The Induction of Thymine Dimers in Ultraviolet-Irradiated Mammalian Cells

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Abstract
Irradiation of monolayers of Chinese hamster cells with UV (2652 A[degree], 5 erg/mm2/sec), showed that thymine dimers are formed as a linear function of dose within the dose range tested (100-2000 erg/mm2). At 100 erg/mm2 at 2652 A", about 0.05% of the thymine dimerized. With this same dose, 0.73 chromosome aberrations per cell are induced in Chinese hamster cells. Since these cells contained 5 x 10-12 gms DNA, it was calculated that 1.5 x 105 thymine dimers are induced by the same dose that induces 0.73 chromosome aberrations per cell. A search for a thymine dimer excision process, similar to that found in UV-resistanct bacteria, indicated no such process existed in Chinese hamster cells. Also, a test to see if these UV-induced thymine dimers could be photoreactivated in vivo demonstrated that there is no photoreativation.