We have used fluorescence in situ hybridization with whole-chromosome probes for human chromosomes 1, 4, 8 and 13 to investigate the extent to which the induction of damage and its repair after exposure to ionizing radiation is distributed randomly among these chromosomes. All the studies were performed with AG1522 human fibroblasts irradiated with 6 Gy maintained in a nondividing state for at least 6 h after irradiation except for the measurements of initial damage. The extent of initial damage was determined by fusion of the cells immediately after irradiation with metaphase HeLa cells to obtain premature chromosome condensation (PCC). Breaks and exchanges were also scored by PCC 24 h after irradiation and in metaphase spreads at the first division after irradiation. The data obtained were consistent with random breakage and repair in these chromosomes. Comparing PCC 24 h after irradiation with first metaphase, there was a deficit in aberrations at metaphase, particularly in unrejoined breaks, implying loss or slowing of cells containing aberrations prior to the first division. An analysis of dicentrics and translocations in chromosome 4 at first and in subsequent divisions showed that there was an equal number of dicentrics and translocations at first metaphase with loss of dicentrics, but no loss of translocations in subsequent divisions. These data are supportive of the hypothesis that the total number of chromosome aberrations in cells can be estimated from a single chromosome pair.

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