Schematic view of DNA polymerase converting ssDNA to dsDNA while an external force f stretches the template (10, 11). A single DNA molecule is strung between a stationary surface and a plastic bead so that it can be micromanipulated with optical or magnetic tweezers. Inset indicates labeling of residues: 0 denotes the template base that pairs, in the active site of the enzyme, with a complementary free nucleotide (dNTP) captured from the ambient solution; +1, +2, … denote successive neighbors upstream, previously paired to form dsDNA; −1, −2, … denote template bases on ssDNA downstream of the active site.