Why is genetic code universal?

There are two possible explanations: chemical constraint and historical accident.

Chemical constraint

On this theory, each particular triplet would have some chemical affinity with its amino acid. GGC, for example, would react with glycine in some way that matched the two together. However, no such chemical relation has been found and there are two reasons for thinking that no such affinity exists.

1. The triplet and amino acid do not physically interact in the translation of the code. They are both held on a tRNA molecule, but the amino acid is attached at one end of the molecule, while the site that recognizes the codon on the messenger RNA is at the other end.

2. There are mutants that change the relation between triplet code and amino acid. These suppress the action of another class of mutants. The fact that the relation between amino acid and codon can be changed in this way shows that the same genetic code has not been forced on all species by some unalterable chemical constraint.

Figure: transfer RNA (tRNA) molecule. The amino acid is held at the other end of the molecule from the anti-codon loop where the triplet code of the messenger RNA molecule is read.