Meiosis and Sexual Reproduction

1. Meiosis and Sexual Reproduction Chapter 12

2. Meiosis • A specialized type of cell reproduction specifically designed to create gametes (ova and sperm or spores) for sexual reproduction • Meiosis halves the parental cell’s chromosome number from diploid to haploid • Occurs only in sexually reproducing eukaryotic species

3. Terms and Concepts • Asexual reproduction • A single parent • No genetic variation • All offspring are genetically identical to each other and the parent • Mechanisms • Mitosis • Prokaryotic fission • Vegetative propagation • Pros and Cons • No energy expended to find a mate (pro) • The phrase “There goes the last female Dodo bird” doesn’t mean extinction (pro) • No genetic variation (con)

5. Fig. 9-1a, p.138

6. Terms and Concepts • Sexual Reproduction • Two parents required • most of the time—self fertilizing flowers • Genetic variation • Variation is important for survival and adaptation (and ultimately evolution) • Mechanisms • Meiosis, gamete formation, and fertilization • Pros and Cons • Genetic variation (pro) • Must expend energy to attract and find a mate (con) • The phrase “There goes the last female Dodo bird” does mean extinction (con)

7. Fig. 9-1b, p.138

8. Fig. 9-1c, p.138

9. Terms and Concepts • Diploid • Two copies of each type of chromosome • Haploid • One copy of each type of chromosome • Homologs • Pair of chromosomes • Somatic cells • Typical diploid cells of an animal • Germ cells • Cells committed to go through meiosis to produce gametes • Gamete • Reproductive cells (Egg/ova, sperm, spores) • Zygote • The first cell of a new individual (result of fertilization)

10. Meiosis • For sexual reproduction the gametes must have 1/2 the number of chromosomes as the normal cells of an organism • For example human cells have 46 chromosomes • (23 pairs) • If the sperm had 46 and the ovum had 46, then the resulting zygote would have 92 chromosomes! • Over time cells would have more and more chromosomes • The process of meiosis produces gametes with ½ the number of chromosomes

11. Fig. 9-12, p.150

12. Meiosis • During meiosis there are two rounds of division • Meiosis I • Separates homologs • Meiosis II • Separates sister chromatids

13. Each homologue in the cell pairs with its partner, then the partners separate. p.141c

14. Meiosis • The process of meiosis is very similar to mitosis, however there are some key differences that account for producing genetically variable, haploid gametes • The following slides will give an overview of meiosis especially pointing out the key differences from mitosis • However, the actual processes of each phase will not be presented as they are the same as in mitosis • Study figure 12.5

15. Meiosis • Meiosis I • Prophase I • Homologs pair up along their length • The event is called synapsis or pairing of homologous chromosomes • The structure of the two chromosomes is called a tetrad • There are four strands of DNA, two sets of sister chromatids • Crossing over • While the homologs are closely associated they can swap segments of DNA • This creates novel combinations of gene traits in both strands • Figure 12.7

16. Fig. 9-6c, p.144

17. Fig. 9-6d, p.144

18. Fig. 9-6e, p.144

19. Fig. 9-6f, p.144

20. Meiosis • Meiosis I • Metaphase I • Homolog pairs line up at the equator (pushed and pulled by the microtubules of the bipolar spindle, just as in mitosis) • Random Alignment (fig 12.7) • Homologs can be attached to either spindle pole • Each homolog can be packaged into either one of the two new nuclei • Increases the number of potential combinations of maternal and paternal alleles in gametes • There are over 8 million possible combinations of the maternal and paternal chromosomes which adds to genetic variability of meiosis

21. 1 2 3 combinations possible or or or Fig. 9-7, p.145

22. Meiosis • Meiosis I • Anaphase I • One of each duplicated chromosome or homolog is pulled towards a spindle pole by the microtubules • Its homolog moves to the opposite pole • This is the step that creates haploid cells by separating the homologous pairs

23. p.141c

24. Meiosis • Meiosis I • Telophase I • One of each type of chromosome has arrived at a spindle pole • For most species the cytoplasm will divide • For a few, cytoplasmic division occurs after both rounds of meiosis • Cells often proceed directly into meiosis II without completely finishing telophase I

