Effect of 1 Gy X-rays in G1 phase on activation of cell cycle checkpoints in human lymphocytes: role played by chromosomal aberrations

The effect of X-irradiation (1 Gy) during G1 on the transition from G1 to S and on the length of G2 over cell cycles subsequent to irradiation was studied in human lymphocytes from six different donors. After irradiation a delay was observed in the onset of S phase, as was an extension of the G2 phase lasting throughout the three to four subsequent cell divisions. The extension of G2 and of the cell cycle as a whole is partly related to the presence of chromosome aberrations in the cell. This is demonstrated by: (i) the presence of a larger number of chromosome aberrations in M1 cells corresponding to sampling times longer after that of irradiation; (ii) the presence of a larger number of chromosome aberrations in cells with a longer G2. The most significant chromosome aberrations in this respect are isochromatid fragments. Lastly, we observed that irradiation during G1 activates another checkpoint governing the way mitosis proceeds. This takes the form of an extension of metaphase; in this case also, in some cells, activation of a possible checkpoint during preanaphase seems to be related to the presence of aberrations.