Chromatin-associated DNA topoisomerase I activity was measured in human diploid fibroblasts during in vitro aging. No diilerence was detected as a function of cell age in the nicking and the closing activities of the DNA-unwinding enzyme. The capacity of type-I topoisomerase to relax superhelical DNA molecules was, however, increased in aged cells. An age-related increase in nucleoprotein content was also observed.

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