25. Meiosis • Meiosis II (both cells created by meiosis I will follow through the steps of meiosis II to divide a second time) • Prophase II • The centriole pairs (centrosomes) separate and create a new bipolar spindle • Microtubules latch onto separate chromatids • Chromosomes are • Still duplicated (two chromatids each), but only one of each type of chromosome (haploid) • Generally still condensed following meiosis I • There is no DNA replication between meiosis I and II

26. Meiosis • Meiosis II • Metaphase II • Chromosomes are lined up at the equator • Anaphase II • Sister chromatids separate and are pulled towards opposite spindle poles • Telophase II • New nuclear envelope forms around all four new haploid nuclei • Cytoplasmic division results in four cells each containing a haploid number of unduplicated chromosomes

27. p.141d

28. Meiosis II Prophase II Metaphase II Anaphase II Telophase II

29. Meiosis • Meiosis introduces genetic variations in traits • Two parents both contribute genes to their offspring • One of each autosomal chromosome and one sex chromosome are passed to offspring • Thus offspring get two of each type of chromosome • Each pair of chromosomes carries the same genes • Genes may not be identical • The differences or traits of the same gene are called alleles

30. Meiosis • Meiosis introduces genetic variations (mixes of different alleles) in traits • Crossing over (figure 12.6) • Prophase I • Random alignment (figure 12.7) • Metaphase I • Remember genetic variation is one of the “pros” of sexual reproduction

31. Meiosis • Results of meiosis • 1 diploid parent cell  4 genetically variable haploid daughter cells

32. Questions • When homologs swap DNA this is called? • T or F: Homologs have a predictable pattern they use to line up during metaphase I. • What is separated during Anaphase I? • What is separated during Anaphase II? • What processes of meiosis contribute to genetic variation? • How many cells are produced by meiosis?

33. From gametes to offspring • Sexual reproduction • Meiosis  four haploid cells • Gamete formation • Sperm formation • Ova/Egg formation • Fertilization • Fusion of the haploid nuclei of the ovum and sperm • Creates a diploid zygote • Adds to variation • Which sperm fertilizes which egg is a matter of chance (see next chapter!)

34. Fig. 9-9, p.147

35. Fig. 9-10a, p.147

36. Fig. 9-12, p.150

37. Sexual Reproduction in Animals • Animals use Gametic Meiosis • Meiosis produces haploid gametes • Gametes do not divide (remain uni-cellular) • Fertilization produces a diploid zygote • Zygote undergoes mitosis to produce a multicellular diploid body (adult animal)

41. Sexual Reproduction in Plants • Plants use Sporic Meiosis (alternation of generations) • Meiosis produces haploid spores • Spores undergo mitosis to produce a multicellular haploid body (gametophyte) • Fertilization produces a diploid zygote • Zygote undergoes mitosis to produce a multicellular diploid body (sporophyte) • Alternates between multi-cellular diploid and multi-cellular haploid bodies

42. multicelled sporophyte zygote Diploid (2n) Phase of Cycle Meiosis Fertilization Haploid (n) Phase of Cycle gametes spores multicelled gametophyte mitosis mitosis mitosis Stepped Art Fig, 21.3, p. 326

43. Sexual Reproduction in Plants • Sporophyte (diploid, multicellular) • Produces flowers which contain the germ cells • Germ cells are located in the carpel (ova) and stamen (pollen)

45. Sexual Reproduction in Plants • Production of the ova • Occurs in flowers within the pistil • Made up of 1 or more carpels • Stigma • Style • Ovary

46. Sexual Reproduction in Plants • Production of the ova • Occurs in flowers within the pistil • The ovary contains 1 or more ovules which produce egg sacs • Meiosis of the ovules produces spores • Spores undergo mitosis to become the egg sac (gametophyte) Meiosis Spores Egg Sac Mitosis

47. an ovule ovary wall megasporocyte integument stalk ovary (cutaway view) Diploid Stage Haploid Stage Double Fertilization Meiosis Fig. 28.6b, p. 454-5

48. Sexual Reproduction in Plants • Production of pollen • Occurs in flowers in the stamen • Anther • Filament

49. Sexual Reproduction in Plants • Production of pollen • The anther contains germ cells in pollen sacks • Meiosis produces spores • Spores undergo mitosis to become the pollen (gametophyte)

50. pollen sac anther (cutaway view) filament Germ cell Diploid Stage Haploid Stage Meiosis Spores Mitosis Pollen