Ribosome structure. Illustration of the subunits found in ribosomes, the structures in cells that are the site of protein synthesis. When not involved in protein synthesis, they exist as two separate components, the large ribosomal subunit (red) and the small ribosomal subunit (yellow). When these two subunits bind together (right), they clamp around messenger ribonucleic acid (mRNA) which binds to the small subunit. The large subunit has two binding sites, with specific forms of transfer RNA (tRNA) bringing amino acids first to binding site A, and then adding them to the growing protein chain via binding site P. For this artwork without labels, see C023/8871.