Fig. 1. Recognition of NTPs and 2′dNTPs by nucleic acid polymerases.

a NTPs and 2′dNTPs equilibrate freely between 3′-endo and 2′-endo conformations in solution. A- and B-form duplexes of RNA and DNA constrain sugar moieties of nucleotides in 3′-endo and 2′-endo conformations, respectively. C1′–C2′–C3′–C4′ bonds of the sugar moieties are colored cyan. b Watson–Crick base pairing of the 3′-endo (cyan) and 2′-endo (magenta) conformers to the template nucleic acid results in markedly different positions of the 3′OH group and the α-phosphate relative to the base-pairing plane. c, d DNAPs and RNAPs stabilize 3′-endo conformers of their cognate substrates. The illustrations reflect the crystallographically documented arrangement of the active site residues but are not accurate projections of 3D structures. Dashed magenta lines depict polar interactions and hydrogen bonds.