Question

Since a 3 nucleotide genetic code would have been more than sufficient for coding 20 amino acids (27 possibilities with 3 bases, vs 64 possibilities with 4 bases), what constraint do you think tipped the evolutionary scale to the excessive capacity of a 4 base code?

Answer 1

A 3 base code gives 4^3 =64 different codons and it's enough for coding of 20 amino acids. But a 4 base code will give 4^4 = 256 different codons.

This number of codons are also enough for coding 20 amino acids but these numbers of codons are more than sufficient for coding of all amino acids. To code this 256 numbers of codons, much more tRNA would have required. And for translation many more tRNA synthetase enzyme would have have been required. These would create unnecessary pressure upon genome because these would increase the total number of genes and thus genome size would increase far more than now.

Nature didn't allow this unnecessary strain upon genome. Thus nature didn't allow this evolutionary event to occur successfully.