Analysis of DNA synthesis in S phase synchronized HeLa cells. Left panel: images of the BrdUrd Comet assay performed at 2.5, 4, 6 and 7 hours from the beginning of DNA synthesis. The pictures, obtained by fluorescence microscopy show identical comet fields with FITC (green) and propidium iodide staining (red). C: control cells. T: treated cells. Right panel: Quantification of DNA synthesis at different time points from the beginning of S phase. (a): representative image of BrdUrd-Comet assay (INGR ×150, FITC staining), the percentage of tail DNA shared uniform distribution among the various comet formations. Tail moment measured following FITC (b) and PI (c) stainings, respectively. Controls (triangles); treated (squares). Results shown are the mean ± SE of 25 cells. (d): Thymidine incorporation per cell at 5 and 6 hours from the beginning of S phase. Control cells (black), treated cells (grey). The error bars show the standard deviation obtained from three independent experiments. Student’s test. *p < 0.05 **p < 0.01.