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Meiosis

Meiosis. Chapter 11.4. Function of Meiosis. Produces gametes (sex cells) Males – sperm Females – egg or ovum Only occurs in gonads (testes or ovaries) Sperm + egg → zygote (fertilized egg). Chromosome Count. Chromosome number = ploidy number (n)

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Meiosis

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  1. Meiosis Chapter 11.4

  2. Function of Meiosis • Produces gametes (sex cells) • Males – sperm • Females – egg or ovum • Only occurs in gonads (testes or ovaries) • Sperm + egg → zygote (fertilized egg)

  3. Chromosome Count • Chromosome number = ploidy number (n) • Haploid: Half the normal number of chromosomes (1 set = n) • Diploid: Normal number of chromosomes (2 sets = 2n) • Sperm (n) + Egg (n) → Zygote (2n)

  4. Human Chromosomes • Humans have 2 sets of 23 chromosomes • n = 23 2n = 46 • 22 pairs of autosomes • 1 pair of sex chromosomes • XX = female XY = male • Meiosis makes haploid gametes from diploid cells

  5. Sex chromosomes

  6. Homologous Chromosomes • Each pair of chromosomes are said to be homologous • One from mom and one from dad • Carry the same genes, but different forms of each gene • Ex – both chromosomes may have eye color gene, but one has blue and one has brown

  7. Meiosis • Two cell divisions • Meiosis I (PMAT I) • Homologous chromosomes separate • Results in 2 haploid cells • Meiosis II (PMAT II) • Sister chromatids separate • Results in 4 haploid cells

  8. Prophase I nuclear env. disappears homologous chromosomes pair up Form a tetrad (tetra = four) Metaphase I tetrads line up along cell’s equator Meiosis I

  9. Anaphase I Tetrad splits, homologous chromosomes separate Telophase I Cell splits into two haploid cells Meiosis I

  10. Meiosis II • Similar to mitosis • Prophase II uneventful • Metaphase II • Replicated chromosomes line up at equator • Anaphase II • Sister chromatids separate • Telophase II • Cells divide = 4 haploid gametes

  11. Gametogenesis • Spermatogenesis • 4 tiny haploid sperm • Oogenesis • 1 large haploid egg, 3 small polar bodies

  12. Creating Variation • Meiosis creates genetic variation in two ways: • Random shuffling of chromosomes in Metaphase I • More than 8 million possible gametes • Crossing over in Prophase I • Homologous chromosomes swap pieces of DNA when tightly paired as tetrad

  13. Crossing Over

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