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would be interesting to establish if the codon length 3 is a consequence of having
at least 20 encoded elements, or otherwise, if 20 is a consequence of having codons
of length 3. Moreover, is the little discordance from the minimality related to other
important aspects of the genetic code, or is it a matter of contingency in the evolutive
process of definition of the code? Finally, we remark the importance of having an
asymmetric treatment of start and stop signals. The begin signal is given by an AUG
codon which encodes also Methionine (a very sharp encoding with only one codon),
while the stop signal is external to the code and uses 3 different codons. We can
easily realize that stop cannot be encoded by using a codon for one amino acid. In
fact, such a choice would reduce the number of effective amino acids. But, why not
employ specific codons only for marking the beginning of translation?
Fig. 4.2
The transducer realized by ribosomes
What abstractly we indicated I-molecules correspond to genes, while M-molecules
correspond to proteins. Genes and proteins interact in the interplay between the pro-
cesses of translation from I-molecules to M-molecules and the regulation role of M-
molecules in the translation of I-molecules. In fact, a specific enzyme (in principle
could be many enzymes) performs the transcription from I-molecules into an inter-
mediate form of molecules, which in real cells are RNA molecules, that we can ab-
stractly indicate as IM-molecules. RNA molecules surely, in the first stages of cell
evolution, were able to play both the roles of I-molecules and M-molecules. This
