A proposed five-state Brownian ratchet model of the multi-subunit RNA polymerase II (Pol II) elongation, adopted from Ref. [92]. The structure of Pol II is provided (upper left). Configurations of the trigger loop (TL, in purple) and bridge helix (BH, in green) around the active site are shown for five kinetic states (I to V) in five windows. The non-template DNA strand is shown in blue, the template DNA strand is shown in cyan, and the synthesizing RNA strand is shown in red. The incoming NTP molecule is shown in orange.