Total numbers of codons are

Twenty different amino acids are required for the synthesis of the cellular complement of proteins; thus, there must be at least 20 distinct codons that make up the genetic code. Since there are only four different nucleotides in mRNA, each codon must consist of more than a single purine or pyrimidine nucleotide. Codons consisting of two nucleotides each could provide for only 16(42 )-distinct codons, whereas codons of three nucleotides could provide 64(43 )-specific codons. It is now known that each codon consists of a sequence of three nucleotides; that is, it is a triplet code (see Table 37-1). The initial deciphering of the genetic code depended heavily on in vitro synthesis of nucleotide polymers, paicularly triplets in repeated sequence. This synthetic triplet ribonucleo- tides were used as mRNAs to program protein synthesis in the test tube, and allowed investigators to deduce the genetic code.Ref: Harpers Illustrated Biochemistry, 30th edition, page no: 414