Molecular events in the reproduction of animal cells. I. The effect of puromycin on the duplication of DNA.

alized as a cyclic process in which the cell prog resses through a sequence of molecular events in volving the timely synthesis or activation of spe cific macromolecules. The present study concerns the mechanisms underlying the initiation of DNA synthesis. With use of synchronized cultures of HeLa (1@) it was demonstrated that DNA replication occurs only after a specific triggering event and that the formation of new DNA is a nonlinear process within a single nucleus. Under the condi tions of these experiments levels of puromycin which inhibit protein synthesis prevented the ac celeration of DNA synthesis without inhibiting the established rate of DNA synthesis. On the basis of the results of these studies and the studies on chromosome labeling in the second paper of this series, it is proposed that the DNA of the triggered IleLa cell exists in two physiologi cal states : a fraction which is competent for repli cation, and another which becomes competent for