Structural information for mammalian DNA pol-beta combined with molecular and essential dynamics studies have provided atomistically detailed views of functionally important conformational rearrangements that occur during DNA repair and replication. This conformational closing before the chemical reaction is explored in this work as a function of the bound substrate. Anchors for our study are available in crystallographic structures of the DNA pol-beta in "open" (polymerase bound to gapped DNA) and "closed" (polymerase bound to gapped DNA and substrate, dCTP) forms; these different states have long been used to deduce that a large-scale conformational change may help the polymerase choose the correct nucleotide, and hence monitor DNA synthesis fidelity, through an "induced-fit" mechanism. However, the existence of open states with bound substrate and closed states without substrates suggest that substrate-induced conformational closing may be more subtle. Our dynamics simulations of two pol-beta/DNA systems (with/without substrates at the active site) reveal the large-scale closing motions of the thumb and 8-kDa subdomains in the presence of the correct substrate--leading to nearly perfect rearrangement of residues in the active site for the subsequent chemical step of nucleotidyl transfer--in contrast to an opening trend when the substrate is absent, leading to complete disassembly of the active site residues. These studies thus provide in silico evidence for the substrate-induced conformational rearrangements, as widely assumed based on a variety of crystallographic open and closed complexes. Further details gleaned from essential dynamics analyses clarify functionally relevant global motions of the polymerase-beta/DNA complex as required to prepare the system for the chemical reaction of nucleotide extension.