Concept 28 Some types of mutations are automatically repaired.

Up until the 1950s, most biologists thought that genes were stable units. The notion that DNA could be damaged and then repaired came from researchers who were trying to explain the odd behavior of their microbes. Cultures that were seemingly killed by exposure to ultraviolet light would recover after sitting by a window, and mutants would curiously pop up long after exposure to a mutagen.

Investigations of organisms from bacteria to humans have uncovered an army of enzymes poised to repair damage from environmental mutagens or errors in DNA replication. Without these enzymes, DNA damage would cause intolerable levels of mutation. Diseases caused by defective repair enzymes shorten life span, illustrating the central role of DNA repair in survival. But the occasional failure to repair DNA or correct errors in replication is also central to survival because these anomalies exist as mutations, and without mutations evolution would be impossible.