RIBOSOME

tiny particle that is present in large numbers in all living cells and which is the site of protein synthesis. Ribosomes are tiny granules that occur both as free particles within cells and as particles attached to the membranes of the endoplasmic reticulum. A ribosome measures about 200 angstroms in diameter and consists of about 40 percent protein and 60 percent RNA. It is composed of four RNA molecules and about 70 different proteins. In shape the ribosome is composed of two subunits, a larger one and a smaller one, each of which has a characteristic shape. Ribosomes are very numerous in a cell and account for a large proportion of its total RNA. Ribosomes are the sites at which information carried in the genetic code is converted into protein molecules. The ribosome is the site at which messenger RNA (mRNA) determines the sequencing of transfer RNA (tRNA) molecules, which in turn bind to amino acids to form proteins. The newly formed proteins detach themselves from the ribosome site and migrate to other parts of the cell for use.