1. Meiosis Sec. 11-3, 11-4

2. All cells must reproduce Prokaryotes-asexual reproduction Eukaryotes-both asexual and sexual reproduction

3. All cells must reproduce Mitosis-asexual reproduction in eukaryotes. –Maintains chromosome number –Produces identical cells –Growth –Repair of old and damaged cells –Reproduction in unicellular organisms

4. All cells must reproduce Meiosis-Sexual reproduction in eukaryotes –Sexual reproduction is favored by natural selection when conditions are changing or uncertain (which turns out to be most of the time). –Reduces chromosome number

5. Meiosis – sexual reproduction Meiosis is used to: –Make gametes These need to have half the number of chromosomes found in a regular body cell. This is necessary so that when male and female gametes combine to make the new individual, it will have the same number of chromosomes as its parents.

6. Ploidy Eukaryotes are typically diploid. “ploid” refers to the number of complete sets of chromosomes for that organism. Di ploid – “di” means two, so diploid organisms have 2 complete sets of chromosomes. Haploid – means the cell has only one complete set of chromosomes.

8. Ploidy We use the symbol 2n to represent diploid. We use the symbol n to represent haploid. 2n = diploid n = haploid

9. Sister chromatids vs. homologous chromosomes 2n → 2 sets of chromosomes –One set came from mom and the other came from dad. –Homologous pair → similar size and shape, but not identical Mom provides one set and dad provides the other set of chromosomes. During the S-phase, the chromosomes are copied –Sister chromatids → identical chromosomes

10. Comparing Mitosis and Meiosis Mitosis –Start with diploid (2n) somatic (body) cell –With 2 sister chromatids per chromosome –Prophase, Metaphase, Anaphase, Telophase, Cytokinesis –End up with two identical, diploid (2n) daughter cells

11. Mitosis

12. Comparing Mitosis and Meiosis Meiosis – the process of forming gametes –Start with diploid (2n) germ (reproductive)cell –With 2 sister chromatids per chromosome – Involves 2 cycles of division – Meiosis I and Meiosis II

13. Comparing Mitosis and Meiosis Meiosis I (first half of meiosis) –Prophase I, Metaphase I, Anaphase I, Telophase I –cytokinesis Meiosis I results in two cells, each haploid (n), still with 2 sister chromatids per chromosome.

14. TELOPHASE IPROPHASE IANAPHASE IMETAPHASE I typically two nuclei (n) Crossing over occurs between homologues. Homologues separate from their partner. Homologous pairs align randomly. Meiosis I

15. Comparing Mitosis and Meiosis Meiosis II (second half of meiosis) –Prophase II, Metaphase II, Anaphase II, Telophase II –cytokinesis Meiosis II results in a grand total of 4 cells, 2 from each of the cells produced in Meiosis I [2*2=4].

16. Comparing Mitosis and Meiosis Meiosis II (second half of meiosis) –These 4 cells that are produced are haploid (n). –They are all genetically different from one another. –In males, all 4 of these go on to become gametes (sperm). –In most female organisms, only one of the 4 cells goes on to form a gamete (egg).

17. Meiosis II TELOPHASE II PROPHASE II ANAPHASE II METAPHASE II no interphase between nuclear divisions Chromosomes align at spindle equator. Sister chromatids of chromosomes separate. four nuclei (n)

18. Genetic variance in meiosis Meiosis produces 4 genetically different daughter cells Sources of differences –Crossing-over –Independent assortment

19. Crossing-over In Prophase I, each homologous pair comes together to form tetrads.

20. Crossing-over Each tetrad has 4 chromatids When the chromosomes are in a tetrad, they overlap one another and exchange equivalent portions of their chromatids. This creates new combinations of alleles. This process is called crossing-over.

21. Independent Assortment In Metaphase I, the tetrads line up across the middle of the cell. The orientation of each tetrad is at random (not lined up with all the father’s chromosomes preparing to go to one side and all the mother’s chromosomes preparing to go to the other). This results in independent assortment.

22. Independent Assortment Independent assortment - pairs of alleles separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another, not together. So, all blond(e)s are not short and all brunettes are not tall.

23. Meiosis – sexual reproduction Meiosis is also used to: –Create more genetic variation in the population. – Genetic variation is a measure of how many different alleles (different forms of the same gene)there are in a group and how common those alleles are.

24. Genetic variation More genetic variation is created in a population by 3 types of genetic recombination. Genetic recombination is the shuffling of alleles into new and different combinations.

25. Genetic recombination Three sources of genetic recombination related to meiosis: – Random mating – gametes unite randomly. – Independent assortment of homologous chromosomes (in Metaphase I and Anaphase I) – Crossing-over in Prophase I

26. Type of cell division Type of reproductio n (Asexual or sexual) Number of daughter cells Chromosome number in daughter cells Genetic identity (identical or not identical) Function of cell division in multicellular organism Mitosis asexual 2 2n identical growth and repair Meiosis sexual 4 n not identical reproduction, formation of gametes