meiosis

• meiosis


  type of cell division that gives rise to genetically dissimilar cells/haploid gametes in which the chromosome number is halved due to separation of homologous chromosomes
• meiosis = the reductive division 


  • in sexual reproduction, 2 haploid sex cells fuse , to form a diploid zygote, which will grow and develop into a new individual
  • meiosis produces haploid gamete, which keeps the chromosome number from doubling each time sexual reproduction occurs
• Homologous pair of chromosomes


  • matching pairss of chromosomes that can possess different versions of the same genes/alleles
  • one member of the pair comes froom the mother and the other from the father
  • are maintained by the exact replication that atkes place prior to each mitotic division
• Prophase I

  Synapsis = pairing of homologous chromosomes

  Crossing over between non-sister chromatids resulting in new combinations of alleles on the chromosomes

  chiasmata = point of the join between non-sister crhomatids
• crossing over & chiasmata (prophase I)

  1. Homologous chromosomes commence pairing as they continue to shorten and thicken by coiling
  2. Breakages cccur in parallel non-sister chromatids at identical points
  3. rejoining of non-sister cchromatids forms chiasmata , resulting in new combination of genes on the chromosomes
  4. Positions of chiasmata become visible later, as tight pairing of homologous chromosomes ends. Chiasmata indicate where the crossing over has occured
  5. once pairs pf homologous chromosomes have separated, the sites of crossing over are not apparent
• Metaphase I

  • spindle forms
  • homologous pairs become attached to individual microtubules of the spindle by thier centromeres
  • homoloogous pairs are arranged at the equator of the spindle
• Anaphase I

  • chromosomes of each homologous pair move to oppsoite poles of the spindle, but with the individual chromatids remaining attached by their centromeres
  • separation of the homologus pairs of chromosomes NOT the sister crhomatids
• Telophase I

  • homologous chromsomes arrived at opposite poles
  • chromosomes tend to uncoil
  • nuclear envelope reforms around both nuclei
  • spindle breaks down
  • the 2 cells continue into meiosis II directly

  Two haploid cells form; chromsomes are still doubled
• Prophase II

  • nuclear envelope breaks down again
  • chromosomes shorten and re-thicken by coiling
  • centrioles (in animal cells only) duplicate and move to oppsite poles of the cell
  • spindle apparatus re-formed at right angles to original spindle
• Metaphase II


  chromosomes line up at the equator of the spindle, attached by their centromeres
• Anaphase II

  • centromeres divide
  • sister chromatids separate and move to oppsite poles of the spindle, with the centromeres first
• Telophase II

  • nuclear envelope forms around the four groups of sister chromatids so four nuclei are formed = four cells, each with half the chromosome number of the original parent cell
  • chromsomes uncoil and become dispersed as crhomatin
  • nucleoli reform

  haploid daughter cells are formed
• Cytokinesis in animals cells (cytoplasm division)

  • cell organelles become evenly distributed between daughter cells
  • in-tucking of the cell surface membrane at the equator of the spindle, 'pinching' the cytoplasm in half
  • occurs by a process known as cleavage
  • first sign of cleavage is the appreance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate
  • cleavage furrow is deepened until the parent cell is pinched into 2, producing 2 completely separated cells
• cytokinesis in plant cells

  • golgi apparatus forms vesicle of new cell wall materials, which collect along the line of the equator of the spindle (cell plate)
  • vesicles will merge, forming the new cell surface membranes and the cellulose cell wall between the 2 cells