Ribosome

A ribosome is a small, dense organelle in cells that assembles proteins. Ribosomes are about 20nm in diameter and are composed of 65% ribosomal RNA and 35% ribosomal proteins (known as a Ribonucleoprotein or RNP). It translates messenger RNA (mRNA) to build a polypeptide chain (e.g., a protein) using amino acids delivered by Transfer RNA (tRNA). It can be thought of as a giant enzyme but, although it contains proteins, its active site is made of RNA, so ribosomes are now classified as "ribozymes".

Ribosomes build proteins from the genetic instructions held within a messenger RNA. Free ribosomes are suspended in the cytosol (the semi-fluid portion of the cytoplasm) or bound to the rough endoplasmic reticulum, or to the nuclear envelope. Since ribosomes are ribozymes, it is thought that they might be remnants of the RNA world. While catalysis of the peptide bond involves the C2' hydroxyl of tRNA's P-site adenosine in a sort of proton shuttle mechanism, the full function (ie, translocation) of the ribosome is reliant on changes in protein conformations.

Ribosomes are an important structure in the cell. Ribosomes were first observed in the mid-1950s by Romanian cell biologist George Palade in the electron microscope as dense particles or granules for which he would win the Nobel Prize. The term ribosome was proposed by scientist Richard B. Roberts in 1958:

During the course of the symposium a semantic difficulty became apparent. To some of the participants, microsomes mean the ribonucleoprotein particles of the microsome fraction contaminated by other protein and lipid material; to others, the microsomes consist of protein and lipid contaminated by particles. The phrase “microsomal particles” does not seem adequate, and “ribonucleoprotein particles of the microsome fraction” is much too awkward. During the meeting the word “ribosome” was suggested; this seems a very satisfactory name, and it has a pleasant sound. The present confusion would be eliminated if “ribosome” were adopted to designate ribonucleoprotein particles in the size range 20 to 100S.

The structure and function of the ribosomes and associated molecules, known as the translational apparatus, has been of research interest since the mid 20th century and is a very active field of study today.