Mutations are permanent, transmissible changes to the genetic material (usually DNA) of an organism. Mutations can be caused by copying errors in the genetic material during cell division, radiation, chemicals or viruses. Mutations often lead to the malfunction or death of a cell, or can cause cancer in higher organisms. Mutations are considered the driving force of evolution, where less favorable mutations are removed by natural selection, but favourable ones tend to accumulate. Neutral mutations do not affect the organism and can accumulate over time, which might lead to a Punctuated Equilibrium.
There are two classes of mutations, spontanous mutations (often called background level), and induced mutations caused by mutagens.
There are three basic types of mutations:
- 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 mutagenous base analogues like bromouracil. Less common is the transversion that exchanges a purine for a pyrimidine or a pyrimidine for a purine. Point mutations can be reversed by another point mutation, either if the same nucleotide is changed back to its original (true reversion), or by second-site reversion (a complementary mutation elsewhere that results in regained gene functionality).
- Insertions add one or more extra nucleotides into the DNA. They are usually caused by transposable elements. They can be reverted by deletions.
- Deletions remove one or more nucleotides from the DNA. They are irreversible.
A spontanous mutation on the molecular level can be:
- Keto <-> Enol
- Amino <-> Imino
- Desamination ap-site (loss of A or G); occurs a 1000 times each day in each mammal
- Desamination base analogs (C->Uracil or A->HX); occurs a 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 baused by oxygen radicals
An induced mutation on the molecular level can be caused by: