EFFECT OF 1 GY X-RAYS IN G(1) PHASE ON ACTIVATION OF CELL-CYCLE CHECKPOINTS IN HUMAN-LYMPHOCYTES - ROLE PLAYED BY CHROMOSOMAL-ABERRATIONS

The effect of X-irradiation (1 Gy) during G(1) on the transition from G(1) to S and on the length of G(2) over cell cycles subsequent to irr adiation was studied in human lymphocytes from six different donors, A fter irradiation a delay was observed in the onset of S phase, as was an extension of the G(2) phase lasting throughout the three to four su bsequent cell divisions, The extension of G(2) and of the cell cycle a s a whole is partly related to the presence of chromosome aberrations in the cell. This is demonstrated by: (i) the presence of a larger num ber of chromosome aberrations in M-1 cells corresponding to sampling t imes longer after that of irradiation; (ii) the presence of a larger n umber of chromosome aberrations in cells with a longer G(2). The most significant chromosome aberrations in this respect are isochromatid fr agments. Lastly, we observed that irradiation during G(1) activates an other checkpoint governing the way mitosis proceeds, This takes the fo rm of an extension of metaphase; in this case also, in some cells, act ivation of a possible checkpoint during pre-anaphase seems to be relat ed to the presence of aberrations.