Meiosis
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Transcript

  • 1. Meiosis
  • 2. Essential terms
    • Gametogenesis
    • Synapsis
    • Crossing over
    • Disjunction / Non disjunction
    • Chiasmata
  • 3. Key points
    • Mitosis – diploid. Identical. Stability.
    • Meiosis – haploid. Combination. Variation.
    • Sexual reproduction ensures genetic continuity and genetic variety, producing offspring's that often differ greatly from patients.
    • Associated with gametogenesis.
    • Counterbalances fertilization - makes sure # of chromosomes remains constant
  • 4. Meiosis
    • Purpose :
    • production of sex cells (gametes)
    • Outcome
    • 2 divisions
    • 4 daughter cells with 1/2 (haploid) sets of chromosomes
  • 5.
    • 1. Chromosomes found in homologous pairs
      • identical but may have varying gene messages
    • b. Humans have 46 (23 pairs)
      • 22 homologous pairs (autosomes)
      • X and Y (23rd pair) sex chromosomes
      • Females (XX); Males (XY)
    • c. Pairs separate during reproduction
      • Offspring receives info from each parent
      • Exchanging genetic material during crossing over (Prophase I)
    • d. Separated during meiosis (Anaphase I)
  • 6.
    • Homologous chromosomes form pairs in form of tetrads (4 chromatids)
    • Involves 2 divisions = Meiosis I and II
  • 7. Interphase
    • chromosomes replicate
    • Consists of 2 identical sister chromatids attached at their centromeres
    • centriole pairs also replicate into two pairs
  • 8. Prophase I
    • Condenstaion. Chromatin thickens and coils. Become visible .
    • Synapsis occurs Homologous chr. pair up forming tetrads.
    • Sister chromatids are attached at centromeres.
    • Each pair = bivalent.
    • Nonsister chromatids features crossing over resulting in chiasmata. = More than 1 can form = genetic variation)
    • • spindle forms from microtubules
    • • nuclear envelope/nucleoli disperse
    • 5 stages: Leptonema, zygonema, pachynema, diplonema and diakenesis.
  • 9. Metaphase I
    • Chromosomes have thickened.
    • Each tetrad interacts with spindle fibers.
    • Movement to the equitorial plate.
    • Homologues are destined to separate towards opposite poles
  • 10. Anaphase I
    • sister chromatids remain attached while homologues move towards the opposite pole. (Mitosis =sister chromatids are moved apart)
    • One half of each tetrad ( one pair of sister chromatid) is pulled toward each pole of the dividing cell.
    • No attraction of the sister chromatids – reduction in the number of chromosomes.
  • 11. Telophase I + Cytokinesis
    • Very short phase compared to mitosis
    • May not always occurs
    • Each pole now has a haploid set of chromosomes composed of two sister chromatids attached at the centromere
    • cytokinesis occurs producing two daughter cells
    • Nuclear membrane forms.
    • No interphase – No DNA replictaion. Already 2 chromatids.
  • 12. 2nd division (II)
    • Prophase II = spindle apparatus forms
    • Chromosomes move towards the metaphase II plate
    • Metaphase II = chromosomes align on metaphase plate
    • kinetochores of sister chromatids point towards opposite poles
  • 13.
    • Anaphase II = centromeres of sister chromatids separate
    • sister chromatids are pulled towards opposite poles
    • Telophase II = I member of each homologue pair is present at each pole.
    • Each chromosome is referred to as monads.
  • 14. Cytokinesis
    • 4 haploid gametes results.
    • Haploid state has been achieved.
    • If crossing over has occurred = each monad is a combination of maternal and paternal genetic information ie. Receives info from grandparents. = increases genetic variation.
  • 15.  
  • 16. Independent assortment
    • Mechanism that allows for genetic variation.
    • Reduction of diploid – diploid means that each gamete will only carry one form of gene for a particular characteristics.
    • Crossing over results in the exchange of genetic info from maternal to paternal chromosomes = possibility of new combination of genes.
    • Random assortment = mixture of maternal and paternal chromosomes = more combination.
  • 17. Nondisjunction and Translocation
    • Too few or too many chromosomes in gametes
    • Translocation: attachment of all or part of chromosome to another chromosome
    • Nondisjunction: failure of chromosomes to separate
    • Down Syndrome
    • 1. Translocation of chromosome #15 to #21, or
    • 2. Nondisjunction with extra #21
    • Nondisjunction of sex chromosomes
    • 1. XO- Turner Syndrome; sterile female
    • 2. XXY- Kleinfelter Syndrome; sterile male
    • 3. XXX- Metafemale; limited fertility