Mutations are permanent, transmissible changes to the
genetic material (usually
DNA) of an
organism. Mutations can be caused by copying errors in the DNA 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 also necessary for
evolution, where less favorable mutations are removed by
natural selection.
There are two classes of mutations, spontanous mutations (often called background level), and induced mutations caused by mutagens.
There are three 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 an extra nucleotide into the DNA. They are usually caused by transposable elements. They can be reverted by deletions.
- Deletions remove a nucleotide from the DNA. They are irreversible.
DNA has so-called hotspots, where mutations occur with up to 100 times more often compared to the normal mutation rate. A hotspot can be at a unusual base, e.g., 5'-methylcytosine?.