What is the first proofreading step?
It occurs before a new nucleotide is added to the growing chain. The correct nucleotide has a higher affinity for the moving polymerase than does the incorrect nucleotide, because the correct pairing is more energetically favorable. Also, after nucleotide binding, but before covalent addition of the nucleotide, the enzyme must undergo a conformational change in which its fingers tighten around the active site. This allows double checking of the exact base-pair geometry before catalytic addition