E36 ts24 is a temperature-sensitive cell cycle mutant which has been derived from the Chinese hamster lung cell line E36. This mutant is arrested in phase S when incubated at the restrictive temperature (40.3 °C) for growth. At this temperature, proliferation of the mutant cells ceases after 10 h. About 2 h earlier, DNA synthesis is arrested. These kinetic studies indicate that the execution point of the mutant cells is in early S phase well beyond the G1/S boundary. The pattern of replication bands in E36 ts24 cells grown for 9 h at 40.3 °C strengthen the kinetic studies and map the execution point to early S phase. The exact point of arrest of the mutant cells in phase S was mapped in early S phase near the execution point. At the point of arrest the cells continue to synthesize DNA at a high rate but practically all of the newly synthesized DNA is degraded. This high rate of DNA degradation is limited to nascent DNA at the point of arrest. In the presence of 5-bromodeoxyuridine (5-BudR), the last E36 ts24 cells which reach mitosis at the restrictive temperature for growth show asymmetric replication bands which illustrate DNA degradation and resynthesis occurring in these cells at 40.3 °C.
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We wish to thank Mrs T. Israeli and Mrs C. Topf for their patience with us and excellent typing of the manuscript and Mrs K. Nissenson for her assistance in preparing the photographs. This investigation was supported by a grant from the US-Israel Binational Science Foundation No. 2895-82 to M. M.