In a method for generating an elongated nucleic acid molecule, a nucleic acid addition of a first nucleic acid molecule attached to a first 3′ or 5′ protecting group to a nucleic acid immobilized on a surface produces an intermediate-length immobilized nucleic acid. The first protecting group is dissociated from the first nucleic acid molecule. A second nucleic acid molecule that is attached to a second associated a 3′ or 5′ associated protecting group is added to the intermediate-length nucleic acid. The second associated protecting group is dissociated from the second nucleic acid molecule. A sequentially-extended elongated immobilized nucleic acid molecule having a desired sequence and length is produced by sequentially extending the intermediate-length immobilized nucleic acid by adding additional nucleic acid molecules with associated protecting groups to the intermediate-length nucleic acid and dissociating the associated protecting group after each addition.