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Problem:. Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?. Answer:. Meiosis. 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.

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  1. Problem: Our cells have 46 chromosomes—so why don’t fertilized eggs have 92 chromosomes?

  2. Answer: Meiosis

  3. Meiosis • The form of cell divisionby 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. 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

  5. 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.

  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. Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes

  8. 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

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

  10. 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.

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

  12. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

  13. OR metaphase plate metaphase plate Metaphase I

  14. Anaphase I

  15. Telophase I

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

  17. Prophase II • same as prophase in mitosis

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

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

  20. Telophase II

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