Prolongation of duration of G2 arrest delays and finally blocks entry into M phase in contrast to stable and reversible G1 arrest: Study of a G1/G2 temperature‐sensitive mutant of rat 3Y1 fibroblasts

Proliferation of 3Y1tsF121 cells was arrested in G1 and G2 phases after a shift up to 39.8°C (restrictive temperature). Both arrests were reversible: after a shift down to 33.8°C (permissive temperature), these cells effectively entered the next phases. However, the entry into M phase of the G2‐arrested cells was delayed depending on the time in arrest. The G2‐arrested cells finally became incapable of entering M phase with a prolonged incubation at 39.8°C. Under the same condition, G1‐arrested cells did not lose their ability to proliferate, and their delay of entry into S phase was slight. Therefore, cells in G2 phase are, in a sense, more unstable than the cells in G1 phase. These results also suggest that the time required for entry into M phase may depend on the preparedness for the initiation of M phase and, that it may be prolonged under the condition where the preparedness for entry into M phase is diminished.