FIG. 1.

Replication fibers and fork rates in wild-type (WT) and Chk1^−/− chicken DT40 cells. (A) Representative images of replication tracts from wild-type and Chk1^−/− DT40 cells pulse-labeled with 25 μM BrdU for 20 min (yellow track) followed by 250 μM IdU for 30 min (red track) and then processed for DNA fiber spreads as described in Materials and Methods. Fork direction is indicated by a black arrow, and the junction between pulse-labels is indicated by a white arrow. (B) Distribution of replication fork rates in wild-type and Chk1^−/− DT40 cells pulse-labeled and processed as described above. (C) Distribution of replication fork rates in wild-type and Chk2^−/− DT40 cells. For each panel, the distribution of fork rates during the first (BrdU) and second (IdU) pulse-labels is shown. Data bars are the means of three independent experiments, with similar results observed for each, and error bars represent 1 standard deviation. The total number of forks scored for each distribution is indicated in parentheses.