A mutation is a change in the genetic material of a cell. There are two broad types:• gene mutations: a change in the base sequence of DNA within a gene
• chromosome mutations: a change in the structure or the number of chromosomesGene mutations mostly arise spontaneously during DNA replication, when a wrong base is inserted or bases are missed or added. The natural rate is low, but it is increased by mutagens, factors that damage DNA or interfere with copying:
• ionising radiation, such as X-rays and gamma rays, and ultraviolet light
• chemicals, such as benzene and substances in tobacco smoke
• some virusesBecause a mutation changes the base sequence, it can change the mRNA, the amino acid sequence and so the shape and function of a protein. Its effect may be:
• harmful: for example a non-functional enzyme, or uncontrolled cell division (cancer)
• beneficial: a new allele that gives an advantage, the raw material of evolution
• neutral: no effect on the phenotypeOnly mutations in cells that form gametes can be passed to offspring. A mutation in a body cell (for example a skin cell) affects that individual only.Worked example: a mutation in a lung cell of a smoker is not inherited by their children, because lung cells do not make gametes.
• chromosome mutations: a change in the structure or the number of chromosomesGene mutations mostly arise spontaneously during DNA replication, when a wrong base is inserted or bases are missed or added. The natural rate is low, but it is increased by mutagens, factors that damage DNA or interfere with copying:
• ionising radiation, such as X-rays and gamma rays, and ultraviolet light
• chemicals, such as benzene and substances in tobacco smoke
• some virusesBecause a mutation changes the base sequence, it can change the mRNA, the amino acid sequence and so the shape and function of a protein. Its effect may be:
• harmful: for example a non-functional enzyme, or uncontrolled cell division (cancer)
• beneficial: a new allele that gives an advantage, the raw material of evolution
• neutral: no effect on the phenotypeOnly mutations in cells that form gametes can be passed to offspring. A mutation in a body cell (for example a skin cell) affects that individual only.Worked example: a mutation in a lung cell of a smoker is not inherited by their children, because lung cells do not make gametes.