Distribution of Dimers in Ultraviolet-irradiated DNA

Abstract

ULTRAVIOLET irradiation causes the formation of dimers between adjacent pyrimidines within a DNA strand. These dimers (predominantly thymine dimers) are highly deleterious to the cell; as evidence of their potential danger, most cells have several mechanisms for removing dimers^1–3. In all cases, the implicit or explicit assumption is that dimers are formed randomly along the DNA chain. The distribution of repair regions in DNA after excision repair suggests that dimers are nonrandomly distributed⁴. In vitro studies also suggest that pyrimidine tracts are sites of increased dimer formation⁵. The preferential formation of dimers in pyrimidine tracts, particularly at low doses of ultraviolet, may have profound biological effects if the pyrimidine tracts are control regions of the DNA such as promoter sites or sites for the initiation of DNA synthesis^6,7.