Gamma Radiation as a Probe of Chromatin Structure: Damage to and Repair of Active Chromatin in the Metaphase Chromosome

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Abstract
60Co .gamma. radiation was employed as a means of preferentially damaging actively transcribing chromatin within interphase and metaphase Chinese hamster V79-379 lung fibroblasts. The single-strand size distribution and break frequency of bulk 3H-labeled DNA were compared to those same parameters for active sequences, i.e., sequences complementary to 125I-labeled poly(A+)RNA. Sequences active during interphase are more sensitive than inactive sequences to single-strand break formation by .gamma. radiation even when the chromatin is condensed in metaphase. Repair of strand breaks in the bulk DNA is slower in metaphase than in interphase cells, but during metaphase, repair is faster in active sequences than in the bulk DNA. Chromatin structure evidently can be probed within intact cells by a method which circumvents isolation of nuclei or chromatin and the use of exogenous nucleases.