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Single gene mutations involve the alteration of a DNA nucleotide. This can happen as a result of one of the following:

• the substitution of one nucleotide with another
• the insertion of one or more nucleotides into a DNA sequence
• the deletion of one or more nucleotides from a sequence

Point mutations

Substitutions result in a change that only affect one codon. This is known as a point mutation. The resulting mutation can be one of three types:

• Missense – these substitutions change the codon so that a different amino acid is produced. This may result in the shape of the protein changing so that it cannot carry out its function. However, some changes in amino acid may not have any significant effect.
• Nonsense – these substitutions change the codon from an amino acid to a stop codon. This results in a shorter protein being produced. The shortened protein is generally non-functional or its function is affected.
• Splice-site – these substitutions affect the boundaries between exons and introns (splice sites). A mutation here can prevent splicing at that site. This will result in a very different protein being made due to the post transcriptional processing as some introns may be included or some exons removed.

Frame-shift mutation

Insertions and deletions result in what are called frame-shift mutations.

This means that when the groups of three nucleotides (codons) are read at the ribosome, the codon reading frame is shifted by one codon. Completely different amino acids are coded for from this mutation onwards.

The resulting protein will be significantly altered and is usually non-functional.