What is a gene mutation?

Gene mutations are changes in the nucleotide sequence of DNA, which can result from errors during DNA replication or damage caused by factors like radiation or carcinogens. There are three types of mutations - substitution, deletion and insertion.

Substitution mutations

  • This occurs when one base in the DNA sequence is replaced by another.
  • For example, if the original sequence is CAA TTT GAA CCC, a substitution could change it to CAA TAT GAA CCC. This is known as a mis-sense mutation.
  • However, a substitution may not always change the amino acid sequence because the genetic code is degenerate, meaning that multiple triplets can code for the same amino acid. For example, TTT and TTC code for lysine.
  • If the mutation results in a STOP codon, a shortened polypeptide will be formed. This is known as a nonsense mutation.
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Insertion mutations

  • This occurs when an extra base is added into the DNA sequence.
  • A frame-shift will occur, shifting every nucleotide base forward by one place.
  • The polypeptide sequence will be changed significantly, causing the tertiary and quaternary shape to be different. 
  • The resulting enzyme or protein will be non-functional.
  • If three bases are added, one amino acid is added to the polypeptide chain but the sequence remains unchanged other than that.
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Deletion mutations

  • This involves the loss of a base from the DNA sequence.
  • A frame-shift will occur, shifting every nucleotide base backwards by one place.
  • The polypeptide sequence will be changed significantly, causing the tertiary and quaternary shape to be different. 
  • A non-functional protein or enzyme is produced as a result. 
  • If three bases are deleted, one amino acid is removed from the polypeptide chain but the sequence remains unchanged other than that.
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