1. Meiosis and Sexual Reproduction Chapter 9

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

3. 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

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

5. 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

6. 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

7. Gamete Formation Gametes are sex cells (sperm, eggs) Arise from germ cells testes ovaries anther ovary

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

9. Meiosis: Two Divisions Two consecutive nuclear divisions –Meiosis I –Meiosis II DNA is not duplicated between divisions Four haploid nuclei are formed

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

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

12. Stages of Meiosis Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

13. Meiosis I - Stages Prophase IMetaphase IAnaphase ITelophase I

14. Prophase I Each duplicated, condensed chromosome pairs with its homologue Homologues swap segments Each chromosome becomes attached to microtubules of newly forming spindle

15. Metaphase I Chromosomes are pushed and pulled into the middle of cell Sister chromatids of one homologue orient toward one pole, and those of other homologue toward opposite pole The spindle is now fully formed

16. Anaphase I Homologous chromosomes segregate from each other The sister chromatids of each chromosome remain attached

17. Telophase I The chromosomes arrive at opposite poles The cytoplasm divides There are now two haploid cells This completes Meiosis I

18. Meiosis II - Stages Prophase IIMetaphase IIAnaphase IITelophase II

19. Prophase II Microtubules attach to the kinetochores of the duplicated chromosomes Motor proteins drive the movement of chromosomes toward the spindle’s equator

20. Metaphase II All of the duplicated chromosomes are lined up at the spindle equator, midway between the poles

21. Anaphase II Sister chromatids separate to become independent chromosomes Motor proteins interact with microtubules to move the separated chromosomes to opposite poles

22. Telophase II The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes The cytoplasm divides There are now four haploid cells

23. Crossing Over Each chromosome becomes zippered to its homologue All four chromatids are closely aligned Non-sister chromosomes exchange segments

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

25. 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

26. 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

27. 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)

28. Possible Chromosome Combinations or 123

29. Plant Life Cycle multicelled sporophyte multicelled gametophytes zygote gametes spores meiosisfertilization mitosis Diploid Haploid

30. Animal Life Cycle multicelled body gametes zygote meiosisfertilization mitosis Diploid Haploid

31. Spermatogenesis Growth Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division spermatids (haploid) secondary spermatocytes (haploid) primary spermatocyte (diploid) spermato- gonium (diploid male germ cell)

32. Oogenesis Growth Mitosis I, Cytoplasmic division Meiosis II, Cytoplasmic division ovum (haploid) primary oocyte (diploid) oogonium (diploid reproductive cell) secondary oocyte (haploid) first polar body (haploid) three polar bodies (haploid)

33. 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

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

35. 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

36. 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

37. Anaphase, Anaphase I, and Anaphase II Anaphase I (Meiosis) –Homologous chromosomes are separated from each other Anaphase/Anaphase II (Mitosis/Meiosis) –Sister chromatids of a chromosome are separated from each other

38. 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