There is disclosed a process for in vitro synthesis and assembly of long, gene-length polynucleotides based upon assembly of multiple shorter oligonucleotides synthesized in situ on a microarray platform. Specifically, there is disclosed a process for in situ synthesis of oligonucleotide fragments on a solid phase microarray platform and subsequent, “on device” assembly of larger polynucleotides composed of a plurality of shorter oligonucleotide fragments.