Mutation

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 the driving force of evolution, where less favorable mutations are removed by natural selection, but favourable ones tend to accumulate.

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:

1. 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).
2. Insertions add an extra nucleotide into the DNA. They are usually caused by transposable elements. They can be reverted by deletions.
3. Deletions remove a nucleotide from the DNA. They are irreversible.

A spontanous mutation on the molecular level can be:

• Tautomerism?
  • 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
• Transition
• Transversion
• 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:

• Chemicals
  • [Base analog]?es
  • Simple chemicals, for example, acids
  • Alkylating agents
  • Polycyclic hydrocarbons?s, for example, the benzpyrene?s found in internal combustion engine exhaust?
  • [DNA crosslinker]?, for example, platinum
  • Oxygen radicals
• Radiation
  • Ultraviolet radiation
  • [Ionizing radiation]?

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 an unusual base, e.g., 5'-methylcytosine?.