Mutation
This article is about the genetic phenomenon. For mutation in linguistic, see sandhi. For the 1998 indie rock album by Beck, see Mutations.
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.
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. Insertions can be reverted by deletions.
- Deletions remove one or more nucleotides from the DNA. They are irreversible.
- Insertions and deletions within a gene's coding region can cause frameshiftss if the number of inserted or deleted nucleotides is not an even multiple of three (though introns and other complicating factors can play a mitigating role even in those cases). This is because codons in a gene are read using a fixed reading frame. Frameshift mutations generally disrupt the function of whatever protein sequence is affected, since the new sequence is essentially random, but can sometimes be reversed by a second frameshift mutation that puts the sequence back in the proper reading frame.
Spontaneous mutations on the molecular level include:
- Tautomerism
- 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
- Transition
- Transversion
- Frameshift (insertion or deletion on one strand), usually through a polymerase error when copying repeated sequences
- Oxidative damage caused by oxygen radicals
Induced mutations on the molecular level can be caused by:
DNA has so-called hotspots, where mutations occur up to 100 times more frequently than the normal mutation rate. A hotspot can be at an unusual base, e.g., 5'-methylcytosine.
It should be noted that, science fiction to the contrary, the overwhelming majority of mutations have no real effect, and the majority of the rest are harmful, if not fatal.
See also: homeobox macromutation
