The standard genetic code contains 64 possible codons.
A codon is a sequence of three RNA bases, and each position can contain one of four bases: A, U, G, or C. The calculation 4 × 4 × 4 produces 64 possible triplets. These codons are read during translation to specify amino acids or stop signals.
Of the 64 codons, 61 specify one of the 20 standard amino acids and three are stop codons. The code is degenerate, meaning that several different codons can specify the same amino acid. Methionine and tryptophan are each represented by only one standard codon.
The genetic code is also nearly universal, although mitochondria and some microorganisms use documented variants. Codons are read in messenger RNA by ribosomes, while transfer RNAs recognize them through complementary anticodons. This codon system is different from the DNA triplets that serve as the original genetic template.