  Motivated by experiments in which a polynucleotide is driven through a
proteinaceous 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. From this description, general
features of the distribution of times for the polymer to pass through the pore
may be deduced. We also introduce a more microscopic model. This model provides
a physically reasonable scenario in which, as in experiments, the polymer's
speed depends sensitively on its chemical composition, and even on its
orientation in the channel. Finally, we point out that the experimental
distribution of times for the polymer to pass through the pore is much broader
than expected from simple estimates, and speculate on why this might be.
