(2) Meiosis and Genetic Variation

• there are two types of cell divisions in eukarytoic organisms :

  • mitosis
  • meiosis
• Mitosis = cell division that makes 2 geneticaly identical diploid daughter cells
• Meiosis = cell division that makes 4 genetically dissimilar haploid daughter cells/ gametes
• meiosis produces gametes in most animals
• gametes fuse together during fertilisation to form new offspring
• as gametes need to fuse together during fertilisation, they must be haploids as they need to fuse together to form diploid cells
• In Fern :

  • gametes are produced by mitosis
  • produces spores by meiosis
• haploid + haploid -(fertilisation)-> diploid
• meiosis produces cells with genetic variation
• 2 ways meiosis creates variation within the genes :

  • independent assortment/ segregation of homologous pairs
  • crossing over of homologous pairs
• meiosis involves 2 nuclear divisions (1/2)
• 2n = diploid
• 1n = haploid
• 1. G1 - organelles replicate
  2. S - DNA replication
  3. G2 - check for DNA errors
  4. Mitosis - prophase, metaphase, anaphase, telophase
• Interphase :

  • chromosomes replicate - double
  • organelles double
  • DNA is checked
• MEIOSIS 1

  Prophase 1 :

  • chromosomes condense
  • homologous pairs form chiasmata
  • crossing over occurs
  • Nuclear membrane breaks down
  • centrosomes move to poles - spindle fibres start to form
• crossing over occurs in prophase 1
• Crossing over :

  • occurs when homologous pairs line up
  • chromatids become twisted around one another
  • causes portions of the chromatid to break off and exchange places on the homologous pair
  • recombination - results in variation
• MEIOSIS 1

  Metaphase 1 :

  • pairs of homologous chromosomes line up along the equator
  • the lining up occurs randomly
• Independent Assortment :

  • when the homologous chromosome pairs line up on the spindle, we cannot predict which gamete will end up with the maternal or paternal chromosome
• MEIOSIS 1

  Anaphase 1 :

  • spindle fibres shorten
  • the homologous pairs get pulled to opposite poles of the cells
• non-disjunction occurs in anaphase 1 of meiosis 1
• MEIOSIS 1

  Telophase 1 :

  • The nuclear membrane reforms
  • chromosomes reform back to chromatin state
• Meiosis 2 occurs straight after meiosis 1
• Prophase 1 vs Prophase 2 :

  • no cross over in P2
  • no interphasein P2
• in meiosis 2 no DNA replication occurs so haploid cell will be produced
• MEIOSIS 2

  Prophase 2 :

  • chromosomes condense
  • the nuclear membrane divides + breaks down
  • spindle fibres start to form
• MEIOSIS 2

  Metaphase 2 :

  • chromosomes line up at the equator
  • spindle fibres attach to each sister chromatids
• MEIOSIS 2

  Anaphase 2 :

  • sister chromatids are pulled apart as the spindle fibres shorten
  • centromeres - non disjunction occurs if centromere fails to divide
• MEIOSIS 2

  Telophase 2 :

  • chromatids reach he poles and become 'chromosome'
  • nuclear envelope reforms
  • Chromosomes recoils into chromatin
• Cytokinesis :

  The cell divide - 4 genetically dissimilar haploid daughter cells/ gametes form
• Prophase - phat (double in size)

  Metaphase - middle (line up in equator)

  Anaphase - apart (spindle fibres shorten)

  Telophase - two new nuclei
• Meiosis 1 = homologous pairs are separated
• Meiosis 2 = sister chromatids are seperated
• due to independent assortment, the same parent cell can produce different combinations of chromosomes in the daughter cells
• 2^n = number of genetically different gametes produced by independent assortment
• n = represents the number of homologous pairs
• 2^23 = 8388608
• (2^n)^2 = takes into account the probability of random fusion of gametes
• recombination = will increase the variation of gametes