Types of Mutations

Point mutations are changes to one base in the DNA code and may involve either:

  • The substitution of a base (e.g. ATG becomes ACG)
  • The insertion of a base (e.g. ATG becomes ATCG)
  • The deletion of a base (e.g. ATG becomes AG)
  • The inversion of bases (e.g. ATG becomes AGT)

Base substitutions may create either silent, missense or nonsense mutations, while insertions and deletions cause frameshift mutations

Effects of Point Mutations

Silent mutations occur when the DNA change does not alter the amino acid sequence of the polypeptide

  • This is possible because the genetic code is degenerate and certain codons may code for the same amino acid

Missense mutations occur when the DNA change alters a single amino acid in the polypeptide chain

  • Sickle cell anaemia is an example of a disease caused by a single base substitution mutation (GAG → GTG ; Glu → Val)

Nonsense mutations occur when the DNA change creates a premature STOP codon which truncates the polypeptide

  • Cystic fibrosis is an example of a disease which can result from a nonsense mutation (this may not be the only cause though)

Frameshift mutations occur when the addition or removal of a base alters the reading frame of the gene

  • This change will affect every codon beyond the point of mutation and thus may dramatically change amino acid sequence

Summary of the Different Types of Point Mutations