The accurate estimation of chromatid breakage, and its relevance to a new interpretation of chromatid aberrations induced by ionizing radiations

Abstract

If cells are subjected to ionizing radiations as they approach mitosis, chromosome changes are initiated which appear at metaphase as visible discontinuities of chromatid structure and as chromatid exchanges between different chromosomes. The discontinuities have previously been interpreted as surviving examples of a much larger number of chromatid breaks produced at the time of irradiation, and the exchanges as the result of new unions between such breaks that occurred close together. Root meristem cells of the broad bean (Vicia faba), which had received 50 or 65 r of X-rays, have been used to test a new interpretation of these changes, according to which each chromatid discontinuity arises from a chromatid exchange between two points close together on one chromosome. On this interpretation, the frequency of chromatid discontinuities at metaphase should be much lower than those of most other aberrations: it is shown that the high value usually obtained is due to the inclusion of a large proportion of short unstained but structurally continuous ‘gaps’, and that the frequency of true breaks is as low as the new hypothesis requires. It is accepted that radiation dosage and dose-intensity experiments indicate that chromosomes sufferprimary eventsat the points where and when they are crossed by the tracks of single ionizing particles; and also that, although themselves unstable, pairs of such primary events that are close enough together in space and time can together enter a second stable state ofexchange initiation. If the new hypothesis is also accepted it is argued to be unlikely that this primary event can be chromatid breakage, as is usually supposed, and unlikely also that exchange initiation can be chromatid reunion. It follows that the chromatid exchange itself is structurally established at some later stage, as the two chromosome points continue their joint development in the affected condition.