degeneracy of the genetic code
/ dee-JEN-er-uh-see /
Sixty-four codons, only twenty amino acids to spell, plus a few stops — the arithmetic leaves a big surplus. So nature does what any sensible code with spare capacity would do: it lets several different codons all mean the same amino acid. That many-codons-for-one-amino-acid arrangement is what biologists call degeneracy, or redundancy, of the genetic code.
The redundancy is not random; it follows a tidy pattern. Most of the freedom lies in the third base of the codon. For example, the four codons GGU, GGC, GGA and GGG all code for glycine — the first two letters GG fix the meaning and the third letter barely matters. Six different codons all spell leucine; six spell serine; six spell arginine. Only methionine (AUG) and tryptophan (UGG) are spelled by a single codon each. This concentration of slack in the third position is closely tied to wobble base-pairing at the ribosome, which lets one transfer RNA read several codons that differ only in their last letter.
Degeneracy is quietly protective. Because the third letter is often a 'don't care', a single-base mutation there frequently changes the codon to a synonym — a silent mutation that leaves the protein unchanged. And the codons for chemically similar amino acids tend to resemble each other, so even a non-silent mutation often swaps in a similar amino acid, softening the blow. The code is therefore not just a dictionary but a shock-absorber, built to limit the damage from inevitable copying errors. Note one subtlety: 'degenerate' here is the mathematical sense of 'many-to-one', not a value judgement — the code is robust, not defective.
Glycine has four codons: GGU, GGC, GGA, GGG. If a gene reads GGU and the third base mutates from U to C, the codon becomes GGC — still glycine. The protein is identical; the mutation is silent.
Synonymous codons mean a third-base change often leaves the amino acid — and the protein — unchanged.
Synonymous codons are not perfectly interchangeable: cells prefer certain codons over their synonyms (codon bias), and the choice can subtly affect how fast and how accurately a protein is made. Silent mutations are usually but not always harmless.