title: Response of Chinese hamster ovary cells to DNA damage after a conditioning exposure to ultraviolet light.
abstract: [3H]Thymidine pulse-labeling and pulse-chasing of DNA in ultraviolet-irradiatied cells was examined in detail by alkaline sucrose gradient velocity sedimentation. In cells exposed to 2.5 J/m2 2 h earlier, a higher percentage of 3H-labeled DNA accumulated at high molecular weight during the 1.5 h after 5.0 J/m2 than in cells exposed to 5.0 J/m2 alone. This increased percentage can be accounted for by the fact that there was already a component of 3H-labeled DNA at high molecular weight at the time of the 5.0 J/m2 exposure in those cells that had been previously exposed to 2.5 J/m2; this component was not observed in cells exposed only to 5.0 J/m2. Moreover, when cells were treated with cycloheximide for the 2 h between the two exposures, there was no component of high molecular weight DNA at the time of the second exposure. These results indicate that the apparently more rapid shift of pulse-chased DNA to high molecular weight in cells exposed twice to ultraviolet light is due to accumulation of high molecular weight DNA precursors that act as substrates for further elongation after the second irradiation.
mesh_terms: D000818:Animals; D002460:Cell Line; D002499:Centrifugation, Density Gradient; D006224:Cricetinae; D003412:Cricetulus; D003513:Cycloheximide / Q000494:pharmacology; D004247:DNA / Q000528:radiation effects*; D004260:DNA Repair; D004261:DNA Replication; D005260:Female; D007553:Isotope Labeling / Q000379:methods; D008970:Molecular Weight; D010053:Ovary; D014316:Tritium; D014466:Ultraviolet Rays
chemical_list: D014316:Tritium; D004247:DNA; D003513:Cycloheximide