Probing DNA superstructure in human quiescent lynphocytes by X-ray-induced double-strand breakage

Summary Human quiescent lymphocytes were lysed onto neutral sucrose gradients in order to sediment subsequently the nuclear DNA released within nucleoids. The position of nucleoids in the centrifuge tubes was detected fluorometrically by using the dye, ethidium bromide, and the height of the fluorescent peak was taken as a measure of DNA content. X-irradiation of lymphocytes, before their lysis, altered the DNA content of nucleoids and their sedimantation rate in accord with the view that(1) nuclear DNA is attached along its length at distance corresponding to 1.7 × 10¹⁰ g/mol, amd that(2) X-ray-induces double-strand breakage releases DNA fragments at random. Incubation at 37° C of irradiated lymphocytes restored the amount of attached DNA as it would be expected from an intracellular repair process for DNA double-strand breaks.