The genetic code used to produce proteins is carried by mRNA, since ribosomes cannot read the DNA blueprint.  There are only four DNA nucleotides, yet there are 21 naturally occuring amino acids found in the structure of proteins.  It was discovered that information about amino acid placement is not carried by single DNA nucleotides, but is instead carried by triplets of mRNA nucleotides called codons.

64 codon combinations are possible based on the original 4 DNA nucleotide triplet pairs (43).  Of these, 61 code for amino acids and 3 (UAA, UAG and UGA) have not associated amino acids, thus serve as stop codons, since they cause ribosomes to stop the process of translation.  The mRNA code is "redundant," since with the exception of the amino acids methionine and tryptophan, there are multiple codons that can code for the same amino acid, with the first two bases of the codon specifying a particular tRNA and the third subject to "wobble" since it does not have to form a bond with a tRNA nucleotide for the amino acid to be placed correctly.