How are dsDNAs/duplex DNAs made?

Just to avoid any potential misconception concerning the production technique applied, building dsDNA by adding complementary base pairs step by step is not possible. The method of choice consists in synthesizing two single strand DNA (ssDNA) oligos separately and align them to form a duplex via complementary base pairing. This drives the formation of more or less stable duplexes/dsDNA.

Each ssDNA oligo is made using a DNA synthesizer, which is basically a computer-controlled reagent delivery system. The first base is attached to a solid support, usually a glass or polystyrene bead, which is designed to anchor the growing DNA chain in the reaction column. DNA synthesis consists of a series of chemical reactions.

Each cycle of reactions results in the addition of a single DNA base. A chain of DNA bases can be built by repeating the synthesis cycles until the desired length is achieved.