Meiosis

• Somatic cells


  Body cells that contain the normal (diploid) number of chromosomes
• Gametes


  Sex cells that contain half (haploid) the normal number of chromosomes
• n = number of chromosomes in a haploid set, 2n = number of chromosomes in a diploid set
• Polyploid cells have more than two chromosomes per set, e.g. 3n (3 chromosomes per set)
• Male gamete


  Sperm, produced in the testes
• Female gamete


  Ovum (ova), produced in the ovaries
• Fertilization


  1. Ovum released from ovary

  2. Sperm and ovum join to form zygote
• Zygote


  Fertilized egg
• Homologous chromosomes


  Pair of chromosomes (one from mother, one from father) that are similar in shape and size and carry genes for the same inherited traits
• Humans have 23 pairs of homologous chromosomes
• Autosomes


  Chromosomes 1-22 that code for most offspring traits
• Sex chromosomes


  Chromosome 23 that determines sex of offspring (XX=female, XY=male)
• Meiosis


  Process by which diploid cells are reduced to haploid gametes
• If meiosis did not occur, the chromosome number in each new generation would double and the offspring would die
• Meiosis


  Two cell divisions (meiosis I and meiosis II) with only one duplication of chromosomes
• Spermatogenesis


  Meiosis in males that produces sperm
• Oogenesis


  Meiosis in females that produces ova
• Spermatogenesis


  Primary spermatocyte undergoes meiosis I and II to produce 4 haploid sperm cells
• Oogenesis


  Only one ovum (egg) is produced from each primary oocyte, polar bodies die
• Interphase I


  Chromosomes replicate, each consisting of two sister chromatids
• Meiosis I


  1. Prophase I

  2. Metaphase I

  3. Anaphase I

  4. Telophase I
• Prophase I


  Longest and most complex phase, homologous chromosomes synapse to form tetrads
• Crossing over


  Segments of non-sister chromatids break and reattach, creating variation
• A diploid cell with 20 chromosomes would produce haploid cells with 10 chromosomes after meiosis
• A cell with 40 chromatids at the start of meiosis would produce cells with 10 chromosomes
• Metaphase I


  Homologous pairs align randomly on the metaphase plate, leading to independent assortment
• Anaphase I


  Homologous chromosomes separate and move to opposite poles
• Telophase I


  Two haploid daughter cells are formed
• Meiosis II


  Similar to mitosis, no interphase II
• Prophase II, Metaphase II, Anaphase II, Telophase II

  Same as corresponding mitosis phases
• Four haploid daughter cells (gametes) are produced by the end of meiosis
• Non-disjunction


  Failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in abnormal chromosome numbers
• Common non-disjunction disorders


  • Down's Syndrome (Trisomy 21)
  • Turner's Syndrome (Monosomy 23 X)
  • Kleinfelter's Syndrome (Trisomy 23 XXY)
  • Edward's Syndrome (Trisomy 18)
• Amniocentesis


  Procedure to detect genetic disorders like non-disjunction
• Karyotype


  Picture of an individual's chromosomes, used to analyze amniocentesis results