The G1 distribution of "G1-less" V79 Chinese hamster cells.

The V79-8 line of Chinese hamster cells has been reported to lack a measurable G1 phase. However, using a combination of time-lapse cinemicroscopy and [3H]thymidine autoradiography, we have found these cells to have a median G1 duration ranging from 1.4 to 2.6 h in different experiments, accounting for more than 15% of the median cycle time. The youngest cell labelled (in seven experiments) was 0.73 h old at the time of fixation suggesting a minimum G1 of between 0.40 and 0.73 h (the duration of the [3H]thymidine pulse being 0.33 h). In those experiments where steady-state proliferation could be established unequivocally, variability in G1 times accounted for all of the variability in cycle times. In addition, the distribution of G1 times (and cycle times) was well described by the two-transition version of the transition probability model. Nevertheless, changes in the average duration of G1 (and hence changes in the transition probabilities) played a comparatively minor role in determining proliferation rate. Instead, the length of S + G2 was markedly influenced by the composition of the culture medium. For purposes of comparison with the 'G1-less' V79-8 line, we have also examined a revertant derived from it (G1+5c) reported to have regained a substantial G1 phase. We are able to confirm that its G1 is indeed longer, the youngest labelled cell being 2.48 h old at the time of fixation. Unlike the parent line, there appeared to be more variability in G1 times than could be explained by two random transitions alone. The proliferation rate of the G1+5c revertant was unusually sensitive to the composition of the culture medium, suggesting the possibility of a metabolic defect.