The invention relates to a device for the amplification of DNA in a reaction mixture, the device (1) comprising a heated chamber (2) including a rotor (3) for holding a plurality of reaction vessels for reaction mixtures, a drive means for the rotor, a microwave energy source (8) with means for controlled delivery of said energy to the reaction mixtures, and a system (14-16) for determining denaturation of double-stranded DNA. The invention also provides a method for the amplification of a nucleic acid strand. In the first step of the method, a reaction mixture is formed comprising the target nucleic acid strand, nucleotides, a primer, a thermostable nucleic acid polymerase, and, if necessary, a reagent for the detection of denaturation of double-stranded DNA. In the second step, the mixture is incubated at a temperature which allows synthesis of a nucleic acid strand complementary to the target nucleic acid strand. The third step comprises denaturing double-stranded DNA formed in the second step by microwave energisation of the reaction mixture with monitoring of the mixture to determine the denaturation end point. The reaction mixture is allowed to cool to a temperature at which primer anneals to the target nucleic acid strand in the fourth step. The second to fourth steps are repeated until a desired level of amplification is achieved.