Synthesis of custom de novo DNA sequences is highly demanded by fast-growing field of synthetic biology. Usually DNA sequences with length more than 1 kb are assembled from smaller synthetic DNA fragments (synthons) obtained by PCR assembly. The ability to synthesize longer synthons sufficiently reduces efforts and time for DNA synthesis. We developed a novel rational oligonucleotide design and programmed approach for the assembly of synthetic DNA synthons up to 1550 bp. The developed procedure was thoroughly investigated by synthesis of cholesterol oxidase gene from Streptomyces lavendulae (1544 bp). Our approach is based on combined design, oligonucleotide concentration gradient, and specialized assembly program that directs assembly reaction to full-length gene in a stepwise manner. The process includes conventional thermodynamically balanced assembly, thermodynamically balanced inside-out elongation, and further amplification. The ability of DNA polymerase to perform programmed assembly is highly influenced by the presence of 5 ' -> 3 '-exonuclease activity. Oligonucleotide probing of PCR assembly products allowed us to shed light on the nature of high molecular weight spurious by-products and to understand the mechanism of their formation. For the first time, we applied light scattering techniques for tracking of oligonucleotide annealing, analysis of gene assembly products, and even for real-time monitoring of gene assembly process.