Search for DNA interchange corresponding to sister chromatid exchanges in Chinese hamster ovary cells

Chinese hamster ovary cells (CHO) grown for one cycle in bromodeoxyuridirie (BrdU) contain a small amount (0.5%) of unusually dense double stranded DNA. This dense DNA has been previously interpreted as being bifilarly substituted with BrdU and hence evidence that sister chromatid exchange (SCE) formation proceeds via the Holliday model of recombination. However, the amount of this dense DNA is 100 times greater than that expected based on the SCE frequency in similarly cultured CHO cells, and is not increased by treating the cells with mitomycin C. Moreover, contrary to expectations for bifilarly substituted DNA, amount of this dense DNA is not reduced by growing BrdU-labeled cells for a second cycle in TdR. Finally, DNA isolated CHO cells contains a minor band (0.5%) with a density 0.025 gm/cc greater than that of the main band, whether or not BrdU been incorporated. These results call into question the identification of this unusually dense DNA as bifilarly substituted and hence its previously postulated relationship to SCE formation.