Tight-binding molecular-dynamics simulations of a typical high-energy grain boundary in silicon show that its equilibrium atomic structure is similar to that of bulk amorphous silicon and contains coordination defects. The corresponding electronic structure is also amorphouslike, displaying gap states mainly localized around the coordination defects, where large changes in the bond-hybridization character are observed. It is proposed that such coordination defects in disordered high-energy grain boundaries are responsible for the experimentally observed gap states in polycrystalline Si. {copyright} {ital 1998} {ital The American Physical Society}