RNA resembles a long chain with the links in the chain made up of individual nucleotide. RNA carries out the instructions encoded in DNA, most biological activities are carried out by proteins. The accurate synthesis of proteins thus is critical to the proper functioning of cells and organisms. The linear order of amino acids in each protein determines its three-dimensional structure and activity.For this reason, assembly of amino acids in their correct order, as encoded in DNA, is the key to production of functional proteins. Three kinds of RNA molecules perform different but cooperative functions in protein synthesis. Synthesis of all protein chains in prokaryotic and eukaryotic cells begins with the amino acid methionine. In most mRNAs, the start (initiator) codon specifying this amino-terminal methionine is AUG.In a few bacterial mRNAs, GUG is used as the initiator codon, and CUG occasionally is used as an initiator codon for methionine in eukaryotes. The three codons UAA, UGA, and UAG do not specify amino acids but constitute stop (terminator) signals that mark the carboxyl terminus of protein chains in almost all cells.The sequence of codons that runs from a specific start site to a terminating codon is called a reading frame. This precise linear array of ribonucleotides in groups of three in mRNA specifies the precise linear sequence of amino acids in a protein and also signals where synthesis of the protein chain starts and stops.
