Mutagens or mutagenic agents are physical or chemical agents that induce changes in the genetic code.

Natural spontaneous mutations occur regularly among cells.  These result from errors during DNA replication as well as recombinant events that occur when segments of DNA from other chromosomes or introduced from parasites such as viruses are added to the strand.

Physical Mutagens

Radiation can alter DNA in two different ways:

Ionizing radiation, such as X-ray and gamma radiation energize electrons, causing them to leave atoms and molecules, forming highly reactive ions.  This can cause DNA molecules to break down and triggers mutations.

Nonionizing radiation such as ultraviolet light causes the braeking of bonds between purines and pyrimindines when two pyrimidine molecules of the same type (T or C) are adjacent to one another on a nucleoside.  These pyrimidine dimers distort the sugar phosphate backbone and prevent proper replication and transcription.

Chemical Mutagens

Nucleotide analogs have a similar chemical structure to a normal nucleotide (as in thymine and 5'-bromouracil, above) and compete with the normal nucleotide during DNA replication and causing point mutations.  Some nucleotide altering chemicals such as nitrous acid alter the structure of a nucleotide.  Nitrous acid removes the amine (nitrogen-containing) group from adenine, converting it to a guanine analog, resulting in a base-pair substitution.

Chemicals such as ethydium bromide, benzopyrene from smoke and yellow and red acridine dyes can cause frameshift mutations in DNA as DNA polymerase copies the alter strand, adding or deleting base pairs around the bulge formed by the bound mutagen.