The sequence of amino acids that make up proteins is encoded in the nucleic acid DNA by a set of triplets of nucleotides called codons. Each codon specifies either a particular amino acid or a signal to stop protein synthesis. A codon consists of three consecutive nucleotides (adenine-uracil-guanine or UAG), and each amino acid is represented by a unique combination of these bases. A sequence of codons is called a codon code. During protein synthesis, the codons on mRNA are translated into amino acids by a small molecule called transfer RNA (tRNA). This molecule binds to mRNA by complementary base-pairs with regions of mRNA that contain codons. The tRNA has an anticodon on one end and an acceptor stem on the other, which recognizes the codons through their matching sequence of three bases. The tRNA then carries the corresponding amino acid to the ribosomal A-site where it is covalently joined to the growing polypeptide chain.
In eukaryotes, the most common start codon is AUG, which codes for the amino acid methionine (Met). A special tRNA known as an initiator tRNA always carries Met or its bacterial equivalent, formyl methionine (fMet) to initiate translation at the correct starting point.
The remainder of the protein synthesis process involves elongation of the polypeptide chain in a three-step cycle. Each new amino acid is added to the chain in the same way that it was started, until a codon specifying a stop signal is encountered.