BREAKAGE OF HUMAN-CHROMOSOME-4, HUMAN-CHROMOSOME-19 AND HUMAN-CHROMOSOME-Y IN G(0) CELLS IMMEDIATELY AFTER EXPOSURE TO GAMMA-RAYS

One of several possible explanations for the fact that the initial num ber of ionizing radiation-induced DNA double-strand breaks (dsbs) per cell per Gy appears to be similar to 5-10 times greater than the numbe r of excess fragments produced in prematurely condensed chromosomes (P CCs) is that dsbs in DNA tightly associated with protein, such as migh t be the case for heterochromatin, are held together and not expressed as discontinuities in the PCC assay. To test this idea the breakage f requencies in chromosome 4, 19 and the heterochromatic and more euchro matic portions of the Y chromosome of non-cycling human fibroblasts we re measured and compared over a wide range of doses by inducing PCC im mediately after irradiation with Cs-137 gamma-rays. Even for doses up to 400 Gy no evidence was seen that Y heterochromatin or large fragmen ts of it remained unbroken. The only significant deviation from the ex pected initial breakage frequency per Gy per unit length of chromosome , in the dose range where a genome average for all chromosomes could b e obtained, was that observed for the euchromatic portion of the Y chr omosome (Y p+cen-->q11.2) where breakage was nearly twice that expecte d. There was no correlation between breakage per unit length and the ( A+T)/(G+C) ratio for these chromosomes or regions.