicated chromosome pairs with homologue • Homologues swap segments • Each chromosome becomes attached to spindle Fig. 10-5, p. 158

18. Metaphase I • Chromosomes are pushed and pulled into the middle of cell • The spindle is fully formed Fig. 10-5, p. 158

19. Anaphase I • Homologous chromosomes segregate • The sister chromatids remain attached Fig. 10-5, p. 158

20. Telophase I • The chromosomes arrive at opposite poles • Usually followed by cytoplasmic division Fig. 10-5, p. 158

21. Prophase II • Microtubules attach to the kinetochores of the duplicated chromosomes Fig. 10-5, p. 158

22. Metaphase II • Duplicated chromosomes line up at the spindle equator, midway between the poles Fig. 10-5, p. 158

23. Anaphase II • Sister chromatids separate to become independent chromosomes Fig. 10-5, p. 158

24. Telophase II • The chromosomes arrive at opposite ends of the cell • A nuclear envelope forms around each set of chromosomes • Four haploid cells Fig. 10-5, p. 158

25. MEIOSIS I newly forming microtubules in the cytoplasm spindle equator (midway between the two poles) one pair of homologous chromosomes plasma membrane PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I Fig. 10-5, p.158

26. there is no DNA replication between the two divisions PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II Fig. 10-5b, p.159

27. newly forming microtubules spindle equator one pair of homologous chromosomes Prophase I Metaphase I Anaphase I Telophase I Meiosis I Stepped Art Fig. 10-5a, p.158

28. Prophase II Metaphase II Anaphase II Telophase II Meiosis II Stepped Art Fig. 10-5b, p.159

29. Crossing Over • Each chromosome becomes zippered to its homologue • All four chromatids are closely aligned • Nonsister chromosomes exchange segments

30. Crossing Over paternal homologue maternal homologue Stepped Art Fig. 10-6, p.160

31. Crossing Over aBoth chromosomes shown here were duplicated during interphase, before meiosis. When prophase I is under way, sister chromatids of each chromosome are positioned so close together that they look like a single thread. Fig. 10-6a, p.160

32. Crossing Over bEach chromosome becomes zippered to its homologue, so all four chromatids are tightly aligned. If the two sex chromosomes have different forms, such as X paired with Y, they still get zippered together, but only in a tiny region at their ends. Fig. 10-6b, p.160

33. Crossing Over cWe show the pair of chromosomes as if they already condensed only to give you an idea of what goes on. They really are in a tightly aligned, threadlike form during prophase I. dThe intimate contact encourages one crossover (and usually more) to happen at various intervals along the length of nonsister chromatids. eNonsister chromatids exchange segments at the crossover sites. They continue to condense into thicker, rodlike forms. By the start of metaphase I, they will be unzippered from each other. fCrossing over breaks up old combinations of alleles and puts new ones together in the cell’s pairs of homologous chromosomes. Fig. 10-6c, p.160

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

35. 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 • 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

36. 1 2 3 combinations possible or or or Possible ChromosomeCombinations Alignment at metaphase I Stepped Art Fig. 10-7, p.161

37. Plant Life Cycle sporophyte zygote diploid fertilization meiosis haploid spores gametes gametophytes Fig. 10-8a, p.162

38. Animal Life Cycle multicelled body zygote diploid fertilization meiosis haploid gametes Fig. 10-8b, p.162

39. three polar bodies (haploid) Oogenesis first polar body (haploid) oogonium (diploid) primary oocyte (diploid) secondary oocyte (haploid) ovum (haploid) Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth Figure 10-10Page 163

40. Spermatogenesis primary spermatocyte (diploid) sperm (mature, haploid male gametes) spermato-gonium (diploid ) secondary spermatocytes (haploid) spermatids (haploid) Meiosis I, Cytoplasmic Division Meiosis II, Cytoplasmic Division Growth cell differentiation, sperm formation Figure 10-9Page 163

41. Fig. 10-10, p.163

42. 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

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

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

45. 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

46. 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

47. 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

48. Meiosis I Prophase I Metaphase I Anaphase I Telophase I Crossing over occurs between homologues. Homologous pairs align randomly. Homologues separate from their partner. Cytoplasm may divide before meiosis II. Fig. 10-11a, p.164

49. Meiosis II no interphase and no DNA replication between the two nuclear divisions Prophase II Metaphase II Telophase II Anaphase II New spindle forms in each nucleus. All chromosomes aligned at the equator. Haploid cells function as gametes or spores. Sister chromatids moved to opposite spindle poles. Fig. 10-11b, p.164

50. Mitosis Prophase Anaphase Telophase Metaphase A spindle forms; tethers chromosomes to spindle poles. All chromosomes aligned at the spindle equator. Sister chromatids moved to opposite spindle poles. Two diploid (2n) nuclei form. Fig. 10-11c, p.164