2.14—DNA replication errors and mutations

A mutation is a change in a DNA base sequence. Its effect depends on the kind of change and on how that altered sequence is translated into a polypeptide; many mutations have little or no observable effect.

Insertion or deletion usually shifts the reading frame, changing every downstream triplet. A substitution changes one base triplet and may be silent, missense or nonsense. Duplication can provide an extra gene copy, while inversion reverses a DNA segment.

A one-base insertion before the second codon changes all later triplets and can produce a non-functional protein. A substitution may instead leave the amino acid unchanged because the genetic code is degenerate, or create a premature stop codon.

A mutation is not automatically harmful, and a changed amino acid is not automatically fatal. The consequence depends on the codon, the protein region, expression and the resulting phenotype.