Figure 2.

Chk1 regulates replication origin density in diploid human fibroblasts. Replication structures of MRC5 human fibroblasts were visualised using the procedures described in Figure 1. Replicon size (A–D) was measured using the separation of sister forks (panel A) after labelling with biotin-dUTP (panels B–D) and structures in untreated cells compared with those in cells treated with caffeine or UCN-01. BrdU labelling was also used to determine the rate of fork elongation (E–G) under the same conditions. Parameters defining average replicon structure under different conditions are shown (H). The organisation of active replication foci in cells treated with caffeine and UCN-01 was compared with untreated cells (I), as described in Figure 1. Scale bars, 5 μm.