10.2 – Meiosis Sexual Reproduction

1. 10.2 – MeiosisSexual Reproduction Function: creates gametes (sperm, egg, pollen) for sexual reproduction.

2. Meiosis: Cell division for sexualreproduction. Females - ovaries Males - testes Sex Cells

3. Homologous Chromosomes • A normal human body cell contains 46 chromosomes in 23 pairs • 23 from mother; 23 from father • Each pair of chromosomes are called HOMOLOGOUS CHROMOSOMES • In a homologous pair, one chromosome is from the mother and one is from the father

7. Haploid vs. Diploid • Haploidcell – a cell with only onecopy of each chromosome (23total in humans) • ½the number of chromosomes as the parent or normal body cells. • n  haploid; n = 23 • Diploidcell – a cell with 2 copies of each chromosome (46total; 23 pairsin humans) • 2n  diploid; 2n = 46

8. Diploid • All body cells are diploid(2n) • Skin, liver, bone, lung, etc. • Mother  23chromosomes • Father  23chromosomes • Bodycells are called SOMATICcells • Diploid body cells divide via mitosisto form two identical diploid daughter cells 46 chromosomes total

10. Haploid • Sex cells are called GAMETES(sperm and egg) • Gametes join to form a ZYGOTE • sperm + egg = zygote • Gametes are haploid (n) • Sperm = n • Egg = n • n + n = 2n

13. Meiosis • Meiosis is the process of making haploid gametes from a diploid cell • Each haploid gamete will have HALFthe genetic material as the diploid parent cell • 2n  n

14. Meiosis • Meiosis happens in two stages: • Meiosis I • Meiosis II • There are twocell divisions • 4 gametes are created

16. Phases of Meiosis 1st Division 2nd Division Prophase II Metaphase II Anaphase II Telophase II Interphase • Prophase I • Metaphase I • Anaphase I • Telophase I Cytokinesis

17. Meiosis: IPMAT cytokinesis; 2nd phase of PMAT producing 4 (haploid-single) daughter cells

18. Chromatin Interphase • During Interphase: • Cell grows in size • DNA is duplicated Nuclear membrane

19. Prophase I • During Prophase I: • Chromatincondenses into chromosomes • Nuclear membrane dissolves • Spindleforms • Homologouschromosomes pair up and CROSSING OVER occurs • Crossing Over: homologous chromosomes pair up and randomly exchange genetic information during Prophase 1

20. Mistakes in Meiosis Genetic Recombination: AKA Crossing Over • Occurs during meiosis, chromosomes actually cross over each other, genetic material gets switched! • This leads to genetic variability! • Can be an endless number of different chromosomes possible!

21. Homologous Chromosomes (Crossing Over) Spindle

22. Metaphase I • During Metaphase I: • Homologouschromosomes are pulled to the middle of the cell by the spindle

23. Homologous Chromosomes Spindle

24. Independent Assortment • Chromosomes line up randomlyduring metaphase • There is a 50% chance of the maternal or paternal chromosomes going to either side of the cell

25. = FATHER = MOTHER

26. Anaphase I • During Anaphase I: • Homologouschromosomes are pulled towards opposite ends of the cell by the spindle

27. Spindle Chromosomes

28. Telophase I • During Telophase I: • Nuclear envelope reforms • Spindle dissolves • Cell stretches and cytoplasm divides (cytokinesis)

29. Chromosomes

30. NO Interphase! • There is NO Interphasebetween Meiosis I and Meiosis II • Each gamete must receive HALFof the parent’s genetic material so no duplication is necessary

31. Prophase II • During Prophase II: • Nuclear membrane dissolves • Spindle re-forms • Meiosis II is the division of the two haploid cells resulting from Meiosis I • 4 haploid cells will be created

32. Chromosomes Spindle

33. Metaphase II • During Metaphase II: • Chromosomes are pulled to the middle of the cell by the spindle

34. Spindle Chromosomes

35. Anaphase II • During Anaphase II: • Sister chromatidsare separated and pulled to opposite ends of the cell by the spindle

36. Chromosomes Spindle

37. Telophase II • During Telophase II • Nuclear membrane re-forms • Spindle dissolves • Cells stretch and prepare for cytokinesis

38. Chromosomes

39. Cytokinesis • The cytoplasm divides resulting in a total of 4 unique haploid cells • All gametes are different because of crossing over during Prophase I and independent assortment during Metaphase I & II

41. Genetic Diversity • Meiosis increases genetic diversity because every sperm or egg is a little bit different. No two offspring are exactly the same • Crossing Over • Independent Assortment • Genetic diversity causes some organisms to be more fit to survive and reproduce (evolution)

42. Nondisjunction • Nondisjunction - an error in Meiosis when homologous chromosomes fail to separate properly during Anaphase I or II • The resulting gametes will have an extra chromosome or be missing a chromosome • 3 copies instead of 2 • 1 copy instead of 2 • Most zygotes will not survive

43. Nondisjunction during Meiosis I Nondisjunction during Meiosis II

44. Non-Disjunction • In humans, a person with an extra 21st chromosomes will have Down’s Syndrome

45. Down Syndrome • Trisomy– when an organism has 3 copies of a chromosome where it should only have two • Trisomy is caused by nondisjunction • Trisomy of the 21st chromosome results in DOWN SYNDROME • 3 copies of the 21st chromosome

46. Trisomy 21 causes Down Syndrome

48. Mitosis vs. Meiosis • Makes EXACT copies • No new genetic diversity • 2n  2n (diploid  diploid) • Happens in body (somatic) cells • One cell division • Asexual • Makes UNIQUE gametes • Increases genetic diversity • 2n  n (diploid  haploid) • Happens in gamete producing cells (GONADS: testes and ovaries) • Two cell divisions • Sexual

49. = Sexual Reproduction Diploid Cell – full set of chromosomes Same as Mitosis 2 daughter cells are identical to parent cell. Chromosomes do NOT duplicate here. Each cell divides- chromosomes separating randomly! Haploid Cells – ½ set of chromosomes Also called Gametes Ex. Sperm, egg, pollen The Result: 4 daughter cells with ½ the number of chromosomes needed for an individual.

50. Sperm are created Eggs are created Resulting Cell when 2 gametes fuse = Fertilization