Take a look at the phosphorous (gold particle) in the right corner that is being oxidized by OH and becomes mechanically separated from the  backbone (single strand break). The remaining H (the white particle) that was initially in OH, forms a bond with the base (base dmage). The distorted base and sugar-ring destroys the base-pair hydrogen bonds and base stacking network.This is a real time simulation performed up to 2.5 pico-second (2.5 10^-12 s) using ReaxFF molecular dynamics. This simulation clearly demonstrates a single strand break on a realistic chunck of DNA for the first time (to the best of my knowledge). Color codes: carbon, oxygen, nitrogen, phosphorus and hydrogen atoms are shown as green, red, blue, gold and white. Ref: Abolfath, van Duin, Brabec, J. Phys. Chem. 2011 (in press).