Mutation

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In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA.^[1] Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitosis, or meiosis or other types of damage to DNA (such as pyrimidine dimers caused by exposure to ultraviolet radiation), which then may undergo error-prone repair (especially microhomology-mediated end joining),^[2] cause an error during other forms of repair,^[3][4] or cause an error during replication (translesion synthesis). Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements.^[5][6][7]

Mutations may or may not produce detectable changes in the observable characteristics (phenotype) of an organism. Mutations play a part in both normal and abnormal biological processes including: evolution, cancer, and the development of the immune system, including junctional diversity. Mutation is the ultimate source of all genetic variation, providing the raw material on which evolutionary forces such as natural selection can act.

Mutation can result in many different types of change in sequences. Mutations in genes can have no effect, alter the product of a gene, or prevent the gene from functioning properly or completely. Mutations can also occur in non-genic regions. A 2007 study on genetic variations between different species of Drosophila suggested that, if a mutation changes a protein produced by a gene, the result is likely to be harmful, with an estimated 70% of amino acid polymorphisms that have damaging effects, and the remainder being either neutral or marginally beneficial.^[8]

Mutation and DNA damage are the two major types of errors that occur in DNA, but they are fundamentally different. DNA damage is a physical alteration in the DNA structure, such as a single or double strand break, a modified guanosine residue in DNA such as 8-hydroxydeoxyguanosine, or a polycyclic aromatic hydrocarbon adduct. DNA damages can be recognized by enzymes, and therefore can be correctly repaired using the complementary undamaged strand in DNA as a template or an undamaged sequence in a homologous chromosome if it is available. If DNA damage remains in a cell, transcription of a gene may be prevented and thus translation into a protein may also be blocked. DNA replication may also be blocked and/or the cell may die. In contrast to a DNA damage, a mutation is an alteration of the base sequence of the DNA. Ordinarily, a mutation cannot be recognized by enzymes once the base change is present in both DNA strands, and thus a mutation is not ordinarily repaired. At the cellular level, mutations can alter protein function and regulation. Unlike DNA damages, mutations are replicated when the cell replicates. At the level of cell populations, cells with mutations will increase or decrease in frequency according to the effects of the mutations on the ability of the cell to survive and reproduce. Although distinctly different from each other, DNA damages and mutations are related because DNA damages often cause errors of DNA synthesis during replication or repair and these errors are a major source of mutation.^[9]