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Related to nucleolus: Smooth endoplasmic reticulum


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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 following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



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.


Busch, H., and K. Smetana. The Nucleolus. New York-London, 1970.
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The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(cell and molecular biology)
A small, spherical body composed principally of protein and located in the metabolic nucleus. Also known as plasmosome.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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During the secondary vitellogenesis stage, the histological sections showed a prominence (80%) of secondary vitellogenic stage oocytes (late vitellin globule oocytes), which occupied a considerable portion of the ovary; a few previtellogenic oocytes (chromatin nucleolus, early chromatin perinucleolus and late chromatin perinucleolus) were found (20%) interspersed among the vitellogenic oocytes.
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In the nucleus, a single central nucleolus was visible.
Nucleolus number visualized at interphase cells after Ag-NOR banding varied from one in large size, which was the prevalent, to eight in small size (Figure 2C).
Nucleus (N) was normal more or less centrally placed, with a single prominent nucleolus with surrounding dense cytoplasm in the hepatocytes.