FREQUENCIES OF POLYCENTRIC CHROMOSOMES FOLLOWING COMBINATIONAL TREATMENTS OF CHINESE-HAMSTER OVARY CELLS WITH RESTRICTION ENDONUCLEASES ANDX-RAYS, BLEOMYCIN OR DNASE-I

Chinese hamster ovary cells were treated with combinations of the rest riction endonucleases AluI, EcoRI or PstI and bleomycin, DNase I or X- rays. With few exceptions the frequencies of polycentric chromosomes c alculated as dicentric chromosomes (DIC) were additive or less than ad ditive after combinational treatments when compared with the effects o f treatments with the single compounds. Less than additivity was found when the frequencies of DIC were higher than 150 DIC per 100 cells in dicating a saturation effect. The data suggest that DNA double-strand breaks (DSB) with different end-structures produce polycentric chromos omes to the same extent as DSB with the same end-structures.