Abstract
IN contrast to the inefficiency of ionising radiations in the induction of sister chromatid exchanges (SCEs)1,2, ultraviolet lignt can cause a striking increase in the SCE frequency in mammalian chromosomes3,4. It has been shown in Chinese hamster cells that the SCE formation requires postirradiation DNA synthesis, and that a single ultraviolet irradiation can induce SCE repeatedly for several cell cycles4. As pyrimidine dimers are the major DNA lesions caused by ultraviolet light and as Chinese hamster cells have a poor ability to excise them5, the delayed formation of SCE seems to be due to unexcised pyrimidine dimers. This possibility was tested here with the use of a rat kangaroo cell line possessing a photoreactivating enzyme6–8 to see if the frequency of ultraviolet-induced SCE was actually reduced when cells were post-treated with visible light.
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