Motivated by experiments in which a polynucleotide is driven through a pro-teinaceous pore by an electric field, we study the diffusive motion of a polymer threaded through a narrow channel with which it may have strong interactions. We show that there is a range of polymer lengths in which the system is approximately translationally invariant, and we develop a coarse-grained description of this regime. In contrast to previous models, this description holds even when some pore degrees of freedom do not remain equilibrated. General features of the distribution of times for the polymer to pass through the pore are then deduced. Finally, we consider more microscopic models and argue that the available data suggests a translocation mechanism involving coupling between the polymer backbone and other slow degrees of freedom.
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