genetic code

The genetic code is the lookup table that says which amino acid each three-letter codon stands for. It is the cipher that lets the four-letter language of nucleic acids be read as the twenty-letter language of proteins — a dictionary every cell shares.

Because there are 64 possible codons but only 20 amino acids (plus a stop signal), the code is redundant: most amino acids are specified by more than one codon. This redundancy is not random — codons for the same amino acid often differ only in the third base, which cushions some mutations against changing the protein.

The code is also nearly universal: the same codons mean the same amino acids in bacteria, plants, and humans, strong evidence that all life shares a common ancestor. A handful of organisms and organelles use slight variations, but the core dictionary is astonishingly conserved across billions of years.