The Induction of Chromatid Deletions in Accord with the Breakage-and-reunion Hypothesis

In the present experiments it has been possible to study large numbers of X-ray induced chromatid deletions, or breads, in Chinese hamster chromosomes and to discern whether or not a sister chromatid exchange also occurs at the point of breadage. Chromatid deletions are only infrequently associated with a sister chromatid exchange. This is contrary to the expectations derived from the exchange hypothesis of Revell. Pn the basis of this hypothesis, in which chromatid deletions are considered to be incomplete exchanges that occur in the necks of little loops in the chromosomes, 40% of the chromatid breaks are expected to be associated with sister chromatid exchanges. The present data are in accord with the conclusions drawn from the earlier autoradiographic experiments of HEDDLE AND BODYCOTE, and show that chromatide breaks can be accounted for on the basis of the breakage-and reunion hypothesis, with the majority being simple breaks and some being incomplete exchanges between two such breaks.