What Is a Genetic Mutation?

A mutation is any alteration in the sequence of DNA, the molecule that carries genetic instructions. Mutations can range from a change in a single "letter" of the genetic code (a point mutation) to the insertion, deletion, or rearrangement of large sections of DNA. They arise in several ways: as random errors when DNA is copied during cell division, or as damage caused by environmental factors called mutagens — such as ultraviolet light, radiation, tobacco smoke, and certain chemicals. Most mutations are neutral, having no noticeable effect. Some are harmful, causing diseases like cancer or inherited disorders such as cystic fibrosis and sickle cell anaemia. But occasionally a mutation is beneficial, giving an organism a survival or reproductive advantage — for example, a mutation that lets bacteria resist antibiotics, or one that gave some humans the ability to digest milk as adults. When beneficial mutations are passed on and spread through a population over generations, they drive evolution. Mutations are therefore both a source of disease and the raw material of life diversity.