A method and apparatus for synthesizing nucleic acid (NA) by sequential addition of nucleotide triphosphates to a single stranded NA molecule (growing-chain) is described. The growing-chain is annealed to an oligonucleotide, wherein the 3′ end of the oligonucleotide is complementary to the 3′ end of the growing-chain and wherein the protruding 5′ end of the oligonucleotide has the correct sequence to serve as a template for the growing-chain to produce a NA product. The growing-chain is then brought into contact with a polymerase in the presence of one or more nucleotide triphosphates under conditions permitting the polymerase to catalyze addition of one or more nucleotide triphosphates to the 3′ end of the growing-chain using the annealed oligonucleotide as a template. After polymerization has occurred, the annealed growing-chain and oligonucleotide are denatured so that the steps can be repeated until the NA product of desired length is synthesized.