A mutation is an alteration in the genetic material of a cell that is transmitted to the cell's offspring. Mutations may be spontaneous (the result of accidents in the replication of genetic material) or induced by external factors (e.g., electromagnetic radiation and certain chemicals). Mutations take place in the genes, which are found in the long, chainlike molecules of deoxyribonucleic acid (DNA). The backbone of the DNA chain is composed of alternating sugar and phosphate groups. Protruding from each sugar group is one of four nitrogenous bases: adenine (A), guanine (G), thymine (T), or cytosine (C). Structurally, each DNA molecule consists of two entwined chains, linked together by bonds b