ACCUMULATION OF DNA GROWING POINTS IN CAFFEINE-TREATED HUMAN-LYMPHOBLASTOID CELLS

DNA bifilarly substituted with bromodeoxyuridine (DNAHH) can be obtained after incubation of human lymphoblastoid cells with heavy analog for short periods. The DNAHH originates from DNA growing points and is derived by branch migration in vitro during or after cell lysis. Treatment of the cells with caffeine resulted in an increase in both the relative and absolute amount of DNAHH as compared to non-treated cultures synthesizing equivalent amounts of DNA. In conjunction with the smaller size of the nascent DNA synthesized in the presence of caffeine, the accumulation of DNAHH likely indicates an accumulation of growing forks, possibly resulting from the decreased rate of chain elongation and the initiation of replication at additional sites of origin. The usefulness of measurement of DNAHH as a probe to estimate the number of replicating forks, i.e. the number of active replicons, is discussed. © 1979.