Apparatus and methods are described for achieving uniform stretching of polynucleotides in hybrid electrophoretic-gel, micro-constricting microfluidic channels. Polynucleotides in normally relaxed configurations are driven by an electric field along a microfluidic channel. The polynucleotides thread through a porous gel barrier formed in the channel and extend into a constriction where an electric field gradient exists. The combined action of the gel and field gradient acts to extend the polynucleotide configuration fully for direct linear analysis of the molecule.