Mutations in DNA

  • Severity of damage: silent << missense < nonsense < frameshift.
  • For point (silent, missense, and nonsense) mutations:
    • Transition—purine to purine (eg, A to G) or pyrimidine to pyrimidine (eg, C to T).
    • Transversion—purine to pyrimidine (eg, A to T) or pyrimidine to purine (eg, C to G).
  • Silent
    • Nucleotide substitution but codes for same (synonymous) amino acid; often base change in 3rd position of codon (tRNA wobble).
  • Missense
    • Nucleotide substitution resulting in changed amino acid (called conservative if new amino acid is similar in chemical structure).
    • Sickle cell disease (substitution of glutamic acid with valine).
  • Nonsense
    • Nucleotide substitution resulting in early stop codon (UAG, UAA, UGA). Usually results in nonfunctional protein.
    • Stop the nonsense!
  • Nonstop
    • Base substitution within stop code results in continued translation.
  • Frameshift
    • Deletion or insertion of a number of nucleotides not divisible by 3, resulting in misreading of all nucleotides downstream. Protein may be shorter or longer, and its function may be disrupted or altered.
    • Duchenne muscular dystrophy, Tay-Sachs disease.
  • Splice site
    • Mutation at a splice site retained intron in the mRNA protein with impaired or altered function.
    • Rare cause of cancers, dementia, epilepsy, some types of β-thalassemia.
  • Conservative
    • Base substitution codes for different amino acid with similar chemical structure

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