A composition and method for controlled in vitro fragmentation of nucleic acids. A transposase forms catalytically active complexes with a modified transposon end that contains within its end sequence degenerate, apurinic/apyrimidinic sites, nicks, or nucleotide gaps, to fragment or shear a target nucleic acid sample in a controlled process. This method yields desired average nucleic acid fragment sizes. The inventive composition and method may be applied for generation of DNA fragments containing shortened transposon end sequences to facilitate subsequent reactions, for production of asymmetrically tailed DNA fragments, etc.