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Production of Gametes

Production of Gametes. MEIOSIS. Meiosis Facts. Meiosis. 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 ). Meiosis.

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Production of Gametes

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  1. Production of Gametes MEIOSIS

  2. Meiosis Facts

  3. Meiosis • 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)

  4. Meiosis • Sex cells divide to produce gametes (sperm or egg) • Gametes have half the number of chromosomes • Meiosis is similar to mitosis with some chromosomal differences

  5. Meiosis • Occurs only in gonads (testes or ovaries) • Male: Spermatogenesis • Female: Oogenesis

  6. 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

  7. 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.

  8. Interphase I • Nucleus and nucleolus visible. Chromatin Nuclear membrane Cell membrane Nucleolus

  9. Meiosis I

  10. Stages of Meiosis I • Cell division thatreduces the chromosome number by one-half • Four phases: a. Prophase I b. Metaphase I c. Anaphase I d. Telophase I

  11. 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 (sister and nonsister chromatids)

  12. Homologous chromosomes Sister chromatids Sister chromatids Tetrad Prophase I - Synapsis

  13. 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

  14. Homologous Chromosomes • Humans have 23 pairs of homologous chromosomes • First 22 pairs of chromosomes called autosomes • Last pair called sex chromosomes • XX female or XY male

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

  16. 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 • Chiasmata (chiasma) are the sites of crossing over

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

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

  19. Spindle fiber Centrioles Aster fibers Prophase I

  20. Metaphase I • Shortest phase • Tetrads align on the metaphase plate • Independent assortment occurs • Chromosomes separate randomly to the poles of the cells

  21. Metaphase I • Independent assortment causes variation in the forming cells • Orientation of homologous pair to poles is random • Formula for determining variation: Formula: 2n Example: 2n = 4 then n = 2 thus 22 = 4 combinations

  22. OR Metaphase plate Metaphase plate Metaphase I

  23. Question: • In terms of Independent Assortment, how many different combinations of sperm could a human male produce?

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

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

  26. Anaphase I

  27. Telophase I • Each pole now has haploid set of chromosomes • 1n = 23 (human) • Cytokinesis occurs and two haploid daughter cells are formed

  28. Telophase I

  29. Meiosis II

  30. Meiosis II • Short or No interphase II • DNA NOT replicated again • Remember: Meiosis II is similar to mitosis • Prophase I, Metaphase II, Anaphase II, and Telophase II

  31. Prophase II • Same as prophase in mitosis • Nuclear envelope breaks down and the spindle apparatus forms

  32. Metaphase plate Metaphase plate Metaphase II • Same as metaphase in mitosis • Chromatids lined up at equator

  33. Anaphase II • Same as anaphase in mitosis • Sister chromatids separate

  34. Telophase II • Same as telophase in mitosis • Nuclei reform • Cytokinesis occurs • Remember: Four haploid daughter cells produced • Gametes = sperm or egg

  35. Telophase II

  36. 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

  37. Genetic Variation

  38. Variation • Important to the survival of populations • Aids in natural selection • Strongest individuals are able to survive and reproduce

  39. Question • What are the three sources of genetic variation in sexual reproduction?

  40. Answer: • Crossing Over (Prophase I) 2. Independent Assortment (Metaphase I) 3. Random Fertilization(sperm joins with egg)

  41. Remember:Variation is helpful to the survival of a species!

  42. Question: • A diploid cell containing 20 chromosomes(2n = 20)at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

  43. Answer: • 10 chromosomes (haploid) • 1n = 10

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

  45. 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

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

  47. Answer: • Four cells with 10 chromosomes each

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