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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.
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the dense light-refracting body within the cell nucleus of eukaryotes. The nucleolus consists mainly of ribo-nucleoproteins, that is, proteins that contain ribonucleic acid (RNA). Usually there are one to three nucleoli in a nucleus. In a few organisms (amoebas, testaceans) and in the growing egg cells of certain vertebrates there are many nucleoli in the nucleus. Sometimes the nucleolus consists of two differently staining parts (amphinucleoli).
The morphology of the nucleolus depends on the functional state of the cell. During mitosis, the nucleolus usually decomposes; an exception is the nucleolus of many protozoans. In the telophase of mitosis, the nucleolus forms anew in special areas of the nucleolar chromosomes, in which the genes that encode riboso-mal RNA are located. The numerous marginal nucleoli of egg cells are not directly connected to the chromosomes; they are formed on nonchromosomal copies of the ribosomal gene, which are first synthesized on the nucleolar chromosome.
The following inclusions can be seen in the nucleolus with an electron microscope: strands of intranucleolar chromatine containing DNA and corresponding either to the nucleolar chromosome itself or to its nonchromosomal copies, a zone of ribonu-cleoprotein fibers 5–10 nanometers (nra) thick, and a zone of ribonucleoprotein granules 15–20 nm thick (usually on the periphery of the nucleolus).
Molecules of preribosomal RNA, with a sedimentation constant of 45S and located in the fibrillar zone of the nucleolus, are synthesized on the genes of the intranucleolar chromatin. The preribosomal-RNA molecules are cleaved to produce molecules of ribosomal RNA with sedimentation constants of 18S and 28S; upon uniting with proteins, the molecules coalesce into granules, which are the precursors of the small and large subunits of the ribosomes. The granules separate from the nucleolus and enter the cytoplasm through the pores of the nuclear membrane. The ribosomes are assembled in the cytoplasm.
REFERENCESBusch, H., and K. Smetana. The Nucleolus. New York-London, 1970.
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I. B. RAIKOV