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Type of DNA Mutations

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Topic updated on 11/08/17 7:35am

Silent 
  • Exchange of one base for another results in same amino acid
    • often an alteration in 3rd position of codon
      • tRNA wobble
  • No change in protein function
Missense 
  • Exchange of one base for another results in changed amino acid 
    • transversion → exchanges a purine to a pyrimidine or a pyrimidine to a purine
      • e.g., C → A or G → T
    • transition → exchanges a purine for another purine or a pyrimidine to another pyrimidine
      • e.g., A → G or C → T
  • Variable change in protein function
    • if the new amino acid is similar to old (leu → ile) the protein will most likely function the same
    • if the new amino acid is different (glu → val) the protein folding/stability will likely be affected
  • e.g., sickle-cell anemia
    • glu → val mutation in β-globin gene
Nonsense 
  • Exchange of one base for another results in a stop codon
  • Loss of function mutation as peptide is truncated
Frameshift 
  • Deletion or addition of 1 or 2 bases resulting in misreading of all nucleotides downstream
  • Loss of function mutation as peptide is completely different
Large Segment Deletion
  • Unequal crossover at meiosis results in loss of large segment of DNA
  • Loss of function mutation
  • e.g., α-thalassemia 
    • deletion of α-globin gene 
Change at splice site
  • Alteration in base sequence at mRNA splicing site results in altered splicing
    • can remove parts of exon
    • can leave parts of intron
  • Variable effect on protein function as number of spliced amino acids varies
  • e.g., β-thalassemia
Triplet repeat expansion
  • Expansion of short nucleotide sequence results in longer polypeptide
    • can be in coding or noncoding region
  • Addition of amino acids affects protein structure/folding and affects function
  • Disease display anticipation
    • earlier disease onset in successive generations
  • e.g., myotonic dystrophy, Huntington's disease, and Fragile X
In-frame 
  • In-frame mutations 
 


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(M1.BC.79) An 8-year-old girl is brought to the pediatrician because she is significantly shorter than her classmates. Her mother notes that she has had thick, oral secretions for the past several months, along with a chronic cough. Her exam is notable for clubbed fingernails. Her pediatrician sends a genetic test for a transmembrane channel mutation, which shows a normal DNA sequence, except for the deletion of three nucleotides that code for a phenylalanine at position 508. What type of mutation has caused her presentation? Topic Review Topic

1. Frameshift mutation
2. In-frame mutation
3. Nonsense mutation
4. Triplet expansion
5. Silent mutation

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