DNA fork displacement rates in synchronous aneuploid and diploid mammalian cells

DNA fork displacement rates were measured in Chinese hamster ovary cells (CHO), human HeLa cells and human diploid fibroblasts. For CHO cells two independent techniques were used: one based on CsCl equilibrium density gradients and the other on 313 nm photolysis of incorporated bromodeoxyuridine (BrdUrd). Both methods indicated that there was no significant variation in fork displacement rates in CHO cells as they progressed through S phase. Asynchronous CHO cultures displayed the same average value (1.0 micron/min) and range of values as found in synchronous cells. In contrast, the rate of DNA fork displacement in HeLa cells, measured by the BrdUrd-313 nm method, increased continuously from 0.8 micron/min in early S to 2.5 micron/min in late S. For human diploid fibroblasts, in early S, the rate was approximately 0.7 micron/min and decreased to a minimum of 0.5 micron/min in mid S. The replication fork displacement rate then increased to a maximum of 0.9 micron/min in late S and declined again before the end of S phase. This pattern of DNA fork displacement rates roughly paralleled the overall thymidine incorporation rate and appears quite different from the patterns found for HeLa and CHO cells.