GENE-SPECIFIC AND STRAND-SPECIFIC DNA-REPAIR IN THE G(1) AND G(2) PHASES OF THE CELL-CYCLE

We have analyzed the fine structure of DNA repair in Chinese hamster o vary (CHO) cells within the G(1) and G(2) phases of the cell cycle. Re pair of inactive regions of the genome has been suggested to increase in the G(2) phase of the cell cycle compared with other phases. Howeve r, detailed studies of DNA repair in the G(2) phase of the cell cycle have been hampered by technical limitations. We have used a novel sync hronization protocol (D. K. Orren, L. N. Petersen, and V. A. Bohr, Mel . Cell. Biol. 15:3722-3730, 1995) which permitted detailed studies of the fine structure of DNA repair in G(2). CHO cells were synchronized and UV irradiated in G(1) or early G(2). The rate and extent of remova l of cyclobutane pyrimidine dimers from an inactive region of the geno me and from both strands of the actively transcribed dihydrofolate red uctase (DHFR) gene were examined within each phase, The repair of the transcribed strand of the DHFR gene was efficient in both G(1) and G(2 ), with no major differences between the two cell cycle phases. Neithe r the nontranscribed strand of the DHFR gene nor an inactive region of the genome was repaired in G(1) or G(2). CHO cells irradiated early i n G(2) were more resistant to UV irradiation than cells irradiated in late G(1). Since we found no major difference in repair rates in G(1) and G(2), we suggest that G(2) resistance can be attributed to the inc reased time (G(2) and G(1)) available for repair before cells commit t o DNA synthesis.