I think all of this is explained by a linguistics 101 course (I have seen a few excerpts of one lecture). A good reason to use symbols in communication is for

I think all of this is explained by a linguistics 101 course (I have seen a few excerpts of one lecture). A good reason to use symbols in communication is for reliability (which is needed in the DNA). With a discrete set of symbols you have most likely no degradation (instead some recovery occurs -- like when you interpret a poorly written letter), and with a small chance you understand a completely different symbol. This stops error propagation. Moreover, symbols allow for easy combination, which expands indefinitely the vocabulary with a fixed cost.