ribosome(redirected from A site)
Also found in: Dictionary, Thesaurus, Medical.
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
..... Click the link for more information. ; nucleic acidnucleic acid,
any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. .
an intracellular particle concerned with protein biosynthesis that is found in the cells of all living organisms (bacteria, plants, and animals); each cell contains thousands or tens of thousands of ribosomes. Ribosomes are close to spherical in shape, although their outlines are complex and cannot be described by a simple geometric figure.
Two major classes of ribosomes are distinguished: 70S and 80S. The 70S ribosome has a molecular weight of about 3 × 106, a diameter of about 200-300 angstroms (Å), and a sedimentation coefficient of about 70 Svedberg units. The larger, 80S, ribosome has a molecular weight of about 4–5 × 106, a maximum diameter reaching 400 Å, and a sedimentation coefficient of about 80 Svedberg units. Ribosomes of the 70S class are characteristic of prokaryotes—cells that do not have a structured nucleus—including bacteria, actinomycetes, and blue-green algae, as well as of the chloroplasts and mitochondria of higher organisms. Ribosomes of the 80S class are found in the cytoplasm of all eukaryotes—organisms that have a structured cell nucleus.
Chemically, ribosomes are nucleoproteins that consist of ribonucleic acid (RNA) and protein. Ribosomes of the 70S class consist of 60-65 percent RNA and 40-35 percent protein, while ribosomes of the 80S class consist of about 50 percent RNA and 50 percent protein. The general principle of the structural organization of ribosomes is that they are composed of two unequal subparticles, or subunits, to which a ribosome may dissociate (for example, upon a decrease in the concentration of Mg2+ ions in a medium) and reassociate according to the formula
70S ⇄ 50S + 30S 80S ⇄ 60S + 40S
The large subunit (50S or 60S) consists of a high-polymer molecule of ribosomal RNA that has a molecular weight of 1.1-1.8 × 106, a low-polymer molecule of ribosomal RNA that has a molecular weight of 40,000, and several tens of protein molecules. The small subunit (30S or 40S) consists of a molecule of ribosomal RNA that exhibits high polymerism and has a molecular weight of 0.6-0.7 × 106 and from 20 (in 30S subunits) to 40 (in 40S subunits) various protein molecules.
The high-polymer ribosomal RNA binds proteins into a single ribonucleoprotein particle. It is experimentally possible to unwind ribosomes—the unit becomes more friable and the RNA unwinds into a strand. During this process, all proteins remain attached to the particle. Under other conditions, the sequential cleavage of proteins from RNA may be achieved; this process is known as the separation of ribosomes. This separation is reversible and under suitable conditions proteins and RNA spontaneously reunite into a ribonucleoprotein, which forms the native structure of a ribosome; this process is known as the self-assembly of ribosomes. The self-assembly of previously synthesized RNA and proteins is also believed to form ribosomes in cells.
Ribosomes have several functions in the synthesis of proteins, including the specific binding and retention of the components of the system that synthesizes proteins; these components include messenger RNA (mRNA), aminoacyl-transfer RNA (aminoacyl-tRNA), peptidyl-tRNA, guanosine triphosphate (GTP), and the protein translation factors EF-T and EF-G. Ribosomes also have catalytic functions, including the formation of peptide bonds and the hydrolysis of GTP. They also assist in the mechanical displacement of substrates, for example, mRNA and tRNA, and in translocation.
The functions of catalysis and the retention of components are distributed between two ribosomal subunits. The small ribosomal subunit contains segments that bind mRNA and aminoacyl-tRNA; it apparently has no catalytic functions. The large subunit contains both a catalytic segment that catalyzes the formation of the peptide bond and a center that participates in the hydrolysis of GTP. During protein biosynthesis, the large subunit retains the growing protein chain in the form of pepti-dyl-tRNA. Each subunit may perform its functions independently of the other subunit. The function of translocation, however, can only be accomplished by the whole ribosome and not by the individual ribosomal subunits.
REFERENCESpirin, A. S., and L. P. Gavrilova. Ribosoma, 2nd ed. Moscow, 1971.
L. P. GAVRILOVA and A. S. SPIRIN