The frequency and distribution of isolabelling in Chinese hamster chromosomes after exposure to x-rays

2 experiments were carried out, following the same scheme. Chinese hamster cells (line CHEF-125) were cultivated for 4 h in the presence of tritiated thymidine (³H]TdR) (Expt. 1, 0.5 μCi/ml; Expt. 2, 0.1 μCi/ml). After removal of the isotope, some of the cultures were X-irradiated in stage S-G₂ preceding M₁ (series S-G₂ I), others in stage S-G₂ preceding M₂ (series S-G₂ II).Analysis of the colchicine-C-anaphase of M₂ cells showed that in both experiments the X-rays produced an increase in the isolabelling regions only if given in the S-G₂ stage preceding M₁. On the other hand, the X-rays produced an increase in sister chromatid exchanges (SCEs) in both series of the second experiment. In the first experiment, where the controls had a high frequency of SCEs, no increase in SCEs was recorded in either of the two series treated with X-rays. The frequency of the isolabelling regions, in relation to the total labelling (Expt. 2), was 1.4% in the control series, 2.5% in series S-G₂ I and 1.3% in series S-G₂ II.Analysis of the distribution of the isolabelled regions along the chromosome showed that the regions furthest from the centromere were those most involved in this phenomenon and that, for the most part, the isolabelling was not directly associated with an exchange.The results have been interpreted, in accordance with the theory put forward in a previous work, to mean that SCEs and isolabelling regions are due to different phenomena, the former resulting from an exchange involving the whole chromatid, the latter from subchromatid exchanges probably involving a single polynucleotide strand.