We developed a method to translate DNA sequences into densely functionalized nucleic acids by using T4 DNA ligase to mediate the DNA-templated polymerization of 5'-phosphorylated trinucleotides containing a wide variety of appended functional groups. This polymerization proceeds sequence-specifically along a DNA template and can generate polymers of at least 50 building blocks (150 nucleotides) in length with remarkable efficiency. The re-sulting single-stranded highly modified nucleic acid is a suit-able template for primer extension using Deep Vent (exo-) DNA polymerase, thereby enabling the regeneration of tem-plate DNA. We integrated these capabilities to perform iter-ated cycles of in vitro translation, selection, and template regeneration on libraries of modified nucleic acid polymers.