INDUCTION AND REJOINING OF BREAKS IN DEOXYRIBONUCLEIC ACID OF HUMAN CELLS IRRADIATED AT VARIOUS PHASES OF CELL CYCLE

The induction and rejoining of single-strand breaks in the DNA of asynchronous and synchronized cultures of human-kidney cells (T-cells) were investigated. The assessment of breaks was carried out with a modified alkaline sucrose-gradient technique. The cells were lysed on top of the gradient in the presence of tri-isopropyl-naphthalenesulphonic acid (TIPNS) and 4-aminosalicyclic acid (PAS). The sedimentation pattern of DNA was studied after X-irradiation with doses up to 40 krad. Breaks could be detected by irradiating cells with doses higher than 5 krad. With synchronous cell populations the smallest induction of breaks was found in the DNA of cells in progress from late S to early G2. More breaks were induced in early S and G1. The rejoining of breaks in the DNA was studied during incubation of the cells at 37° after X-ray exposures to 5 and 20 krad. The capacity of rejoining breaks seemed to fluctuate slightly in the different phases of the cell cycle, being most effective in early S and minimal in G2 phase. In preliminary experiments, carried out to study the influence of protective agents, it was found that the presence of cysteamine during irradiation protected against radiation-induced breaks. © 1968.