[dylukes]

Sheer random chance.

I had this same question while working on structural resolution of a certain RNA polymerase.

I asked "okay so if this ratcheting mechanism allows the protein to zipper along the nucleotide strand... what's pushing it along?"

The answer is that nothing is pushing it along. It's just random movement and enough time (milliseconds) plus a ratcheting mechanism to ensure it doesn't go backwards.

Our low level processes are not so different from viruses in this respect!