the triplet codon

If the genetic code is a dictionary, the codon is a single word in it. When the cell reads a messenger RNA to build a protein, it does not take the bases one at a time — it reads them in groups of three. Each such group of three bases is a codon, and it is the basic unit of meaning.

Why exactly three? Count the possibilities. There are twenty amino acids to specify. Reading one base at a time gives only 4 meanings — far too few. Two bases at a time gives 4 x 4 = 16 — still not enough for twenty. Three bases gives 4 x 4 x 4 = 64, which finally covers all twenty with room to spare. So the triplet is the smallest word length that works, and life settled on it. A codon is read off the mRNA in the 5'-to-3' direction; the codon AUG, for instance, is the bases adenine-uracil-guanine in that order, and it is matched during translation by a complementary three-base anticodon on a transfer RNA, which physically delivers the right amino acid.

The triplet nature of the codon was not obvious — it had to be proven. The decisive evidence came from Crick and Brenner, who showed that adding or removing one or two bases from a gene wrecks the protein, but adding or removing three together (or three separate single insertions) restores a readable message — exactly what you would expect if the reading frame moves in steps of three. The codon is why a single-base insertion is so destructive: it is not one wrong letter, it is a derailment that scrambles every word downstream.