Architecture of DNA polymerases. (a) A DNA polymerase is composed of thumb (green), finger (blue) and palm domain (red). Replicative polymerases often include an intrinsic proofreading 3´−5´ exonuclease (light purple). The ternary complex of an A-family DNA polymerase (PDB: 1LV5) is shown as an example. (b) Primary structures of seven DNA polymerase families. (c-e) Topology diagrams of the palm domain in A, B, Y, and RT families, C and X families, and PrimPol. (f) Side-by-side view of the catalytic centers of Pol η and Pol β.