Mutations arise spontaneously at low frequency owing to the chemical instability of purine and pyrimidine bases and to errors during DNA replication. Natural exposure of an organism to certain environmental factors, such as ultraviolet light and chemical carcinogens (e.g., aflatoxin B1), also can cause mutations.
A gene mutation is a permanent change in the DNA sequence of a gene. Mutations can occur in a single base pair or in a large segment of a chromosome and even span multiple genes. Mutations can result from endogenous (occurring during DNA replication) or exogenous (environmental) factors.
