Mutation

A mutation is any lasting change in the sequence of DNA, or of RNA in viruses whose genes are made of RNA. Mutations arise spontaneously from copying mistakes during DNA replication that escape repair, or are induced by mutagens such as ultraviolet light, X-rays and certain chemicals; some viruses can cause them too. They range from point mutations, which alter one base, through insertions and deletions of a few bases, to the movement of large segments between chromosomes.

The effect depends on where the change falls. A silent mutation changes a codon but not its amino acid, a missense mutation swaps one amino acid for another, and a nonsense mutation creates an early stop codon. Insertions or deletions can shift the reading frame. Most mutations have no noticeable effect, some are harmful and a few are helpful. Mutations in egg or sperm cells (germline mutations) can be inherited, while those in body cells (somatic mutations) affect only the cells descended from the changed one and can lead to cancer.

Mutation is the original source of new alleles, supplying the variation that natural selection acts on. Clinical geneticists now prefer the neutral word variant, because many DNA changes cause no disease.