Structure of a DNA polymerase polypeptide that contains a 3'–5' proofread

a | Structure of the RB69 DNA polymerase.  A model for the coordinated action of the polymerase and exonuclease activities of polymerases can be constructed by comparing the structure of RB69 in its polymerizing and editing modes.  A mismatched base pair prevents the fingers from rotating towards the palm to bind the incoming deoxyribonucleoside 5'-triphosphate.  This leaves the 3'-mismatched end available for binding to the exonuclease active site, which removes the wrong nucleotide. During the switch between polymerizing and editing modes, the DNA moves towards the exonuclease active site with a rotation in the axis of the double helix.  This movement is aided by the tip of the thumb subdomain, which maintains contact with the DNA during the movement, and guides it on a path between the two sites.
b | The structure of a minimal replication complex, which consists of RB69 (right) complexed to the ringlike sliding clamp (left).  The complex is shown on the DNA (middle).