Meiosis is the type of cell division that produces gametes, such as sperm and egg cells, which have half the number of chromosomes as regular body cells. It involves two cell divisions - Meiosis I and Meiosis II. In Meiosis I, homologous chromosomes pair up and separate, reducing the chromosome number by half. Meiosis II then separates the sister chromatids, resulting in four haploid daughter cells each with a single set of chromosomes. Meiosis introduces genetic variation through independent assortment of chromosomes and crossing over, providing the raw material for natural selection.

1. Edited by Hasimah Azit

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

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

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

5. Similar to mitosis interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres . Centriole pairs also replicate.

6. Nucleus and nucleolus visible. nuclear membrane nucleolus cell membrane chromatin

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

8. Longest and most complex phase (90%). Chromosomes condense. Synapsis occurs: homologous chromosomes come together to form a tetrad . Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids) .

9. Homologous chromosomes sister chromatids sister chromatids Tetrad

10. Pair of chromosomes ( maternal and paternal ) that are similar in shape and size. Homologous pairs (tetrads) carry genes controlling the same inherited traits. Each locus (position of a gene) is in the same position on homologues. Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes

11. Paternal Maternal eye color locus eye color locus hair color locus hair color locus

12. Crossing over (variation) may occur between nonsister chromatids at the chiasmata . Crossing over : segments of nonsister chromatids break and reattach to the other chromatid . Chiasmata (chiasma) are the sites of crossing over .

13. variation nonsister chromatids chiasmata: site of crossing over Tetrad

14. XX chromosome - female XY chromosome - male

15. centrioles spindle fiber aster fibers

16. Shortest phase Tetrads align on the metaphase plate . 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

17. metaphase plate OR metaphase plate

18. Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres .

20. Each pole now has haploid set of chromosomes . Cytokinesis occurs and two haploid daughter cells are formed.

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

23. same as prophase in mitosis

24. same as metaphase in mitosis metaphase plate metaphase plate

25. same as anaphase in mitosis sister chromatids separate

26. Same as telophase in mitosis . Nuclei form. Cytokinesis occurs. Remember: four haploid daughter cells produced. gametes = sperm or egg

28. 2n=4 sex cell diploid (2n) n=2 n=2 meiosis I n=2 n=2 n=2 n=2 sperm haploid (n) meiosis II

29. MEIOSIS

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

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

33. The 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

34. mitosis meiosis