5-bromodeoxyuridine-DNA strand symmetry and the repair of photolytic breaks in Chinese hamster cell chromosomes.

Experiments described in this report quantiate the black light sensitivities of Chinese hamster lung (CHL) cells containing BrdUrd-DNA of defined composition. Cesium chloride equilibrium gradient centrifugation provides estimates both of the percent thymidine replacement by BrdUrd and of the symmetry (unifilar versus bifilar) of BrdUrd incorporation into the chromosomal DNA duplexes. Radiation damage to BrdUrd-substituted CHL cell DNA and its repair in situ also have been assessed by alkaline sucrose gradient sedimentation. We observe that animal cell sensitivities to visible light (300-400 nm wavelength) depend markedly upon the symmetries of BrdUrd-substitution within the cells Dna. Cells that contain only unifilar BrdUrd-DNA are resistant to black light, whereas cells that contain bifilar BrdUrd-DNA are extremely photosensitive. The former cell populations repair single-stranded nicks (breaks in phosphodiester bonds) in their DNA within 24 hr of irradiation; the latter cell populations, however, are not able to repair light-induced, double-stranded breaks in their DNA.