A system for multistep DNA-templated synthesis is controlled by the sequential formation of DNA junctions. Reactants are attached to DNA adapters which are brought together by hybridization to DNA template strands. This process can be repeated to allow sequence-controlled oligomer synthesis while maintaining a constant reaction environment, independent of oligomer length, at each reaction step. Synthesis can take place in a single pot containing all required reactive monomers. Different oligomers can be synthesized in parallel in the same vessel, and the products of parallel synthesis can be ligated, reducing the number of reaction steps required to produce an oligomer of a given length.