Change in a single nucleotide of .
Point mutations can be silent mutations, missense mutations, nonsense mutations, , etc. See also and , and note the potential for contrast with the concept of substitution.
Often single-nucleotide substitutions are solely described as point mutations and then contrasted with insertions and deletions, even if the latter involve only a single base. The latter, but not base substitions, can give rise to frameshift mutations.
Point mutations can be selectively , neutral, or beneficial, as listed in order of decreasing likelihood. See also ().
For point mutations that are , only three possible point mutations are possible. For an entire gene, the total number of single-base substitutions that are possible therefore is three times the total number of nucleotides making up that gene. The likelihood of a specific point mutation – such as A → G occurring in a specific location in genome – therefore is roughly equal to the mutation rate as measured for the entire genome (and as determined for point mutations) divided by three times the number of nucleotides making up the genome.
I use the word "roughly" immediately above because mutation rates are not constant across genomes and different mutations differ in likelihood. (pyrimidine to pyrimidine mutations, i.e., T → C and C → T, or purine to purine mutations, i.e., G → A and A → G), for example, are more likely than (purine to pyrimidine or pyrimidine to purine mutations).
For an entire genome, the is equal to the size of the genome in nucleotides times three. To these numbers we can then add one insertion per existing nucleotide as well as one deletion, for a total of five possible point mutations for every nucleotide present.