Ultraviolet Light and DNA

In recent years basic research has provided fundamental knowledge about how ultraviolet light (UV) affects deoxyribonucleic acid. In microorganisms under appropriate circumstances UV causes damage to nucleic acid bases with pyrimidine dimer formation. The injury is rapidly repaired by an excision-repair process. This involves enzymatic excision of the dimers and repair of the defect by new formation of nucleic acid in short chains which are reattached by enzymatic (polynucleotide ligase) activity.

Ultraviolet-light injury to mammalian cells causes dimer formation but the precise method of repair remains uncertain.¹ There is evidence of unscheduled DNA synthesis in interphase nuclei after UV irradiation, which is portrayed most graphically in autoradiograms made after pulse-labeling human fibroblast cell cultures with tritiated thymidine. Normally, a small number of cells in the DNA-synthesizing phase of the cell cycle take up tritiated thymidine and appear densely overlaid with silver grains in developed autoradiograms, but after UV almost