Evidence for double replication of chromosomal DNA segments as a general consequence of DNA replication inhibition.

We have previously presented evidence that a transient inhibition of DNA synthesis by a pulse of 1-beta-D-arabinofuranosylcytosine (ara-C) results in a disruption of the pattern of replication of the chromosomal DNA of cultured human cells, resulting in some DNA segments being replicated more than once in a single S phase. Further evidence is presented in this paper that this effect is not a specific property of the ara-C molecule in that a similar effect is produced in cells by a pulse of 9-beta-D-arabinofuranosyladenine (ara-A) and also by a pulse of cycloheximide. The activated form of ara-A and ara-C (the triphosphates) both inhibit DNA synthesis at the level of the polymerase. Double replication following an ara-A pulse demonstrates that double replication after an ara-C pulse is not caused by some specific property of the ara-C molecule which might be unrelated to any effect on DNA synthesis. However, cycloheximide is an inhibitor of mammalian protein synthesis and inhibits DNA synthesis only indirectly, probably through a consequent deficiency of DNA-packaging proteins. Hence, the occurrence of double replication of chromosomal DNA segments following a pulse of cycloheximide is consistent with this phenomenon being a general and nonspecific consequence of the freezing of DNA replication forks.