WHAT IS THE OPTIMUM SIZE FOR THE GENETIC ALPHABET

An important question in biology is why the genetic alphabet is made o f just two base pairs (G.C and A.T). This is particularly interesting because of the recent demonstration [Piccirilli, J. A., Krauch, T., Mo roney, S. E. & Benner, S. A. (1990) Nature (London) 343, 33-37] that t he alphabet can in principle be larger. It is possible to explain the size of the present genetic alphabet as a frozen character state that was an evolutionary optimum in an RNA world when nucleic acids functio ned both for storing genetic information and for expressing informatio n as enzymatically active RNA molecules-i.e., ribozymes. A previous mo del [Szathmary, E. (1991) Proc. R. Soc. London Ser. B 245, 91-99] has described the principle of this approach. The present paper confirms a nd extends these results by showing explicitly the ways in which copyi ng fidelity and metabolic efficiency change with the size of the genet ic alphabet.