Inter- and intra-chromosomal distribution of chromatid breaks induced by X-rays during G2 in human lymphocytes

Cultures of blood from healthy adults were irradiated 48 h after stimulation with 240 R of X-rays and fixed after various time intervals (0-2 h, 2-4 h, 4-6 h). 3HTdR was added to several cultures after irradiation. Mitotic and labelling indices were used to distinguish between two cell samples inside the irradiated G2 population: D- cells reaching mitosis without mitotic delay and a high frequency of chromatic breaks and D+ cells with mitotic delay and which, during the delay, repair most of the damage produced. After R banding 450 chromatid deletions were located in each of the two cell samples. The D+ cells showed a higher frequency of breaks than the D- cells with decreasing chromosome size, in the telomeric and centromeric region and in the junction between the R+ and R- bands. These results can be interpreted as indicative of a non-random distribution of repair processes both between and within chromosomes.