1. 1 Production of Gametes MEIOSIS

2. 2 Meiosis Facts

3. 3 Meiosis cell division gametes, half chromosomes,The form of cell division by which gametes, with half the number of chromosomes, are produced Diploid (2n)  haploid (n)Diploid (2n)  haploid (n) Called a reduction division (because # of chromosomes is reduced)Called a reduction division (because # of chromosomes is reduced) Meiosissexual reproductionMeiosis is sexual reproduction Two divisionsmeiosis I meiosis IITwo divisions (meiosis I and meiosis II)

4. 4 Meiosis Germ Line cells (aka sex cells) divide to produce gametes (sperm or egg) Gametes have half the number of chromosomes Meiosis is similar to mitosis with some chromosomal differences

5. 5 Meiosis Occurs only in gonads (testes or ovaries) Male: Spermatogenesis Female: Oogenesis

6. 6 Spermatogenesis 2n=46 Human sex cell Diploid (2n) n=23 Meiosis I n=23 Sperm Haploid (n ) Meiosis II

7. 7 Interphase I mitosisSimilar to mitosis interphase Chromosomes(S phase)Chromosomes replicate (S phase) chromosome chromatids centromeresEach duplicated chromosome consist of two identical sister chromatids attached at their centromeres CentrioleCentriole pairs also replicate.

8. 8 Interphase I NucleusnucleolusNucleus and nucleolus visible. Nuclear membrane Nucleolus Cell membrane Chromatin

9. 9 Meiosis I

10. 10 Stages of Meiosis I Cell division chromosome one-halfCell division that reduces the chromosome number by one-half Four phasesFour phases: a.Prophase I b.Metaphase I c.Anaphase I d.Telophase I

11. 11 Prophase I Longest and most complex phase (90%)Longest and most complex phase (90%) ChromosomesChromosomes condense Synapsishomologous chromosomes tetradSynapsis occurs: homologous chromosomes come together to form a tetrad Tetradchromosomes chromatidsTetrad is two chromosomes or four chromatids (sister and nonsister chromatids)

12. 12 Prophase I - Synapsis Homologous chromosomes Sister chromatids Tetrad

13. 13 Homologous Chromosomes chromosomesmaternal paternalPair of chromosomes (maternal and paternal) that are similar in shape and size (tetrads)Homologous pairs (tetrads) carry genes controlling the same inherited traits locus(position of a gene)Each locus (position of a gene) is in the same position on homologues

14. 14 Homologous Chromosomes homologous chromosomesHumans have 23 pairs of homologous chromosomes autosomesFirst 22 pairs of chromosomes called autosomes sex chromosomesLast pair called sex chromosomes XX female or XY maleXX female or XY male

15. 15 Homologous Chromosomes PaternalMaternal eye color locus eye color locus hair color locus hair color locus

16. Gene Loci 16

17. 17 Crossing Over Crossing over (variation) chromatidschiasmataCrossing over (variation) may occur between nonsister chromatids at the chiasmata Crossing over chromatids chromatidCrossing over: segments of nonsister chromatids break and reattach to the other chromatid Chiasmata (chiasma) crossing overChiasmata (chiasma) are the sites of crossing over

18. 18 Crossing Over - Variation nonsister chromatids chiasmata: site of crossing over variation Tetrad

19. 19 Sex Chromosomes XX chromosome - femaleXY chromosome - male

20. 20 Prophase I Centrioles Spindle fiber Aster fibers

21. 21 Metaphase I Shortest phaseShortest phase Tetrads metaphase plateTetrads align on the metaphase plate Independent assortment occursIndependent assortment occurs Chromosomes separate randomly to the poles of the cellsChromosomes separate randomly to the poles of the cells

22. 22 Metaphase I Independent assortment causes variation in the forming cells Orientation of homologous pair to poles is random Formula for determining variation: Formula: 2 n Example: 2n = 4 then n = 2 then n = 2 thus 2 2 = 4 combinations thus 2 2 = 4 combinations

23. 23 Metaphase I Metaphase plate OR Metaphase plate

24. 24 Question: In terms of Independent Assortment, how many different combinations of sperm could a human male produce?In terms of Independent Assortment, how many different combinations of sperm could a human male produce?

25. 25 Answer Formula: 2 nFormula: 2 n Human chromosomes:Human chromosomes: 2n = 46 2n = 46 n = 23 n = 23 2 23 = ~8 million combinations 2 23 = ~8 million combinations

26. 26 Anaphase I Homologous chromosomesHomologous chromosomes separate and move towards the poles Sister chromatids centromeresSister chromatids remain attached at their centromeres

27. 27 Anaphase I

28. 28 Telophase I haploid chromosomesEach pole now has haploid set of chromosomes 1n = 23 (human)1n = 23 (human) CytokinesisCytokinesis occurs and two haploid daughter cells are formed

29. 29 Telophase I

30. 30

31. 31 Meiosis II

32. 32 Meiosis II Short or No interphase IIShort or No interphase II DNA NOT replicated againDNA NOT replicated again Remember: Meiosis II mitosisRemember: Meiosis II is similar to mitosis Prophase I, Metaphase II, Anaphase II, and Telophase IIProphase I, Metaphase II, Anaphase II, and Telophase II

33. 33 Prophase II prophasemitosisSame as prophase in mitosis Nuclear envelope breaks down and the spindle apparatus formsNuclear envelope breaks down and the spindle apparatus forms

34. 34 Metaphase II metaphasemitosisSame as metaphase in mitosis Chromatids lined up at equatorChromatids lined up at equator Metaphase plate

35. 35 Anaphase II anaphasemitosisSame as anaphase in mitosis Sister chromatids separateSister chromatids separate

36. 36 Telophase II telophase mitosisSame as telophase in mitosis Nuclei reform CytokinesisCytokinesis occurs Remember: Four haploid daughter cells producedRemember: Four haploid daughter cells produced Gametes = sperm or eggGametes = sperm or egg

37. 37 Telophase II

38. 38 Meiosis 2n=4 sex cell diploid (2n) n=2 Meiosis I n=2 Sperm Haploid (n) Meiosis II

39. 39

40. 40 Genetic Variation

41. 41 Variation  Important to the survival of populations  Aids in natural selection  Strongest individuals are able to survive and reproduce

42. 42 Question What are the three sources of genetic variation in sexual reproduction?What are the three sources of genetic variation in sexual reproduction?

43. 43 ***Answer: 1.Crossing Over (Prophase I) (Prophase I) 2. Independent Assortment (Metaphase I) 3. Random Fertilization (sperm joins with egg)

44. 44 Remember: Variation is helpful to the survival of a species!

45. 45 Question: 20 chromosomes(2n = 20) chromosomesA diploid cell containing 20 chromosomes (2n = 20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

46. 46 Answer: 10 chromosomes (haploid)10 chromosomes (haploid) 1n = 101n = 10

47. 47Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type.

48. 48

49. Trisomy Genetic disorders such as Down’s Syndrome (Trisomy 21) are the result of nondisjunction 49

50. 50 Fertilization spermegg zygoteThe fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote

51. 51 Question: 40 chromatids chromosomesA cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

52. 52 Answer: Four cells with 10 chromosomes eachFour cells with 10 chromosomes each