MEIOSIS IN GENETIC NURSINGS (1).ppt.pptx

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1. MEIOSIS

2. Meiosis A special form of germ cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n) → haploid (n) Gametes have half the set of chromosomes Meiosis is sexual reproduction. Two divisions ( meiosis I and meiosis II ). Sex cells produce gametes (sperm or egg) . Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis

3. Spermatogenesis 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

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

5. Prophase I 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.

6. Prophase I - Synapsis Homologous chromosomes sister chromatids sister chromatids Tetrad

7. Homologous Chromosomes 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.

8. Homologous Chromosomes Paternal Maternal eye color locus eye color locus hair color locus hair color locus

9. Crossing Over Crossing over (variation) may occur between nonsister chromatids at the chiasmata . Crossing over : segments of nonsister chromatids break and reattach to the other chromatid .

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

11. Sex Chromosomes XX chromosome - female XY chromosome - male

12. Prophase I centrioles spindle fiber aster fibers

13. Metaphase I Shortest phase Tetrads align on the metaphase plate .

14. Metaphase I metaphase plate OR metaphase plate

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

16. Anaphase I

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

18. Telophase I

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

20. Prophase II same as prophase in mitosis

21. Metaphase II same as metaphase in mitosis metaphase plate metaphase plate

22. Anaphase II same as anaphase in mitosis sister chromatids separate

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

24. Telophase II

25. Meiosis 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

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

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

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

29. Answer: 10 chromosomes (haploid)

30. Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type.

31. Fertilization 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

32. Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes ?

33. Answer: 10 chromosomes