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incorporated into membranes, or packaged for storage, or exported outside of the cell. Ribosomes exist either as a single ribosome (that is, one ribosome translating an mRNA) or as polysomes (two or more ribosomes sequentially translating the same mRNA in order to make multiple copies of the same protein).

Ribosomes have the critical role of mediating the transfer of genetic information from DNA to protein. Ribosomes translate this code using an intermediary, the messenger RNA, which is a copy of the DNA that can be interpreted by ribosomes. To begin translation, the small subunit first identifies, with the help of other protein factors, the precise point in the RNA sequence where it should begin linking amino acids, the building blocks of protein. The small subunit, once bound to the mRNA, is then joined by the large subunit and translation begins. The amino acid chain continues to grow until the ribosome reaches a signal that instructs it to stop.

Many of the antibiotics used in humans and other animals to treat bacterial infections specifically inhibit ribosome activity in the disease-causing bacteria, without affecting ribosome function in the host-animal's cells. These antibiotics work by binding to a protein or RNA target in the bacterial ribosome and inhibiting translation. In recent years, the misuse of antibiotics has resulted in the natural selection of bacteria that are resistant to many of these antibiotics, either because they have mutations in the antibiotic's target in the ribosome or because they have acquired a mechanism for excluding or inactivating the antibiotic. see also Cell, Eukaryotic; Ribozyme; RNA; Translation.

Janice Zengel

Bibliography

Frank, Joachim. "How the Ribosome Works." American Scientist 86 (1998): 428-439

Garrett, Robert A., et al, eds. The Ribosome: Structure, Function, Antibiotics, and Cellular Interactions. Washington, DC: ASM Press, 2000

Karp, Gerald. Cell and Molecular Biology: Concepts and Experiments, 3rd ed. New York: John Wiley & Sons, 2002.

catalysts substances that speed up a reaction without being consumed (e.g., enzyme)

Ribozyme

Ribozymes are RNA molecules that catalyze chemical reactions. Most biological processes do not happen spontaneously. For example, the cleavage of a molecule into two parts or the linkage of two molecules into one larger molecule requires catalysts, that is, helper molecules that make a reaction go faster. The majority of biological catalysts are proteins called enzymes. For many years scientists assumed that proteins alone had the structural complexity needed to serve as specific catalysts in cells, but around 1980 the research groups of Tom Cech and Sidney Altman independently discovered that some biological catalysts are made of RNA. These two scientists were honored with the Nobel Prize in chemistry in 1989 for their discovery.

eukaryotic desribing an organism that has cells containing nuclei

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