Spontaneous mutations are the result of endogenous mutagens that create oxy-radical DNA damage, base deaminations and depurinations, or polymerase infidelity. Induced mutations arise from the actions of exogenous mutagens such as chemical, physical, or viral agents.
Exogenous mutagens increase the occurrence of mutations above the background or spontaneous rate.
Since mutations are changes in DNA, a knowledge of DNA structure is necessary to understand mutagenesis. The DNA helix is stabilized by hydrogen bonds between base pairs in each strand, by hydrophobic interactions among successive base pairs, and by hydrophilic interactions between the sugar-phosphate backbone and the aqueous exterior. While the nucleotide composition is dependent on the DNA source, in all DNAs, guanine pairs with cytosine through three hydrogen bonds and adenine pairs with thymine through two hydrogen bonds. DNA damage may occur through a variety of mechanisms. A base may be altered through adduct formation after interaction with reactive electrophiles.