MutationFor mutation in linguistic, see sandhi.
Mutations (or mutagenesis, both words originating in the Latin word mutare, to change) are permanent, transmissible changes to the genetic material (usually DNA or RNA) of an organism. Mutations can be caused by copying errors in the genetic material during cell division and by exposure to radiation, chemicals, or viruses. Mutations often lead to the malfunction or death of a cell and can cause cancer in higher organisms. Mutations are considered the driving force of evolution, where less favorable mutations are removed by natural selection, but favorable ones tend to accumulate. Neutral mutations do not affect the organism and can accumulate over time, which might result in what is known as Punctuated Equilibrium; a modern variation on classic evolutionary theory.
Two classes of mutations are spontaneous mutations (often called background level) and induced mutations caused by mutagens.
Three basic types of mutations are:
- Point mutations are usually caused by chemicals or malfunction of DNA replication and exchange a single nucleotide for another. Most common is the transition that exchanges a purine for a purine or a pyrimidine for a pyrimidine (C <-> T, A <-> G). A transition can be caused by nitrous acid, base mispairing, or mutagenic base analogs such as bromouracil. Less common is a transversion, which exchanges a purine for a pyrimidine or a pyrimidine for a purine. A point mutation can be reversed by another point mutation, in which the nucleotide is changed back to its original state (true reversion) or by second-site reversion (a complementary mutation elsewhere that results in regained gene functionality). Point mutations are called missense or nonsense mutations, depending on whether the erroneous codon codes for an amino acid or a stop.
- Insertions add one or more extra nucleotides into the DNA. They are usually caused by transposable elements. If the number of nucleotides inserted is not a multiple of three, the result is a framing error. Insertions can be reverted by deletions.
- Deletions remove one or more nucleotides from the DNA. They are irreversible.
- Keto <-> Enol
- Amino <-> Imino
- Deamination ap-site (loss of A or G); occurs 1000 times each day in each mammal
- Deamination base analogs (C->Uracil or A->HX); occurs 100 times each day in each mammal
- Frameshift (insertion or deletion on one strand), usually through a polymerase error when copying repeated sequences
- Oxidative damage caused by oxygen radicals