The stepwise mechanism of transcription fidelity. Cartoon schematically shows RNA polymerase active centre and the steps of discrimination against various erroneous substrates. From top to bottom: incorporation of complementary nucleotide triphosphate (cNTP), misincorporation of ncNTP, c2'dNTP and c3'dNTP. Discrimination against unNTP (unusable substrates) takes place in the open state of the active centre and is not shown. The approximate discrimination contributions, found in our study, are shown for each step.