Molecular recombination and the repair of DNA double-strand breaks in CHO cells

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Abstract
Molecular recombination and the repair of DNA double-strand breaks (DSB) have been examined in the G-0 and S phase of the cell cycle using a temperature-sensitive CHO cell line to test i) if there are cell cycle restrictions on the repair of DSB's; ii) the extent to which molecular recombination can be induced between either sister chromatids or homologous chromosomes and iii) whether repair of DSB's involves recombination (3). Mitomycin C (l–2μg/ml) or ionizing radiation (50 krad) followed by incubation resulted in molecular recombination (hybrid DNA) in S phase cells. Approximately 0.03 to 0.10% of the molecules (number average molecular weight: 5.6 × 106 Daltons after shearing) had hybrid regions for more than 75% of their length. However, no recombination was detected in G-0 cells. Since the repair of DSB was observed in both stages with more than 50% of the breaks repaired in 5 hours, it appears that DSB repair in G-0 cells does not involve recombination between homologous chromosomes. The Possibility is not excluded that repair in G-0 cells involves only small regions (less than 4 × 106 Daltons).