Nanosecond dynamics simulations for DNA polymerase beta (pol beta)/DNA complexes with three mismatched base-pairs, namely GG, CA, or CC (primer/template) at the DNA polymerase active site, are performed to investigate the mechanism of polymerase opening and how the mispairs may affect the DNA extension step; these trajectories are compared to the behavior of a pol beta/DNA complex with the correct GC base-pair, and assessed with the aid of targeted molecular dynamics (TMD) simulations of all systems from the closed to the open enzyme state. DNA polymerase conformational changes (subdomain closing and opening) have been suggested to play a critical role in DNA synthesis fidelity, since these changes are associated with the formation of the substrate-binding pocket for the nascent base-pair. Here we observe different large C-terminal subdomain (thumb) opening motions in the simulations of pol beta with GC versus GG base-pairs. Whereas the conformation of pol beta in the former approaches the observed open state in the crystal structures, the enzyme in the latter does not. Analyses of the motions of active-site protein/DNA residues help explain these differences. Interestingly, rotation of Arg258 toward Asp192, which coordinates both active-site metal ions in the closed "active" complex, occurs rapidly in the GG simulation. We have previously suggested that this rotation is a key slow step in the closed to open transition. TMD simulations also point to a unique pathway for Arg258 rotation in the GG mispair complex. Simulations of the mismatched systems also reveal distorted geometries in the active site of all the mispair complexes examined. The hierarchy of the distortions (GG>CC>CA) parallels the experimentally deduced inability of pol beta to extend these mispairs. Such local distortions would be expected to cause inefficient DNA extension and polymerase dissociation and thereby might lead to proofreading by an extrinsic exonuclease. Thus, our studies on the dynamics of pol beta opening in mismatch systems provide structural and dynamic insights to explain experimental results regarding inefficient DNA extension following misincorporation; these details shed light on how proofreading may be invoked by the abnormal active-site geometry.