Glossary

A point mutation is a mutation that only affects a single nucleotide of nucleic acid. Point mutations most commonly involve the substitution of one base for another (which changes the complementary base as well in DNA). The term point mutationt also includes insertions or deletions of a single base pair. Point mutations rarely cause a huge change in the underlying organism, which is fortunate as they are by far the most common type of mutation.

Single base substitutions can be divided into transitions and transversions based on whether or not they change the nucleobase's structure. Transitions, which change a nucleotide from a purine to purine or pyrimidine to pyrimidine, occur far more often than transversions, especially within coding regions of the genome.

When a point mutation does occur in a coding region, it can be assigned to one of the following categories, according to the consequence of the mutation (see the figure below for an illustration of different mutations applied to $\textrm{TTC}$):

• Silent mutation: does not change the corresponding amino acid, and thus has no measurable effect on the genome.
• Missense mutation: changes the codon causing the amino acid change. If the properties of amino acid (polarity, charge etc.) remain the same, then the mutation is called conservative, in which case it may not affect the protein's function. Otherwise, the mutation is called non-conservative, and it can lead to the loss of protein function and result in disease.
• Nonsense mutation: exchanges a normal amino acid codon for a stop codon, which results in the protein's truncation. Usually this shortening also leads to the loss of the protein's function.