Gene-specific and strand-specific DNA repair in the G₁ and G₂ phases of the cell cycle

We have analyzed the fine structure of DNA repair in Chinese hamster ovary (CHO) cells within the G₁ and G₂ phases of the cell cycle. Repair of inactive regions of the genome has been suggested to increase in the G₂ phase of the cell cycle compared with other phases. However, detailed studies of DNA repair in the G₂ phase of the cell cycle have been hampered by technical limitations. We have used a novel synchronization protocol (D. K. Orren, L. N. Petersen, and V. A. Bohr, Mol. Cell. Biol. 15:3722-3730, 1995) which permitted detailed studies of the fine structure of DNA repair in G₂. CHO cells were synchronized and UV irradiated in G₁ or early G₂. The rate and extent of removal of cyclobutane pyrimidine dimers from an inactive region of the genome and from both strands of the actively transcribed dihydrofolate reductase (DHFR) gene were examined within each phase. The repair of the transcribed strand of the DHFR gene was efficient in both G₁ and G₂, with no major differences between the two cell cycle phases. Neither the nontranscribed strand of the DHFR gene nor an inactive region of the genome was repaired in G₁ or G₂. CHO cells irradiated early in G₂ were more resistant to UV irradiation than cells irradiated in late G₁. Since we found no major difference in repair rates in G₁ and G₂, we suggest that G₂ resistance can be attributed to the increased time (G₂ and G₁) available for repair before cells commit to DNA synthesis.