The three domains of the hepatitis C helicase, NS3, are represented in blue, green and yellow. The blue and green domains walk along the double-stranded DNA (ladder-like structure, in black), one base pair (rungs) at a time, likely consuming one ATP (in red) per base pair. The yellow domain lags behind. These small reactions build tension in the DNA-protein complex, which unwinds only after three nucleotide pairs are unhitched from one another. When the enzyme encounters an insurmountable barrier it bounces back and restarts the unwinding process.