A polymer (chain of N monomers) passes through a narrow nanopore driven by an electric field and thermal fluctuations. The translocation coordinate s (number of monomers through pore) follows a biased random walk on a free-energy landscape F(s). The ratchet effect: asymmetric barriers from chain entropy and field drive unidirectional transport. Translocation time scales as τ ~ N^(1+ν) where ν≈0.588 (Flory exponent).