Meiosis-Reduction Division

1. Meiosis-Reduction Division Ch. 9-5 and 9-6

2. Gametes • Form from cell division of sex cells • Meiosis is cell division to produce gametes • Meiosis has two divisions of the nucleus (Meiosis I and Meiosis II) and produces cells with half the number of chromosomes (haploid)

3. from mother from father child too much! Meiosis • Reduces the genetic material by half • Why is this necessary? meiosis reduces genetic content

4. Homologous Chromosomes • Carry the same genes • Pair during Meiosis I • Separate in the formation of gametes • One copy of each pair is from the mother and one is from the father. Figure 1.2

5. Sexual Reproduction • Meiosis and sexual reproduction increases genetic diversity in a population • Variation is important in a changing environment • Evolution is the genetic change in a population over time

6. Comparison of Mitosis and Meiosis Table 3.1

7. Meiosis Interphase precedes meiosis I Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II Figure 2.13

8. Spindle fibers Nucleus Nuclear envelope Prophase I (diploid) Metaphase I (diploid) Anaphase I (diploid) Telophase I (diploid) Meiosis I : the reduction division Figure 3.4

9. Prophase I • Late prophase • Chromosomes condense • Spindle forms • Nuclear envelope fragments • Early prophase • Homologs pair • Crossing over occurs Figure 3.4

10. A A a a B B b b C C c c D D d d E E e e F F f f Recombination (crossing over) • Occurs in prophase of meiosis I • Homologous chromosomes exchange genes • Generates diversity Figure 3.5

11. Recombination (crossing over) A a a A B • Exchange between homologs • Occurs in prophase I b b B c C C c D D d d E E e e F F f f Figure 3.5 Letters denote genes and case denotes alleles

12. Recombination (crossing over) a A a A B b B b c c C C • Creates chromosomes with new combinations of alleles for genes A to F D D d d E E e e F F f f Figure 3.5

13. Metaphase I • Homolog pairs align • along the equator of the cell Figure 3.4

14. Independent Assortment The homolog of one chromosome can be inherited with either homolog of a second chromosome. Figure 3.6

15. Anaphase I • Homologs separate and move to opposite poles • Sister chromatids remain attached at their centromeres Figure 3.4

16. Telophase I • Nuclear membrane reforms • Spindle disappears • Cytokinesis divides cell Figure 3.4

17. Prophase II (haploid) Metaphase II (haploid) Anaphase II (haploid) Telophase II (haploid) Four nonidentical haploid daughter cells Meiosis II : like mitosis; sister chromatids separate Figure 3.4

18. Prophase II • Nuclear envelope fragments • Spindle forms Figure 3.4

19. Metaphase II • Chromosomes align • along equator of cell Figure 3.4

20. Anaphase II • Centromeres divide • Sister chromatids separate Figure 3.4

21. Telophase II • Nuclear envelopes reform • Chromosomes decondense • Spindle disappears • Cytokinesis divides cells Figure 3.4

22. Results of Meiosis • Gametes • Four haploid cells • Contain one copy of each chromosome and one allele of each gene • Each cell is unique Figure 3.4

23. Meiosis I (reduction division) Meiosis II (equational division) Diploid Haploid Haploid Meiosis: Cell Division in Two Parts Figure 3.3 Result: one copy of each chromosome in a gamete.

24. Table 3.1