The effect of 5-fluorouracil on DNA chain elongation in intact bone marrow cells

The effect of 5-fluorouracil (FUra) on DNA elongation was assessed in intact bone marrow cells that had been pulsed for 1 hr with [³H]-dThd in the absence or presence of FUra, chased in fresh media from 0 to 3 hr, and then analyzed on alkaline sucrose gradients. While DNA from control cells elongated at an average rate of 86 nucleotides per sec over a 3 hr interval, DNA from FUra-treated cells did not elongate and in contrast decreased in size over the same interval. In a parallel study to examine what happens to the FUra that was incorporated into DNA, bone marrow cells were pulsed for 1 hr with 50 μM [³H]-FUra, and then chased in fresh media from 0 to 2 hr. An aliquot of cells from each time point was lysed on an alkaline sucrose gradient to assess the size of [³H]-FUra-containing DNA, while another aliquot of cells from each time point was analyzed for radioactivity remaining in total DNA. The percentage of replicon-size DNA (≥ 100S) containing radiolabel decreased over the 2 hr chase while the percentage of small molecular weight DNA (≤ 7.2S) increased over the same interval. These changes in DNA size were accompanied by a decrease in radioactivity in total DNA. These studies suggest that excision of FUra from nascent DNA chains may prevent further elongation of DNA.