[Conformational changes before and after DNA synthesis increase cytogenetically lethal radiation damage]

Chinese hamster ovary fibroblasts were synchronized by mitotic selection and showed cytogenetic lethally acting radiation damages, which depended on age at irradiation time. The development of cell cultures was followed by counting of the killed cells in the 3rd daughter generation of the irradiated ones and by evaluation of the colony size spectra 73.5 h after innoculation. At a dose of 300 rads two narrow but high peaks of radiation sensitivity were induced, which appeared at the beginning and at the end of the S-phase of the cell cycle and therefore are reduced to conformation changes of the DNA. Consequently the DNA-super-structures linking pieces of the nucleoprotein-fibre during fastening to, and detaching from, the nuclear membrane are assumed to be the loci of the increased radiation sensitivity.