Uninterrupted DNA synthesis during S-phase in untransformed diploid hamster fibroblasts

Untransformed Syrian hamster fibroblasts in exponential growth were exposed to a pulse of [3H]-thymidine for 5 min, followed immediately by bromodeoxyuridine, and serial samples were taken up to 16 h. Preparations were autoradiographed and stained for replication banding. No cell with replication bands was found without significant [3H]-thymidine uptake, although the extent of uptake varied between sub-phases of S. Thus there is no indication of a total cessation of synthesis at any period during S-phase.