The Genetic Code and Codons

Proteins are built from 20 amino acids, but mRNA only has 4 different bases: A, U, C, G. A single base could only specify 4 amino acids, and two bases in a row give just 4x4 = 16 combinations, still short of 20. Three bases in a row give 4x4x4 = 64 possible codons, which is more than enough to cover all 20 amino acids with room to spare. That extra room is not wasted: it explains why the code is redundant, meaning most amino acids are coded for by more than one codon.

A codon chart organizes all 64 codons by their three positions and shows which amino acid each one specifies, plus three stop codons (UAA, UAG, UGA) that end translation and one start codon (AUG, methionine) that begins it. The ribosome reads mRNA three bases at a time, from a fixed starting point, never skipping or overlapping. Redundancy usually shows up in the third position of a codon: GAA and GAG both code for glutamic acid, for instance, so a change in that last base often has no effect on the protein made. This cushions the cell against certain mutations.

Because the rules mapping codons to amino acids are the same in bacteria, oak trees, and humans, this shared code links directly to translation itself, where ribosomes, tRNAs, and codons physically interact, and to mutation, where a single base change can shift which amino acid gets inserted. The universality of the code is strong evidence that all life descends from one common ancestor that already had this exact system in place.