1. MEIOSIS

2. Meiosis cell divisiongametes, halfchromosomes,The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n)Diploid (2n)  haploid (n) Meiosissexual reproduction.Meiosis is sexual reproduction. Two divisionsmeiosis I meiosis IITwo divisions (meiosis I and meiosis II).

3. Meiosis Sex cells gametes(sperm or egg)Sex cells divide to produce gametes (sperm or egg). GameteshalfchromosomesGametes have half the # of chromosomes. Occurs only in gonads (testes or ovaries).Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis MeiosismitosisMeiosis is similar to mitosis with some chromosomal differences.

4. Spermatogenesis 2n=46 human sex cell diploid (2n) n=23 meiosis I n=23 sperm haploid (n) meiosis II

5. Interphase I mitosisSimilar to mitosis interphase. Chromosomes(S phase).Chromosomes replicate (S phase). chromosome chromatids centromeresEach duplicated chromosome consist of two identical sister chromatids attached at their centromeres. CentrioleCentriole pairs also replicate.

6. Meiosis I (four phases) Cell division chromosome one-half.Cell division that reduces the chromosome number by one-half. four phasesfour phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

7. Prophase I Longest and most complex phase (90%).Longest and most complex phase (90%). ChromosomesChromosomes condense. Synapsis homologous chromosomes tetradSynapsis occurs: homologous chromosomes come together to form a tetrad. Tetradchromosomes chromatidsTetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

8. Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

9. Homologous Chromosomes chromosomesmaternalpaternalPair of chromosomes (maternal and paternal) that are similar in shape and size. (tetrads)Homologous pairs (tetrads) carry genes controlling the same inherited traits. homologous chromosomes.Humans have 23 pairs of homologous chromosomes. autosomes a.22 pairs of autosomes sex chromosomes b.01 pair of sex chromosomes

10. Homologous Chromosomes PaternalMaternal eye color locus eye color locus hair color locus hair color locus

11. Crossing Over Crossing over (variation) chromatidsCrossing over (variation) may occur between nonsister chromatids Crossing over chromatids chromatidCrossing over: segments of nonsister chromatids break and reattach to the other chromatid. This results in a large variation of possible traits

12. Crossing Over - variation nonsister chromatids chiasmata: site of crossing over variation Tetrad

13. Prophase I centrioles spindle fiber aster fibers

14. Metaphase I Shortest phaseShortest phase Tetradsmetaphase plateTetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example:2n = 4 then n = 2 thus 2 2 = 4 combinations thus 2 2 = 4 combinations

15. Metaphase I metaphase plate OR metaphase plate

16. Question: In terms of Independent Assortment - how many different combinations of sperm could a human male produce?In terms of Independent Assortment - how many different combinations of sperm could a human male produce?

17. Answer Formula: 2 nFormula: 2 n Human chromosomes:2n = 46Human chromosomes:2n = 46 n = 23 n = 23 2 23 = ~8 million combinations2 23 = ~8 million combinations

18. Anaphase I

19. Telophase I haploid chromosomesEach pole now has haploid set of chromosomes. CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

20. Telophase I

21. Meiosis II No interphase IINo interphase II DNA replication (or very short - no more DNA replication) Remember: Meiosis II mitosisRemember: Meiosis II is similar to mitosis

22. Prophase II prophasemitosissame as prophase in mitosis

23. Metaphase II metaphasemitosissame as metaphase in mitosis metaphase plate

24. Anaphase II anaphasemitosissame as anaphase in mitosis sister chromatids separatesister chromatids separate

25. Telophase II telophasemitosisSame as telophase in mitosis. Nuclei form. CytokinesisCytokinesis occurs. Remember:four haploid daughter cells produced.Remember:four haploid daughter cells produced. gametes = sperm or egg

26. Telophase II

27. Meiosis 2n=4 sex cell diploid (2n) n=2 meiosis I n=2 sperm haploid (n) meiosis II

28. Variation Important to population as the raw material for natural selection.Important to population as the raw material for natural selection. Question:Question: What are the three sexual sources of genetic variation?

29. Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!

30. Question: 20 chromosomes(diploid) chromosomesA cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

31. Answer: 10 chromosomes (haploid)10 chromosomes (haploid)

32. Fertilization spermeggzygoteThe fusion of a sperm and egg to form a zygote. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote