Poly-ADP-ribosyltransferase that mediates poly-ADP-ribosylation of proteins and plays a key role in DNA repair. Mediates glutamate, aspartate, serine or tyrosine ADP-ribosylation of proteins: the ADP-D-ribosyl group of NAD(+) is transferred to the acceptor carboxyl group of target residues and further ADP-ribosyl groups are transferred to the 2'-position of the terminal adenosine moiety, building up a polymer with an average chain length of 20-30 units. Serine ADP-ribosylation of proteins constitutes the primary form of ADP-ribosylation of proteins in response to DNA damage. Mainly mediates glutamate and aspartate ADP-ribosylation of target proteins in absence of HPF1. Following interaction with HPF1, catalyzes serine ADP-ribosylation of target proteins; HPF1 conferring serine specificity by completing the PARP1 active site. Also catalyzes tyrosine ADP-ribosylation of target proteins following interaction with HPF1. PARP1 initiates the repair of DNA breaks: recognizes and binds DNA breaks within chromatin and recruits HPF1, licensing serine ADP-ribosylation of target proteins, such as histones, thereby promoting decompaction of chromatin and the recruitment of repair factors leading to the reparation of DNA strand breaks. In addition to proteins, also able to ADP-ribosylate DNA: catalyzes ADP-ribosylation of DNA strand break termini containing terminal phosphates and a 2'-OH group in single- and double-stranded DNA, respectively. NAD(+) + (ADP-D-ribosyl)n-acceptor = nicotinamide + (ADP-D-ribosyl)n+1-acceptor + H(+). L-seryl-[protein] + NAD(+) = H(+) + nicotinamide + O-(ADP-D-ribosyl)-L-seryl-[protein] L-aspartyl-[protein] + NAD(+) = 4-O-(ADP-D-ribosyl)-L-aspartyl-[protein] + nicotinamide L-glutamyl-[protein] + NAD(+) = 5-O-(ADP-D-ribosyl)-L-glutamyl-[protein] + nicotinamide L-tyrosyl-[protein] + NAD(+) = H(+) + nicotinamide + O-(ADP-D-ribosyl)-L-tyrosyl-[protein] Localizes to sites of DNA damage. The N-terminal disordered region does not act as a key DNA-binding domain. The WGR and PARP catalytic domains function together to recruit PARP1 to sites of DNA breaks. The N-terminal disordered region is only required for activation on specific types of DNA damage. The WGR domain bridges two nucleosomes, with the broken DNA aligned in a position suitable for ligation. The bridging induces structural changes in PARP1 that signal the recognition of a DNA break to the catalytic domain of PARP1, promoting HPF1 recruitment and subsequent activation of PARP1, licensing serine ADP-ribosylation of target proteins. Poly-ADP-ribosylated on glutamate and aspartate residues by autocatalysis. Belongs to the ARTD/PARP family.