Meiosis

• Nondisjunction: The failure of homologous chromosomes to separate properly during meiosis (leads to disorders like Edwards, Klinefelter's and Downs syndrome)
• Keys of meiosis,

  • produces sperm and egg cells (Gametes)
  • produces 4 genetically different haploid cells (n=23)
  • goes through PMAT twice
  • used for creating sex cells
• Keys of mitosis,

  • produces body cells (somatic cells)
  • produces 2 genetically identical diploid cells (2n=46)
  • goes through PMAT once
  • used for cell reproduction
• Chromatin: unwound DNA Present during interphase (during prophase becomes condensed chromosomes)
• Interphase I,

  • DNA is in form of chromatin
  • DNA & centrioles replicate (2n)
  • Nucleus & nucleolus are visible
• Prophase I:

  • longest & most complex phase
  • Chromosomes condense
  • nuclear membrane begins to dissolve
  • centrioles split and begin moving to poles
  • spindle fibers form
  • chromosomes form tetrads (pairs of homologus chromosomes so 4 in total)
  • chromosomes may crossover & exchange genetic material (creating variation)
• Metaphase I,

  • shortest phase
  • spindle fibers attach to centromeres of homologous pairs
  • tetrads line up along equator
• Anaphase I:

  • tetrads are pulled apart to opposite ends (segregation)
• Telophase I:

  • nuclear membrane reforms
  • each pole now has a diploid number of chromosomes (2n)
• Meiosis II,

  • (no interphase II & no replication)
  • similar to mitosis ends with cytokenisis producing 4 different haploid cells (n=23)
• Spermatogenesis: male reproductive division
• Oogenesis: female reproductive division
• Abnormalities during meiosis cause genetic disorders like non disjunction
• Karyotypes: A diagram that shows the number of chromosomes in a cell.

  • pairs 1-22: homologous autosomes (somatic body chromosomes)
  • pair 23: sex chromosomes & contain genes that code for sexual characteristics
• Female karyotypes (XX): homologous pair 2 sets of genes for every trait
• Male karyotype (XY): non-homologous (no pair), each one contains different genes only 1 copy of genes
• sex determination: all eggs carry one chromosome, but the sperm carries two, so males determine gender of offspring
• During oogenesis 1 viable egg is produces & 3 usless polar bodies which are reabsorbed into bodu
• During spermatogenesis 4 sperm are produced