Sub-chromatid exchanges and the strandedness of chromosomes

8-Ethoxycaffeine (EOC)-induced sub-chromatid exchanges and the type of aberrations they give rise to in the second (T2) division after treatment, were studied in root-tips of Allium cepa and Vicia faba. When roots were fixed 2 to 3 hours after 1-hour treatments with EOC, sub-chromatid exchanges were the predominant type of aberration. In roots fixed 5 hours after the treatments, the sub-chromatid exchanges were mostly replaced by chromatid exchanges. The total number of sub-chromatid exchanges obtained was much higher than the number of chromatid exchanges. The appearance of the sub-chromatid exchanges at metaphase (Fig. 2) is hardly compatible with the idea that half-chromatids are involved in the exchange. Rather, the configurations observed suggest that a varying amount of material connects the chromatids at the point of the exchange, and that there is a gradual transition from a situation where most of the material goes in the “old” direction so that the chromatids involved are connected only by two thin “threads”, to a situation where all the material goes in the “new” direction (i.e., a full chromatid exchange). Cells with sub-chromatid exchanges were labelled by making them tetraploid with colchicine or binucleate with theophylline. When these cells divided for the second time after the EOC treatment (T2 division) they contained exchanges which were exclusively of the chromosome type. The frequency of these exchanges was very high and corresponded to the high frequency of sub-chromatid exchanges in the T1 division. The observation that “sub-chromatid” exchanges appear as chromosome-type exchanges in the T2 division suggests that the structures involved in the exchange are whole chromatids and supports the folded-fiber model of chromosome structure.