Evidence for the formation of hybrid DNA during mitotic recombination in Chinese hamster cells.

Direct evidence is provided for the formation of hybrid DNA during mitotic recombination in CHO cells. The cells were labeled for one round of replication in medium containing BUdR, so that the density of the DNA was heavy light (HL) and then returned to light medium. Further DNA synthesis, during either repair or chromosome replication, can only result in HL or fully light (LL) DNA; however, the formation of hybrid DNA as part of the process of recombinational repair will produce some fully heavy (HH) DNA. A small fraction of DNA containing regions of HH DNA has been detected on neutral CsC1 gradients, and the amount of this DNA is increased by treatment of the cells with mitomycin C. Increasing doses of mitomycin C produce smimlar increases in both the amount of HH DNA and the frequency of sister chromatid exchanges measured cytologically. This correlation provides evidence that the HH DNA is hybrid DNA, formed as an intermediate in recombinational repair.