Temporal sequence of mutation for 6-thioguanine resistance in synchronised Chinese hamster cells

The temporal sequence of mutation to 6-thioguanine resistance was studied in Chinese hamster ovary cells synchronised in S phase. The number of resistant mutants induced following exposure to pulses of bromodeoxyuridine and subsequent selection was significantly greater if BUdR pulses were administered during the first, rather than the second half of S phase. The results are interpreted as indicating that the gene responsible for 6-thioguanine resistance is replicated during the first half of S phase.