Manipulating chromosome structure and metaphase status with ultraviolet light and repair synthesis inhibitors

DNA repair occurs in metaphase-arrested cells in response to ultraviolet irradiation. In the presence of the repair synthesis inhibitors hydroxyurea and 1-beta-D-arabinofuranosylcytosine the chromosomes of such cells, as seen in Carnoy-fixed preparations, are decondensed. The extent of decondensation is related to both the u.v. dose and the duration of incubation in the presence of inhibitors. For any particular cell type there is a reasonable correlation between the amount of decondensation and the number of single-strand DNA breaks generated by the repair process under the same inhibitory conditions, though the chromosome changes continue after the number of single-strand breaks has reached a plateau. The dose response of chromosome decondensation varies between different cell types but is in general correlated with differences in levels of single-strand breaks accumulated under comparable inhibitory conditions. Decondensation can be detected after 0.5 Jm-2 in repair-competent human cells. In human cells defective in excision repair there is much less chromosome decondensation in response to the same u.v. dose and time of repair inhibition. However, a simian virus 40-transformed muntjac cell displays pronounced chromosome decondensation but has limited incision ability. Both chromosome decondensation and single-strand break accumulation in the presence of inhibitors are reversed when DNA precursors are provided, but reversal after higher u.v. doses and longer periods of incubation leads to recondensed chromosomes that are fragmented. Elution of the DNA from such cells through polycarbonate filters under non-denaturing conditions reveals that double-strand DNA breaks are generated during the period of incubation with inhibitors. Although the chromosomes of repair-inhibited metaphase cells are decondensed in fixed preparations, their morphology appears normal in intact cells. The cells also retain a capacity to induce prematurely condensed chromosomes (PCC) when fused with interphase cells: compared with control mitotic cells, the speed of induction is sometimes reduced but the final amount of PCC produced is similar.