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 cellular reactions. In most eukaryotic cells, cellular poly(ADP-ribose) formation is the most acute response to such damage. Recently, such events have been amenable to study with soluble cell-free extracts of human cells. These investigations clarify the modulating role on DNA repair by poly (ADP-ribose), and suggest that the primary function of this unusual polymer is to act as an antirecombinant agent. Similar biochemical studies of subsequent repair events have revealed a branched pathway for the ubiquitous DNA base excision-repair process. The alternative pathway provides the cell with back-up functions for individual steps in this essential form of DNA repair.