Cell cycle studies of mononucleate and cytochalasin-B--nduced binucleate fibroblasts.

The progress of mononucleate and cytochalasin-B-induced binucleate cells through the G1 period of the growth cycle has been studied. DNA synthesis was initiated in synchrony in both of the nuclei of cells rendered binucleate by a brief pulse of cytochalasin B. Furthermore, populations of these cells traversed the G1 period significantly faster than their mononucleate counterparts. That this effect was due to the presence of two nuclei in a common cytoplasm and not to the increased size of binucleate cells was suggested by studies of cells with altered nucleocytoplasmic ratios. No correlation was observed between protein content per cell and the rate of progress through G1. Evidence for the existence of nuclear-nuclear cooperation that results in a shortening of the G1 period is discussed.