Why are there only 20 amino acids?

NB this question is discussion based and could be approached from many different angles and include more ideas than I have covered below. I have split this question into 2 parts. Firstly you could dress why having only 20 amino acids is desirable, a more obvious approach to the questions for students as it is touched upon by many A level syllabus'. I would them move onto the molecular basis behind this, building on A level knowledge of translation and protein tertiary structure to prompt the student to answer the question correctly.DNA is read in codons, a triplet of bases encodes 1 amino acid. This means that there are 4³= 64 potential codons, 4 of which code for stop and start codons, which leaves in theory 60 different amino acids which could be encoded. However only 20 amino acids are synthesised in humans. This means that genetic information is redundant – often one amino acids relates to 2 or 4 codons, with the 3^rdbase in the codon being variable. So many codons coding for so few amino acids may seem uneconomical, but this degeneracy of the DNA code is vital in ensuring that DNA is translated with high fidelity (accuracy). Redundancy of information safeguards the DNA against point mutations. 2 in 3 point mutations are synonymous. A synonymous mutation means that although one base in the codon is substituted for another, the same amino acid is still produced. So having 64 codons encoding 20 amino acid is a good strategy in minimising the damage of point mutations to ensure that DNA is translated with high fidelity. Knowing this, we must ask where did this genetic redundancy come from; on a molecular level how can different codons produce the same amino acid? tRNA acts to ‘translate’ the mRNA template into amino acids, with a bespoke tRNA molecule transporting a specific amino acids to a specific codon on the ribosome so that they line up with their complementary mRNA codon. This is A level knowledge. But how is this specificity achieved? At GSCE you are taught that all cellular reactions and processes are catalysed by enzymes and that such enzymes are highly specific. The addition of amino acids to tRNA is also an enzyme catalysed reaction, a synthetase enzyme recognises the tertiary structure of the tRNA and if it is complementary will add a specific amino acid. However, there is only a set repertoire of stable tertiary conformations that the tRNA molecules can adopt, so that they also fit in with the rest of the protein translation machinery at the ribosome. If you increased the number of amino acids beyond 20 then it would be difficult to create suitable tRNA molecules that could be distinguished between one another by the synthetase enzyme so that the correct amino acid could be added. The addition of amino acids to tRNA would lose its specificity. So our amino acid repertoire is limited by the recognition of different tRNA molecules, but the resulting degeneracy of the genetic code is in fact valuable as it safeguards our DNA against point mutations.