DNA mutations can be induced by several methods:
Chemical Mutagens
- Base analogues (BrdU, caffeine)
- Base-modifying chemicals (nitrosoguanidine, nitrous acid)
- Intercalators (inducing a frame shift)
Base analogues (such as BrdU and caffeine) are able to be inserted into the DNA sequence, taking the place of a base. This can cause issues with transcription and DNA replication. Base modifying chemicals, like nitrosoguanidine and nitrous acid, are able to alter the chemical structures of bases, changing the DNA sequence. In the case of nitrosoguanidine, a transition mutation occurs from GC to AT. Intercalators cause a frame shift. These are frequently used to stain DNA during gel electrophoresis, using chemicals like ethidium bromide or Midori green. Ethidium bromide is a highly carcinogenic compound, due to it permanently intercalating within the DNA sequence. Midori green is less harmful, only temporarily intercalating, and so favoured in laboratories due to its improve safety profile.
Spontaneous
- Introduction of a non-canonical base by wobble base pairing
- UV, Gamma radiation
- Tautomer / Isomerisation (imino, deamination)
Spontaneous mutations occur without physical intervention, switching a base. This can be by radiation (including UV and gamma), causing isomerisations (such as converting a base to the imino form, or deamination. The introduction of a non-canonical base can occur through wobble base pairing.
Slip-strand mispairing Leads to an increase or decrease in the number of repeats in a section of DNA, due to the temporary detachment and reattachment of the DNA polymerase during replication. Can be used in bacteria to turn genes off and on.
Biological (transposons) Transposons can insert into genomes at different sites, as well as move between sites. This can disrupt genes where the transposon inserts into a coding region.
Insertion / Deletion by homologous / non-homologous recombination Homologous recombination allows the exchange of genetic material between sites that display homology (a region of similar DNA sequence). This is important in horizontal gene transfer and meiosis, where genetic variation is able to be easily introduced. Non-homologous recombination is much less frequent than homologous recombination, where a recombination event occurs between regions that do not display homology. This can be used in random mutagenesis, and non-homologous end joining may occur during CRISPR, with the two ends of the section excised by Cas9 re-joining together and not allowing the insert to undergo homology directed recombination.