X-ray crystallographic structures of human DNA polymerase beta with nonhydrolyzable analogs containing all atoms in the active site required for catalysis provide a secure starting point for a theoretical analysis (quantum mechanics/molecular mechanics) of the mechanism of chemistry without biasing of modeling assumptions as required in previous studies. These structures provide the basis for a detailed quantum mechanics/molecular mechanics study of the path for the complete transfer of a monophosphate nucleoside donor to the sugar acceptor in the active site. The reaction is largely associative with the main energetic step preceded by proton transfer from the terminal primer deoxyribose O3' to Asp-256. The key residues that provide electrostatic stabilization of the transition state are identified and compared with those identified by mutational studies.