The firing of origins is reduced in response to UVA-induced ROS.
Asynchronous or S-phase synchronised (condition S4R, see Material and Methods) cells were exposed to 80 and 160 kJ/m2 of UVA in MEMi. IdU and CldU were then sequentially added to the medium 4 h (asynchronous cells) or 2 h (S-phase cells) after UVA radiation. The forks density is defined as the ratio between the number of CldU-IdU-CldU signals (Red-Green-Red signals) and the length of replicated DNA. The percentage of cells in S-phase at the labeling time was determined by flow cytometry and was found to be 47% for the asynchronous population and 80% for S-phase synchronised cells. The data represent the mean ± sd of 4 independent experiments for A and 3 independent experiments for B (excepted for the dose of 80 kJ/m2 of UVA for which only 2 data were available).