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 microgram/ml) and actinomycin D (3.0 microgram/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.