A code is a set of rules governing the order of symbols in communication. This defines a code, regardless of the nature of the symbols, be they alphabetic letters, voice sounds, dots and dashes, DNA bases, amino acids, nerve impulses, or what have you. Codes are generally expressed as binary relations or as geometric correspondences between a domain and a counterdomain; one speaks of mapping in the latter case. Thus, in the International Morse Code, 52 symbols consisting of sequences of dots and dashes map on 52 symbols of the alphabet, numbers and punctuation marks; or in the genetic code, 61 of the possible symbol triplets of the RNA domain map on a set of 20 symbols of the polypeptide counterdomain.
In intercellular communication the domains and counterdomains are the signal molecules and their receptors, and the code is like the base-pair rules of the first-tier code of the DNA, a simple rule between pairs of molecules of matching surfaces.
Why There are no Double-Entendres in Biological Communication: The basic information for the encoding in intercellular communication (a high-class encoding complying with Shannons Second Theorem) is all concentrated in the interacting molecular surfaces. And this information is what makes the communications unambiguous. We can now define an unambiguous communication: a communication in which each incoming message or signal at a receiver (or retransmitter) stage is encoded in only one way; or, stated in terms of mapping, a communication in which there is a strict one to one mapping of domains, so that for every element in the signal domain there is only one element in the counterdomain.
The table in Figure 7.9 tells us at a glance that a given amino acid may have more than one coding triplet: UUA, UUG, CUU, CUC, CUA, CUG, for instance, are all synonyms for leucine. A code of this sort is said to be degenerate. That is OK despite the epithet, so long as the information flow goes in the convergent direction, as it normally does. The counterdomain here consists of only one element, and so a given triplet codes for no more than one amino acid. Thus, there is synonymity, but no ambiguity in the communications ruled by the genetic code.
-The Touchstone of Life: Molecular Information, Cell Communication and the Foundations of Life, by Werner R. Loewenstein, Oxford University Press, 1999