FIGURE 5-33 Proofreading by DNA polymerase. All DNA polymerases have a similar three-dimensional structure, which resembles a half-opened right hand. The “fingers” bind the single-stranded segment of the template strand, and the polymerase catalytic activity (Pol) lies in the junction between the fingers and palm. As long as the correct nucleotides are added to the 3′ end of the growing strand, it remains in the polymerase site. Incorporation of an incorrect base at the 3′ end causes melting of the newly formed end of the duplex. As a result, the polymerase pauses, and the 3′ end of the growing strand is transferred to the 3′ →5′ exonuclease site (Exo) about 3 nm away, where the mispaired base, and probably other bases, are removed. Subsequently, the 3′ end flips back into the polymerase site and elongation resumes. See C. M. Joyce and T. T. Steitz, 1995, J. Bacteriol. 177:6321, and S. Bell and T. Baker, 1998, Cell 92:295.