TRANSLATION AND RIBOSOMES
Ribosomes bring together all the mechanical machinery necessary for translation. They couple the tRNAs to their proper codons on the mRNA, facilitate the formation of peptide bonds between amino acids, and translocate the mRNA so that the next codon can be read. Each ribosome includes a large and a small subunit; each subunit contains ribosomal RNA (rRNA) and more than 50 proteins. A single mRNA molecule can be translated by groups of ribosomes called polyribosomes.

Protein synthesis is generally divided into three distinct stages:

• Initiation
• Repeating cycles of elongation
• Termination

In hesitation, small ribosomal subunit protein, plus initiation factors and the initiator tRNA, binds to AUG, the initiation codon, in the 5' region of the mRNA. This is followed by binding of the large ribosomal subunit. Elongation is a cyclic process in which amino acids are added one by one to the growing polypeptide chain. Elongation proceeds in a 5' to 3' direction along the mRNA as the polypeptide chain grows. Termination occurs when the ribosome reaches one of three special termination, or stop, codons, triggering release of the completed polypeptide chain.