Dynamics of polynucleotide transport through nanometre-scale pores

The transport of biopolymers through large membrane channels is a ubiquitous process in biology. It is central to processes such as gene transfer by transduction and RNA transport through nuclear pore complexes. The transport of polymers through nanoscopic channels is also of interest to physicists and chemists studying the effects of steric, hydrodynamic, and electrostatic interactions between polymers and confining walls. Single-channel ion current measurements have been recently used to study the transport of biopolymers, and in particular single-stranded DNA and RNA molecules, through nanometre-size channels. Under the influence of an electric field, the negatively charged polynucleotides can be captured and drawn through the channel in a process termed 'translocation'. During translocation, the ion current flowing through the channel is mostly blocked, indicating the presence of the polymer inside the channel. The current blockades were found to be sensitive to the properties of the biopolymers such as their nucleotide composition, length, and secondary structure, and to physical parameters such as the driving field intensity, temperature, and ionic strength. These blockades are therefore a rich source of information regarding the dynamics of polynucleotides in the pore. The translocation process is separated into its two main steps: (a) polymer 'capture' in which one of the polymer's ends is threaded a small distance through the channel, and (b) polymer sliding through the channel. The experimental and theoretical efforts to elucidate polymer capture and the transport dynamics of biopolymers in nanoscopic pores are reviewed in this article.