Types of Mutation

Mutations can be classified along several independent dimensions simultaneously, and a single mutation is usually described using more than one of these categories at once. By origin, a mutation is spontaneous if it arises without any identifiable mutagen, or induced if it results from deliberate or incidental exposure to a known mutagen. By the cell type affected, a mutation is somatic if it occurs in a non-reproductive cell (and so is not passed to offspring) or germ-line if it occurs in a reproductive cell (and so can be inherited). By its effect on gene function, a mutation can be loss-of-function (eliminating the gene's normal activity entirely, also called a null or knockout mutation), hypomorphic (merely reducing, or 'leaking', some of the normal activity), hypermorphic (increasing the normal activity beyond its usual level), or gain-of-function (causing the gene to be expressed at an incorrect time or in the wrong cells altogether, sometimes called ectopic expression). At the molecular level, a point mutation is either a nucleotide substitution, which is further split into a transition (a purine replaced by the other purine, A to G, or a pyrimidine replaced by the other pyrimidine, T to C) or a transversion (a purine replaced by a pyrimidine, or vice versa, such as A to T or C to G), or it is an insertion (one or more extra nucleotides added) or a deletion (one or more nucleotides lost). Finally, by its effect once the mRNA is translated, a mutation is silent (synonymous) if the new codon still specifies the identical amino acid, missense (non-synonymous) if the new codon specifies a different amino acid, nonsense if the new codon becomes on …