Figure 4.

Two rules of complementarity guide base pairing in DNA and RNA (collectively xeno nucleic acid [XNA]). The rules are (1) size complementarity (large purines pair with small pyrimidines), and (2) hydrogen-bonding complementarity (hydrogen bond donors, D, pair with hydrogen bond acceptors, A). Rearranging the nucleobase D and A groups gives artificially expanded genetic information systems (AEGIS). Chemical issues in the “first-generation” AEGIS (left pairs) are indicated in magenta. These motivated the synthesis of a second-generation AEGIS (right pairs). Electron density presented to the minor groove (green lobes) is recognized by polymerases (Benner et al. 1998).