Molecular events in the reproduction of animal cells

Synchronized cultures of HeLa cells were caused during a single cycle of DNA synthesis to replicate given fractions of their DNA with 5-bromodeoxyuridine (BdeU) instead of thymidine. It was observed that the introduction of 5-bromodeoxyuridine into the early replicating DNA complexes resulted in a striking decline in the cloning efficiency whereas the introduction of 5-bromodeoxyuridine into the late replicating DNA was without effect. The data suggest that the early replicating DNA of the HeLa nucleus plays an important role in survival.