Fig. 2. Effects of incubation for 4 hours at 39°C on DNA synthesis. (A,B) DNA synthesis activity of CHO-K1 and tsTM18 cells were measured by [³H]thymidine incorporation as described in the Materials and methods. The average and standard deviation of at least three independent experiments are indicated. Incorporation rate at 39°C relative to that at 34°C of replication was 1.34 for CHO-K1 and 0.77 for tsTM18 cells, respectively. The temperature shift appears to reduce DNA synthesis in tsTM18 cells. (C-F) Images of replication were obtained as described in the Materials and methods. Single equatorial sections were collected with a confocal microscope. Scale bar, 10 µm.