Codon degeneracy

Because at least 21 codes are required (20 amino acids plus stop) and the next largest number of bases is three, then 4³ gives 64 possible codons, meaning that some degeneracy must exist.

A nucleotide substitution at a 4-fold degenerate site is always a synonymous mutation with no change on the amino acid.

[2]: 521–522 There are three amino acids encoded by six different codons: serine, leucine, and arginine.

[2]: 531–532 A practical consequence of redundancy is that some errors in the genetic code cause only a synonymous mutation, or an error that would not affect the protein because the hydrophilicity or hydrophobicity is maintained by equivalent substitution of amino acids (conservative mutation).

For example, a codon of NUN (where N = any nucleotide) tends to code for hydrophobic amino acids, NCN yields amino acid residues that are small in size and moderate in hydropathy, and NAN encodes average size hydrophilic residues.

[5][6] These tendencies may result from the shared ancestry of the aminoacyl tRNA synthetases related to these codons.