Codons
DNA specifies protein through an mRNA. Hidden within the mRNA lies the ‘triplet code’, a series of three nucleotides, called codons that code for a single amino acid. There are only 20 amino acids that occur in naturally derived proteins. The mRNA contains four different nucleotides namely: adenine (A), uracil (U), guanine (G) and cytosine (C). Thus, 20 amino acids are coded by only four unique bases in mRNA. The codons of the mRNA read from 5′ → 3′ direction corresponds to the amino acid sequence of the protein read from its N-terminus to C-terminus.
The variation in the number of nucleic acid bases and the number of amino acids proves that there cannot be a code of one base per amino acid. Even if two nucleotides code an amino acid (a doublet code), it could not account for 20 amino acids. As with four bases and a doublet code, there would only be 16 possible combinations (42 = 16) and thus, they could not encode all 20 amino acids. However, a triplet code produces 64 (43 = 64) possible combinations or codons and thus could satisfactorily code all the 20 amino acids but a triplet code introduces the problem of there being more than three times the number of codons than amino acids.