meiosis and genetic variation

• gene mutations are any change to one or more nucleotide bases or changes to the sequence of bases in dna
• gene mutations can occur spontaneously and the frequency in occurrence is increased by mutagenic agents
• the order of bases within a gene determines the amino acid sequence of a protein so mutations result in an altered primary structure - which determines later stages of folding (change to the overall 3D tertiary structure ) = result in non-functioning protein
• substitution mutation is when one nucleotide base is replaced by another nucleotide base - number of bases remains the same and there is no frame shift

  
• consequences of substitution mutation:

  1. formation of one of the three stop codons = polypeptide chain being prematurely stopped
  2. formation of the codon for a different amino acid = structure of polypeptide differ in a single amino acid (sickle cell anaemia)
  3. formation of a different codon for the same amino acid = genetic code is degenerate so mutations have no effect
• deletion mutation is when one or more bases is removed changing the codon at the point of mutation and all following codons, resulting in a frameshift to the left

  altered codons potentially code for a different amino acid sequence resulting in non-functional protein

  
• non-disjunction:

  • chromosomes in meiosis 1 do not seperate equally
  • chromatids in meiosis 2 do not seperate equally
• chromosome non-disjunction can occur in 2 forms:

  1. changes in the whole sets of chromosomes = 3 or more sets of chromosomes rather than usual 2 (diploid)
  2. changes in the number of individual chromosomes = one additional or one too few chromosomes (downs syndrome)
• meiosis produces 4 genetically different haploid daughter cells through 2 nuclear divisions
• independent segregation is way in which the chromosomes align themselves on the spindle fibre on equator is completely random, resulting in a huge number of possibilities of chromosomal combinations in the gametes
• crossing over:

  1. occurs in meiosis 1 when homologous pairs line up and become twisted around each other
  2. puts tension on chromatids to break at chiasmata.
  3. broken parts of the chromatid recombine with another chromatid
  4. forming chromosomes with different combinations of alleles
• increases genetic variation:

  • crossing over
  • independent segregation
  • random fertilisation
• meiosis: mitosis:

  • 2 nuclear divisions - 1 nuclear divisions
  • haploid cell - diploid cells
  • genetic variation - create genetically identical cells
• stages in meosis:

  MEIOSIS 1 = homologous chromosomes pair up whereby crossing over takes place. The cell then divides whereby each daughter cell contains one chromosome from each homologous pair.

  MEIOSIS 2 = chromatids move apart producing 4 haploid daughter cells
• crossing over

  A) 1

  
• independent segregation

  A) assortment

  B) segregation

  
• meiosis:

  1. DNA replication
  2. two divisions
  3. separation of homologous chromosomes at meiosis 1
  4. separation of sister chromatids at meiosis 2
  5. produces 4 genetically different haploid cells