The invention relates to a method for detecting a double-stranded region in a nucleic acid by (1) providing two separate, adjacent pools of a medium and a interface between the two pools, the interface having a channel so dimensioned as to allow sequential monomer-by-monomer passage of a single-stranded nucleic acid, but not of a double-stranded nucleic acid, from one pool to the other pool; (2) placing a nucleic acid polymer in one of the two pools; and (3) taking measurements as each of the nucleotide monomers of the single-stranded nucleic acid polymer passes through the channel so as to differentiate between nucleotide monomers that are hybridized to another nucleotide monomer before entering the channel and nucleotide monomers that are not hybridized to another nucleotide monomer before entering the channel.