Cutaway view of the RNA polymerase II elongation complex

Direction of polymerase motion is left to right as shown.

DNA entering the enzyme is gripped by protein “jaws” (upper jaw not shown in this cutaway model).

The 3’ end of growing RNA is adjacent to one of the catalytically essential Mg2+ ions.

The wall forces the DNA to turn.

rNTPs probably enter the active site, as shown, through a funnel structure and pore.

The 5’ end of the growing RNA chain is diverted from the DNA template by a protein loop called the rudder, which limits the length of RNA hybridized to template DNA.

The rudder and lid, which guide the exit of RNA, emanate from a large clamp that swings from back to front, as shown, over the catalytic site and contribute to the binding of nucleic acids, and hence, to the high processivity of transcription.