Changes in the rate of DNA replication fork movement during S phase in mammalian cells.

DNA fiber autoradiography was used to measure the rate of replication fork movement and the size of replication units as a function of time during the S phase of synchronized Chinese hamster ovary cells. The rate of fork movement increased by about threefold from early S to later S phase, with the most dramatic change occurring in the first hour of S phase. On the other hand, the size of replication units did not vary significantly during S phase.