The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA (A, G, C, and T or U). The four bases make up the “letters” of the code. The letters are combined in groups of three to form “words” called codons. There are 64 possible codons, and each codon codes for one amino acid or for a start or stop signal. The codon AUG is the start codon that establishes the reading frame of the code. After the AUG start codon, the next three bases are read as the second codon. The next three bases after that are read as the third codon, and so on. The sequence of bases is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons.