REVELL REVISITED

The controversy of the Classic versus the Exchange theories for the origin of simple chromatid breaks is outlined. Using BrdU harlequin sister-chromatid differentiation four Revell ratios can be defined and these have been obtained and tested as a block in V79 hamster cells. The values are quite different from the simple predictions. However, values similar to those observed (taken as a block) can be readily simulated from Revell theory by making the assumption that intra-chromatid events are dominant (0.7:0.3). They can also be obtained from contingency modelling of Classic theory using the same assumption plus the additional constraint that there is no contribution from isolated single lesions (Poisson class 1). If this latter assumption is correct, then the frequency of breaks involving a colour-jump (ratio III) should not decline to zero as the dose falls. A dose-response experiment shows that it does not, but remains approximately constant at about 12%, even in the unirradiated control. An added complication arises when we discover for the TB/BB situation, that whilst neither breaks nor gaps show any excess BB involvement (sensitisation), lesions involved in interchanges show at least a 2-fold BB excess. Clearly, the chromatid discontinuities we are scoring are not behaving as would be expected of a residue of unrejoined primary breaks (Classic theory) and we infer also that they are not 'simple'.