Meiosis and Sexual Reproduction: Genetic Diversity and Chromosome Reduction

1. Chapter 12 Meiosis and Sexual Reproduction

2. Question? • Does Like really beget Like? • The offspring will “resemble” the parents, but they may not be “exactly” like them. • This chapter deals with reproduction of life.

3. Heredity • The transmission of traits from parents to offspring. • Comment - Humans have been aware of heredity for thousands of years.

4. Genetics • The scientific study of heredity. • Comment - Genetics is only about 150 years old.

5. Genes • The DNA for a trait. • Locus - the physical location of a gene in a chromosome. • Top part of chromosome is the “p” and the bottom is the “q”

6. Reproduction • A method of copying genes to pass them on to offspring. • Two main types: • Asexual reproduction • Sexual reproduction

7. Asexual Reproduction • Parent passes all of its genes to its offspring. • Uses mitosis. • Also known as cloning. • Comment - many organisms reproduce this way.

8. Asexual Bud

9. Advantages • Only need 1 parent. • Offspring are identical to the parent. • Good genetic traits are conserved and reproduced.

10. Disadvantages • No new DNA combinations for evolution to work on. • Clones may become extinct if attacked by a disease or pest.

11. Sexual Reproduction • Two parents contribute DNA to an offspring. • Comment - most organisms reproduce this way, but it hasn’t been proven in some fungi and a few others.

12. Advantages • Offspring has a unique combination of DNA which may be an improvement over both parents. • New combination of DNA for evolution to work with.

13. Disadvantages • Need two parents. • Good gene combinations can be lost. • Offspring may not be an improvement over the parents.

14. Question ? • Do parents give their whole DNA copy to each offspring? • What would happen to chromosome number if they did?

15. Chromosome Number • Is usually constant for a species. • Examples: • Humans - 46 • Corn - 20 • Onions - 16 • Dogs - 72

16. Life Cycle - if Mitosis Female 46 Male 46 egg 46 sperm 46 Zygote 92 mitosis mitosis Mitosis

17. Result • Chromosome number would double each generation. • Need a method to reduce the chromosome number.

18. Life Cycle - if Meiosis Female 46 Male 46 egg 23 sperm 23 Zygote 46 mitosis mitosis Meiosis

19. Result • Chromosome number will remain the same with each sexual reproduction event. • Meiosis is used to produce the gametes or sex cells.

20. Meiosis - Purpose • To reduce the number of chromosomes by half. • Prevents doubling of chromosome numbers during sexual reproduction.

21. Sexual Life Cycle • Has alternation of meiosis and fertilization to keep the chromosome numbers constant for a species.

23. Ploidy • Number of chromosomes in a "set" for an organism. • Or, how many different kinds of chromosomes the species has. • Usually shown as N = …… • Humans N = 23

24. Diploid • 2 sets of chromosomes. • Most common number in body or somatic cells. • Humans 2N = 46 • Corn 2N = 20 • Fruit Flies 2N = 8

27. Haploid • 1 set of chromosomes. • Number in the gametes or sex cells. • Humans N = 23 • Corn N = 10 • Fruit Flies N = 4

28. Polyploids • Multiple sets of chromosomes. • Examples • 3N = triploid – Ex: seedless watermelons – get from crossing a diploid male with a tetraploid female • 4N = tetraploid • Common in plants, but often fatal in animals.

29. Life Cycle Variations

30. Meiosis/Mitosis Preview of differences • Two cell divisions, not one. • Four cells produced, not two. • Synapsis and Chiasmata will be observed in Meiosis

31. Meiosis/Mitosis Preview of differences • 1st division separates PAIRS of chromosomes, not duplicate chromosomes. • Interkinesis is present.

32. Meiosis • Has two cell divisions. Steps follow the names for mitosis, but a “I” or “II” will be added to label the phase.

34. Prophase I • Basic steps same as in prophase of Mitosis. • Synapsis occurs as the chromosomes condense. • Synapsis - homologous chromosomes form bivalents or tetrads.

35. Prophase I • Chiasmata – (a point of overlap of paired chromatids at which fusion and exchange of genetic material take place during prophase of meiosis) observed. • That’s fancy for “crossing over” • Longest phase of division.

36. Metaphase I • Tetrads or bivalents align on the metaphase plate. • Centromeres of homologous pairs point toward opposite poles.

37. Anaphase I • Homologous PAIRS separate. • Duplicate chromosomes are still attached at the centromeres.

38. Anaphase I • Maternal and Paternal chromosomes are now separated randomly.

40. Telophase I • Similar to Mitosis. • Chromosomes may or may not unwind to chromatin. • Cytokinesis separates cytoplasm and 2 cells are formed.

41. Interkinesis • No DNA synthesis occurs. • May last for years, or the cell may go immediately into Meiosis II. • May appear similar to Interphase of Mitosis.

42. Meiosis II • Steps are the same as in Mitosis. • Prophase II • Metaphase II • Anaphase II • Telophase II

43. Meiosis - Results • 4 cells produced. • Chromosome number halved. • Gametes or sex cells made. • Genetic variation increased.

45. Sexual Sources of Genetic Variation 1. Independent Assortment of Chromosomes during Meiosis. 2. Random Fertilization. 3. Crossing Over.

46. Independent Assortment • There are 23 pairs of chromosomes in humans. • The chance to inherit a single chromosome (maternal or paternal) of each pair is 1/2.

48. Gamete Possibilities • With 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are: 223 or 8,388,608

49. Random Fertilization • The choice of which sperm fuses with which egg is random.

50. Random Fertilization • Therefore, with 8,388,608 kinds of sperms and 8,388,608 kinds of eggs, the number of possible combinations of offspring is over 64 million kinds.