ENZYMES ACTING AT STRAND INTERRUPTIONS IN DNA

Endogenous and environmental DNA-damaging agents often generate single -strand interruptions in DNA. The lesions trigger a complex set of cel lular reactions. In most eukaryotic cells, cellular poly(ADP-ribose) f ormation is the most acute response to such damage. Recently, such eve nts have been amenable to study with soluble cell-free extracts of hum an cells. These investigations clarify the modulating role on DNA repa ir by poly (ADP-ribose), and suggest that the primary function of this unusual polymer is to act as an antirecombinant agent. Similar bioche mical studies of subsequent repair events have revealed a branched pat hway for the ubiquitous DNA base excision-repair process. The alternat ive pathway provides the cell with back-up functions for individual st eps in this essential form of DNA repair.