Wavelengths of ultraviolet (UV) light that are most effective in stimulating mutagenesis or death lie near 260 nm, where DNA light absorption is maximal. UV-irradiated DNA contains small amounts of many different altered DNA constituents, called photoproducts. Prominent among them are intrastrand dimers consisting of two pyrimidine bases joined by a cyclobutane ring structure formed from carbons 5 and 6 on each pyrimidine ring (Figure 25.9a). Dimerization draws the adjacent thymine residues together, distorting the helix in such a way that replicative polymerization past this site is blocked. The ability of an organism to survive ultraviolet irradiation is related directly to its ability to remove thymine dimers from its DNA. Cyclobutane thymine dimers are removed from UV-irradiated DNA by photoreactivation.

The photoproduct that is the principle form of UV-induced mutation is shown in Figure 25.9b. This structure is also a dimer, linked via C-6 of the 5' pyrimidine (either thymine or cytosine) and C-4 of the 3' pyrimidine (usually cytosine).