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1. A genetically-engineered terminal deoxynucleotidyl transferase (TdT), wherein a wild-type TdT has been mutated at a single amino acid residue site to incorporate an azide or cyclooctene non-naturally occurring amino acid selected from the group consisting of: 4-Azido-L-phenylalanine (AzF), N-Propargyl-Lysine (PrK), Cyclooctene-L-Lysine (TCO-K) or Cyclooctyne-Lysine (SCO-K), wherein the non-naturally occurring amino acid is modified with a bifunctional azobenzene derivative comprising two orthogonal functional domains, wherein the first functional domain comprises a click reactive group for attachment to an affinity tag peptide for purification, and the second functional domain comprises a click reactive group whereby the bifunctional azobenzene derivative is attached to the non-naturally occurring amino acid, resulting in a TdT modified with the bifunctional azobenzene derivative capable of a reversible conformational change for controlled addition of a mononucleotide to the 3′ end of a single-stranded polynucleotide.
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