Complex manifold of oligonucleotide extension reactions. Oligonucleotides, both free (top) and template-bound (bottom) may react with activated nucleotides (LG-pN) resulting in 3'-elongation. The equilibrium between free and template-bound oligonucleotides is a function of both length and complementarity to the template. Similar sets of reactions may occur in parallel at all positions on the template. Over many cycles of template-binding and unbinding, longer oligonucleotides that are complementary to the template will accumulate. Ultimately, oligonucleotide ligation and gap-filling reactions may lead to complete assembly of a full-length complementary strand.