meiosis and mitosis

• Mitosis is the process by which cells divide to produce two genetically identical daughter cells.
• Prophase

  • Chromosomes condense and are now visible when stained
  • The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
  • The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
  • Spindle fibres (protein microtubules) begin to emerge from the centrosomes (which consist of two centrioles in animal cells)
  • The nuclear envelope (nuclear membrane) breaks down into small vesicles
• Metaphase

  • Centrosomes reach opposite poles
  • Spindle fibres (protein microtubules) continue to extend from centrosomes
  • Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
  • Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
  • Each sister chromatid is attached to a spindle fibre originating from opposite poles
• Anaphase

  • The sister chromatids separate at the centromere (the centromere divides in two)
  • Spindle fibres (protein microtubules) begin to shorten
  • The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)
• Telophase

  • Chromosomes arrive at opposite poles and begin to decondense
  • Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
  • The spindle fibres break down
• cytokinesis is the division of the cytoplasm into two daughter cells
• meiosis is a type of cell division that produces four haploid daughter cells from one diploid parent cell
• • Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during meiosis I
• • The different combinations of chromosomes in daughter cells increases genetic variation between gametes
• • In meiosis I, homologous chromosomes pair up randomly and are pulled towards the equator of the spindle
  • The alignment of one homologous pair is unaffected by the alignment of any other pair
  • The homologous chromosomes are then separated and pulled apart to different poles
  • The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
• crossing over is the process by which non-sister chromatids exchange alleles
• • Process of crossing over:
  • During meiosis I homologous chromosomes pair up and are in very close proximity to each other
  • The paired chromosomes are known as bivalents
  • The non-sister chromatids can cross over and get entangled
  • These crossing points are called chiasmata
  • The entanglement places stress on the DNA molecules
  • As a result of this, a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
• • This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
  • There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
  • Crossing over is more likely to occur further down the chromosome away from the centromere