substitution

A substitution is a mutation in which one DNA base is replaced by a different base, like swapping a single letter in a word for another without changing the word's length. The sequence stays the same length; only the identity of one rung on the DNA ladder changes.

Geneticists distinguish two chemical classes. A transition swaps a base for another of the same shape family — a purine for a purine (A to G) or a pyrimidine for a pyrimidine (C to T). A transversion swaps across the families, a purine for a pyrimidine or the reverse. Transitions are generally more common than transversions because of how the chemistry of damage and copying errors plays out.

Because substitutions preserve the reading frame, their effect depends on which codon is hit. The new triplet may still specify the same amino acid (silent), specify a different amino acid (missense), or specify a stop (nonsense). This is why a substitution can range from completely invisible to seriously disruptive.