Abstract
Chinese hamster cells (line CHEF-125) were cultured for 1 or 4 h in the presence of tritiated thymidine ( 3HTdR). Immediately after the end of the treatment with 3HTdR or 4 h afterwards, some cultures were irradiated with X-rays, while others served as controls. Analysis of colchicine-C anaphase of M 1 and M 2 cells showed that: ( a) in the M 1 chromosomes the X-rays produced a significant departure from a 1:1 ratio of the number of silver grains counted on the sister chromatids; and ( b) in the M 2 chromosomes the X-rays increased significantly the frequency of sister-chromatid exchanges and of isolabelling regions. Subchromatid exchanges involving a single polynucleotide strand may be induced by X-rays. These exchanges would cause inequality of labelling over M 1 sister chromatids and isolabelling in the M 2 chromosomes.
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