The induction of chromosomal aberrations by X irradiation during S-phase in cultured diploid Syrian hamster fibroblasts.

The induction of chromosomal aberrations by 4.0 Gy of 25 kV X-rays in cell throughout S-phase has been investigated in untransformed diploid Syrian hamster fibroblasts. Using a method of subdividing S into cytologically defined stages (on the basis of replication band patterns displayed after bromo-deoxyuridine incorporation) it is shown that: (1) This dose does not perturb, measurably, the intracellular programme of synthesis at the chromosome band level, so that the cell classification criteria remain valid after radiation. (2) Mitotic delay and perturbation appears to be less for cells in very early S, but there is no evidence of a massive cell mixing of S cells. (3) S-phase is, in general, much less sensitive to aberration induction at all sub-phases than G2. (4) Both chromosome and chromatid-type aberrations are found in pre- S and S cells, but chromatid-types predominate in the latter at all sub-phases. (5) The frequency of chromatid-types, especially interchanges falls in early S. Syrian hamsters have large blocks of non-centromeric late-replicating autosomal chromatin, and unambiguous chromatid changes can be observed in these prior to detectable synthesis. Although the resolution afforded by the BrdU method is much better than that with radioactive labelling, it is still very poor compared with events at the molecular level, and it is felt that this finding cannot be taken as conclusive evidence that chromosomes appear double to radiation prior to synthesis.