As RNA polymerase moves along the DNA, it continues to untwist the double helix, exposing about 10-20 DNA nucleotides at a time for pairing with RNA nucleotides (Figure 17.9). The enzyme adds nucleotides to the 3' end of the growing RNA molecule as it continues along the double helix. In the wake of this advancing wave of RNA synthesis, the new RNA molecule peels away from its DNA template, and the DNA double helix re-forms. 
