Disclosed are devices for amplifying a preselected polynucleotide in a sample by conducting a polynucleotide polymerization reaction. The devices comprise a substrate microfabricated to define a sample inlet port and a mesoscale flow system, which extends from the inlet port. The mesoscale flow system includes a polynucleotide polymerization reaction chamber in fluid communication with the inlet port which is provided with reagents required for polymerization and amplification of a preselected polynucleotide. In one embodiment the devices may be utilized to implement a polymerase chain reaction (PCR) in the reaction chamber (PCR chamber). The PCR chamber is provided with the sample polynucleotide, polymerase, nucleoside triphosphates, primers and other reagents required for the polymerase chain reaction, and the device is provided with means for thermally controlling the temperature of the contents of the reaction chamber at a temperature controlled to dehybridize double stranded polynucleotide, to anneal the primers, and to polymerize and amplify the polynucleotide.