RNA polymerase (RNAP) is the key enzyme that directs gene transcription.We have systematically studied the elongation cycle of a prototypical RNAP from bacteriaophage T7.The RNAP had been experimentally suggested to work largely as a Brownian ratchet during the elongation.Based on the single molecule measurements, our previous kinetic modeling revealed that a small translocation bias of T7 RNAP aids nucleotide selection during its ratcheting [1].Next we constructed a theoretical framework to analyze how the nucleotide selection proceeds efficiently for fidelity control through multiple kinetic checkpoints in the absence of proofreading [2].