Chap 13: Meiosis and Sexual Life Cycles

1. Points of Emphasis • Know: • 1. all the bold-faced terms • the stages of meiosis • What is mean by “alternation of generation.” • The differences between meiosis and mitosis • Where genetic variation comes from. You’ve got to know why we are all different Chap 13: Meiosis and Sexual Life Cycles

2. Figure 13.1 The asexual reproduction of a hydra Budding:one example of asexual reproduction. Very little difference between offspring and parent. Hydra: relative of the jellyfish

3. Figure 13.x1 SEM of sea urchin sperm fertilizing egg Sexual Reproduction: produces greater variation because a unique combination of genes due to the process of meiosis.

4. Figure 13.3 Preparation of a human karyotype

5. Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes

6. Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes

7. Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization

8. Three Sexual Life Cycles • Mitosis and meiosis occurs in all sexually reproducing organisms but their timing differs. • Animals (humans) • Gametes are the only haploid cells • Meiosis occurs only in the production of sperm and egg cells. • No further division occurs after the sperm and egg cells are produced. • Zygote, the product of the sperm and egg, is diploid and divides to produce the multicellular organism.

9. Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization

10. Three Sexual Life Cycles, cont’d • Fungi and some algae • Gametes (n) fuse to make the the 2n zygote • Meiosis occurs before an organism can develop and these 1n cells divide to produce a haploid multicellular organism. • Then mitosis occurs to produce the gametes. This maintains the number of chromosomes at 1n. • The only diploid or 2n stage is the zygote, not the multicellular organism. • So in fungi, both 1n cells and 2n cells can undergo mitosis.

11. Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization

12. Three Sexual Life Cycles, cont’d • Plants and some Algae: Alternation of Generations • Both diploid and haploid can be multicellular • Sporophyte stage is the diploid stage • Sporophyte (2n) undergoes meiosis producing 1n spores • Spores divide into a multicellular individual, the gametophyte • The gametophyte undergoes mitosis and produces gametes. • Gametes fuse diploid zygote new sporophyte.

13. Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization

14. Meiosis undergoes one replication but two cellular divisions Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number A homologous pair can have differences in their DNA sequence (different alleles)

15. Figure 13.7 The stages of meiotic cell division: Meiosis I

16. Figure 13.7 The stages of meiotic cell division: Meiosis II

17. The Genetic Consequences of Chromosomal Reduction • Daughter cells of mitosis are genetically identical to parent cells whereas in meiosis, the daughter cells are genetically different. • This is allowed by: • Crossing Over • Paired homologues are held tightly together during synapsis. • This forms a tetrad, 4 sister chromatids intertwined • Chiasmata, where the nonsister chromatids cross over.

18. Chromosomal Reduction, cont’d • At metaphase I of meiosis, it is the homologous pairs that line up at the metaphase plate. • At anaphase I, sister chromatids DO NOT separate. Instead the homologous pairs separate. • The second meiotic division is where the sister chromatids separate.

19. Figure 13.8 A comparison of mitosis and meiosis

20. Figure 13.8 A comparison of mitosis and meiosis: summary

21. Origins of Genetic Variation • So if our DNA is 99% identical, why are we different? • Crossing Over • Genetic exchange or recombination occurs at the chiasmata • This genetic exchange is between your father’s genes (alleles) and your mother’s genes (alleles) which are from your grandparents and so on and so on. • This new combination that results in the sister chromatids that have new combinations from many generations • These sister chromatids can align at the metaphase plate in metaphase II in all sorts of combinations with all the other sister chromatids of the other chromosomes (independent assortment) • When these sister chromatids separate in anaphase II of meiosis, you have very different egg cells and sperm cells.

22. Figure 13.10 The results of crossing over during meiosis

23. Random Fertilization • Independent Assortment of Chromosomes • Just take two chromosomes from each parent, F1, F21 from your father and M1 and M21 from your mother. • These chromosomes replicate so now you have two sister chromatids of each, held together by a centromere. • When they line up at the metaphase plate, your father’s F1 and mother’s F1 must line up across from each other, but F1 doesn’t have to be on the same “side or half” as F21 so you could have • F1 – M1 or F1 - M1 • F21 – M21 M21 – F21 • And recalling that genetic recombination has occurred during crossing over that even adds to the variation. Origins of Genetic Variation, cont’d

24. Figure 13.9 The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I

25. Evolutionary Adaptation A population evolves or changes over time by differential success in surviving and reproducing in the environment. Genetic variations most suitable for the environment are the ones that accumulate.