Types of mutations

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  • Types of mutations
    • Missense mutation
      • new nucleotide alters the codon so as to produce a altered amino acid in the protein product
        • Sickle cell anaemia
          • base 17 of beta-chain of haemoglobin is mutated from A to T
          • codon 6 changes form GAG to GTG resulting in amino acid valine instead of glutamic acid
    • Point mutation
      • where a single base becomes replaced by another
      • Transition
        • one purine or pyrimidine is replaced by the other
          • A is replaced by G or C is replaced by T
      • Transversion
        • a purine is replaced by a pyrimidine or visa versa
          • A is replaced by C or T etc.
    • Nonsense mutation
      • new nucleotide changes a codon that specified an amino acid to one of the STOP codons(TAA, TAG or TGA)
        • CF
          • most common mutation is delta F508 or G542X
      • the earlier in the gene that this occurs, the more truncated the protein product and the more likely that it will be unable to function
    • Silent mutation
      • most amino acids are encoded by several different codons so if one is changed, the same amino acid is still produced
    • Splice site mutations
      • one or more introns remain in the mature mRNA and can disrupt the protein product
        • PKU
          • single base change, G to A, at the 5' splice donor site of intron 12 in the phenylalanine hydroxylase gene
          • splice machinery (enzymes) does not see the intron 12 donor site and uses intron 11 donor site instead
    • Frameshift
      • Indels cause frameshift. Often create new stop codons, resulting in a nonsense mutation
      • Indels of 3 nucleotides or mutiples of three may be less serious due to reading frame being preserved
        • Trinucleotide repeat diseases e.g. Huntington's disease or Fragile X syndrome
    • Indels
      • Insertion
        • between one and several thousand base pairs may be added to DNA sequence of gene
      • Deletion
        • between one and several thousand base pairs may be removed from DNA sequence of gene

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