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Meiosis

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

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Meiosis

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  1. Meiosis • The form of cell divisionby which, with half the number of chromosomes,are produced. • Diploid (2n)  haploid (n) • Meiosis is sexual reproduction. • Two divisions (meiosis Iand meiosis II).

  2. Meiosis • 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.

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

  4. Interphase I • 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.

  5. Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

  6. 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

  7. Prophase I • Synapsis occurs: homologous chromosomescome together. • Homologous chromosomes carry similar information. You get one from your mom and one from your dad.

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

  9. Homologous Chromosomes • Pair of chromosomes (from mom and dad) that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each 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

  10. eye color gene eye color gene hair color gene hair color gene Paternal Maternal Homologous Chromosomes

  11. Crossing Over • Crossing overmay occur between chromatids. • Crossing over: segments of chromatids break and reattach to the other chromatid. • Crossing over adds variation.

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

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

  14. spindle fiber centrioles aster fibers Prophase I

  15. Metaphase I • Shortest phase • Tetrads align on the metaphase plate. • INDEPENDENT ASSORTMENT OCCURS: INCREASES VARIATION.

  16. OR metaphase plate metaphase plate Metaphase I

  17. Question: • In terms of Independent Assortment -how many different combinations of sperm could a humanmale produce?

  18. Answer • Formula: 2n • Human chromosomes: 2n = 46 n = 23 • 223 = ~8 million combinations

  19. Anaphase I • Homologous chromosomesseparate and move towards the poles. • Sister chromatids remain attached at their centromeres.

  20. Anaphase I

  21. Telophase I • Each pole now has haploid set of chromosomes.

  22. Telophase I

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

  24. Prophase II • same as prophase in mitosis

  25. metaphase plate metaphase plate Metaphase II • same as metaphase in mitosis

  26. Anaphase II • same as anaphase in mitosis • sister chromatids separate

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

  28. Telophase II

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

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

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

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

  33. Answer: • 10 chromosomes (haploid)

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

  35. sperm n=23 n=23 egg 2n=46 zygote Fertilization • The fusion of a sperm and egg to form a zygote. • A zygote is a fertilized egg

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