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Small particles, present in large numbers in every living cell, whose function is to convert stored genetic information into protein molecules. In this synthesis process, a molecule of messenger ribonucleic acid (mRNA) is fed through the ribosome, and each successive trinucleotide codon on the messenger is recognized by complementary base-pairing to the anticodon of an appropriate transfer RNA (tRNA) molecule, which is in turn covalently bound to a specific amino acid. The successive amino acids become linked together on the ribosome, forming a polypeptide chain whose amino acid sequence has thus been determined by the nucleic acid sequence of the mRNA. The polypeptide is subsequently folded into an active protein molecule. Ribosomes are themselves complex arrays of protein and RNA molecules, and their fundamental importance in molecular biology has prompted a vast amount of research, with a view to finding out how these particles function at the molecular level. See Genetic code, Protein, Ribonucleic acid (RNA)
Ribosomes are composed of two subunits, one approximately twice the size of the other. In the bacterium Escherichia coli, whose ribosomes have been the most extensively studied, the smaller subunit (30S) contains 21 proteins and a single 16S RNA molecule. The larger (50S) subunit contains 32 proteins, and two RNA molecules (23S and 5S). The overall mass ratio of RNA to protein is about 2:1. Cations, in particular magnesium and polyamines, play an important role in maintaining the integrity of the ribosomal structures. The ribosomes are considerably larger in the cytoplasm of higher organisms (eukaryotes). Nevertheless, all ribosomal RNA molecules have a central core of conserved structure, which presumably reflects the universality of the ribosomal function. See Cell (biology), Organic evolution
The process of protein biosynthesis is essentially very similar in both prokaryotes and eukaryotes; what follows is a brief summary of what happens in E. coli. The first step is that an initiator tRNA molecule attached to the amino acid N-formyl methionine recognizes its appropriate codon on a mRNA molecule, and binds with the mRNA to the 30S subunit. A 50S subunit then joins the complex, forming a complete 70S ribosome. A number of proteins (initiation factors, which are not ribosomal proteins) are also involved in the process. At this stage, the initiator aminoacyl tRNA occupies one binding site, the P-site (peptidyl site), on the ribosome, while a second tRNA binding site (the A-site, or aminoacyl site) is free to accept the next aminoacyl tRNA molecule. In the subsequent steps, the elongation process aminoacyl tRNA molecules are brought to the A-site as ternary complexes together with guanosine triphosphate (GTP) and a protein factor (elongation factor Tu). Once an aminoacyl tRNA is in the A-site, the initiator amino acid (or at later stages the growing polypeptide chain) is transferred from the P-site tRNA to the A-site aminoacyl tRNA. It is not clear whether this peptidyl transferase activity requires the active participation of a ribosomal component. After peptide transfer has taken place, the peptide is attached to the A-site tRNA, and an “empty” tRNA molecule is at the P-site. The peptidyl tRNA complex must now be translocated to the P-site, in order to free the A-site for the next incoming tRNA molecule. Here again protein factors and GTP are involved, and it is probable that the empty tRNA occupies a third ribosomal site (exit or E-site) before finally leaving the complex. Studies show that elongation factor G hydrolyzes GTP to bring about translocation of the tRNA molecules. The protein chain is completed by the appearance of a “stop” codon in the mRNA. This is recognized by yet another set of protein factors, which causes the completed polypeptide chain to be released from the ribosome. At any one time a number of ribosomes are engaged in the reading of a single mRNA molecule, which leads to the appearance of polyribosomes (polysomes). See Molecular biology