A,C, G and T - stand in for the four chemical bases that store information in DNA. A sequence of these same four letters, or repeating in a particular order,genetically defines an organism. Within the genome sequence are shorter, three-letter codons that represent one of the 20 regularly used amino acids, or with three of the possible 64 three-letter codons reserved for stop signals. These amino acids are the building blocks of proteins that carry out a myriad (a very large number) of functions. For example,the amino acid alanine can be represented by the three-letter codon GCU and the amino acid cysteine by the three-letter codon UGU. In some organisms, the three-letter codon UGA, and which normally signals the conclude of a protein-coding gene,is hijacked to code for a scarce genetically encoded amino acid called selenocysteine.
Source: phys.org