Proofreading by RNA polymerase. (a) In kinetic proofreading, the polymerase stalls after incorporating a mismatched base into the growing RNA chain, enabling pyrophosphorolysis to remove the incorrect base. (b) In nucleolytic proofreading, the polymerase backtracks on the DNA, melting several nucleotides of the RNA (i.e., breaking the DNA-RNA base pairs), then an intrinsic nuclease removes the section of melted RNA.