Meiosis & Sexual Life Cycles

1. Meiosis & Sexual Life Cycles Chapter 13

2. Cell Reproduction • Mitosis • produce cells with sameinformation • identical daughter cells • exact copies (clones) • same amount of DNA • same number of chromosomes • asexual reproduction What are disadvantages of asexual reproduction? What are advantages?

3. Asexual Reproduction • Single-celled eukaryotes reproduce asexually (mitosis + cytokinesis) • Yeast, Paramecium, Amoeba • Simple multicellular eukaryotes reproduce asexually by budding • Hydra Budding in Hydra

4. Budding in Yeast Cell Division in Amoeba

5. Reproduction in multicellular organisms • How is it possible to maintain such genetic continuity in a multicellular organism? Michael & Kirk Douglas Baldwin brothers Martin & Charlie Sheen, Emilio Estevez

6. Sexual Reproduction • But how does a complex multicellular organism reproduce? • joining of gametes • egg (or ovum) • sperm What has to happen to make gametes?

7. Role of Meiosis in Sexual Cycles • Alternating processes • fertilization & meiosis alternate • meiosis reduces chromosome number • diploid haploid • 2n n • fertilization restores chromosome number • haploid diploid • n 2n

8. Meiosis & fertilization must alternate for sexual reproduction to continue • Alternation of generationdominant multicellular stage (n vs. 2n) differs amongst organisms

9. Multicellular Haploid (n) organism = gametophyte • Multicellular Diploid (2n) organism = sporophyte • Alternation of generations: life history having a multicellularhaploid stage which alternates with a multicellulardiploid stage • Evolutionary trends: Earlier organisms have dominant gametophyte (n) individual • • gametophyte generation is major stage in life of mosses, visible stage in fungi, and is an independent plant in ferns • However, gametophyte is only an inconspicuous structure within flower or other reproductive organs of "higher" plants

10. Visualizing Chromosomes

11. 23 pairs of chromosomes

12. Human Female Karyotype

13. Human male karyotype

14. What does a karyotype reveal? • Mutation in chromosome number or gross chromosomal abnormalities Down’s syndrome or Trisomy 21

15. Cri du chat

16. Homologous Chromosomes • Paired chromosomes • both chromosomes of a pair carry genes controlling same inherited characters • homologous = same information 2 copies = diploid = 2n

17. Sexual reproduction: Fertilization From Dad – male gamete From Mom – female gamete • 1 copy • Haploid • 1n What would happen if meiosis didn’t occur? -2 copies -Diploid -2n Zygote

18. Making gametes -2 copies -diploid -2n -1 copy -haploid -1n

19. Meiosis = reduction division • Meiosis • special cell division in sexually reproducing organisms • reduce 2n 1n • diploid haploid (half) • makesgametes (sperm, eggs) Warning: meiosis evolved from mitosis, so stages & “machinery” are similar but the processes are radically different. Do not confuse!

20. Double division of Meiosis DNA replication 1st division of meiosis separates homologous pairs 2nd divisionof meiosis separates sister chromatids

21. Steps of Meiosis • Meiosis 1 • prophase 1 • metaphase 1 • anaphase 1 • telophase 1 • Meiosis 2 • prophase 2 • metaphase 2 • anaphase 2 • telophase 2 1st division of meiosis separates homologous pairs (2n 1n) 2nd division of meiosis separates sister chromatids (1n 1n) *just like mitosis*

22. Preparing for Meiosis 2n Single-stranded • 1st step of meiosis • Replication of DNA • Why bother? • meiosis evolved after mitosis • convenient to use “machinery” of mitosis • DNA replicated in S phase of meiosis interphase (just like in mitosis) 2n Double-stranded

23. Preparing for meiosis 2n single stranded • 1st division of meiosis separates homologous pairs • Synapsis – pairing of homologous chromosomes– forms atetrad(4 sister chromatids) 2n double stranded Prophse 1 Synapsis Metaphase 1 2n double stranded Telophase 1 Tetrad 1n double stranded

24. Meiosis 2 • 2nd division of meiosis separates sister chromatids What does this division look like? 1n double stranded Metaphase 2 1n single stranded cytokinesis

27. Mitosis vs. Meiosis

28. Mitosis (asexual) 1 division daughter cells genetically identical to parent cell produces 2 cells 2n 2n produces cells for growth & repair no crossing over Meiosis 2 divisions daughter cells genetically different from parent produces 4 cells 2n 1n produces gametes crossing over Mitosis vs. Meiosis

29. The value of meiosis • Meiosis introduces genetic variation • gametes of offspring do not have same genes as gametes from parents • genetic recombination • random assortment in humans produces 223 (8,388,608) different combinations Frommom From dad New gametes made by offspring

30. Random Fertilization • Any 2 parents will produce a zygote with over 70 trillion (223 x 223) diploid combinations

31. And more variation • During Prophase 1 • homologous pairs swap pieces of chromosome • sister chromatids intertwine • crossing over

32. Crossing Over • 3 steps • cross over • breakage of DNA • re-fusing of DNA • New combinations of traits What are advantages of sexual reproduction? What are disadvantages?

33. Genetic variation • Meiosis & crossing over introduce great genetic variation to population - drives evolution Consider the greater variation with 23 pairs of chromosomes = mixing and matching

34. Sources of genetic variability • Genetic variability in sexual reproduction • independent assortment • homologous chromosomes in Meiosis 1 • sister chromatids in Meiosis 2 • crossing over • between homologous chromosomes in prophase1 • random fertilization • random ovum by a random sperm

35. Spermatogenesis • Continuous & prolific process in adult male • Each ejaculation contains 100–650 million sperm • Occurs in seminiferous tubules

36. Structure of sperm

37. Oogenesis • At birth an ovary contains all of the primary oocytes it will ever have • ~300 eggs released • Unequal cytokinesis • 1 egg • 3 polar bodies • polar bodies degenerate What is the advantage of 1 egg released at a time?