Codon does not have

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. The terms first, second, and third nucleotide refer to the individual nucleotides of a triplet codon read 5'-3', left to right. A adenine nucleotide; C, cytosine nucleotide; G, guanine nucleotide; Term, chain terminator codon; U, uridine nucleotide. AUG, which codes for Met, serves as the initiator codon in mammalian cells and also encodes for internal methionines in a protein. Thus codon does not have thymine.Ref: Harpers Illustrated Biochemistry, 30th edition, page no: 414