Variation In G1 Transit Time Relative to the Cycloheximide and Actinomycin D Drug Restriction Points

The transit time distribution at various points in the cell cycle of synchronized Chinese hamster ovary cells was determined from the mitotic index, [³H]thymidine labeling index and increase in cell number monitored at regular intervals after mitotic selection. Variation in G₁ transit time compared with that for the total cell cycle indicates that variation in cell cycle transit time occurs mainly during G₁ phase. the cycloheximide (5.0 μg/ml) and actinomycin D (3.0 μg/ml) restriction points occur 0.2 and 1.7 hr prior to entry into S phase, respectively. the transit time distributions are further characterized by the moments of the distributions. the variance (2nd moment about the mean) of the transit time distribution at the actinomycin D restriction point is similar to the variance of the transit time distribution at the G₁/S border, thus variation in cell cycle transit time originates earlier than 1.7 hr prior to entry into S phase (i.e., the first 3/4 of G₁). If G₁ transit time variability and cell cycle control are related, then the results presented here indicate that the major regulatory events do not occur during late G₁ phase.