(A) Chemical mechanism of PARP1 highlighting in red important groups of NAD+ as related to initiation (C-1′′), elongation (2′-OH), and branching (2′′-OH). (B) Cartoon representation of the four reactions catalyzed by PARP1 (in light blue): initiation, elongation, branching, and hydrolysis. ADP-ribose (ADPR) monomers (red circles) form PAR chains or free ADPR. (C) Cartoon representation of the different nucleosome constructs utilized in this study. The number in the name indicates the length of DNA (e.g., Nuc147 has 147 bp of DNA), and the small numbers next to the linkers or ends indicate the length of these regions.