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(yo͞okâr`ē-ōt'), a cell or organism composed of cells that have a membrane-bound nucleus and organelles (mitochondria, chloroplasts; see cellcell,
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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, in biology) and genetic material organized in chromosomes in which the DNA is combined with histonehistone
, any of a class of protein molecules found in the chromosomes of eukaryotic cells. They complex with the DNA (see nucleic acid) and pack the DNA into tight masses of chromatin, which have the structure of coiled coils, much like a tangled telephone cord.
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 proteins. Eukaryotes are contrasted with the prokaryotes (see MoneraMonera,
taxonomic kingdom that comprises the prokaryotes (bacteria and cyanobacteria). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts; see cell, in biology).
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). Eukaryotes formed through the merger of prokaryotes, which predate them in the fossil record by some 2 billion years. In the five-kingdom system of classificationclassification,
in biology, the systematic categorization of organisms into a coherent scheme. The original purpose of biological classification, or systematics, was to organize the vast number of known plants and animals into categories that could be named, remembered, and
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, the eukaryotes have comprised the taxonomic kingdoms ProtistaProtista
or Protoctista
, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms.
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, FungiFungi
, kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites, symbionts, or saprobes (see saprophyte).
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, Plantae (see plantplant,
any organism of the plant kingdom, as opposed to one of the animal kingdom or of the kingdoms Fungi, Protista, or Monera in the five-kingdom system of classification.
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), and Animalia (see animalanimal,
any member of the animal kingdom (kingdom Animalia), as distinguished from organisms of the plant kingdom (kingdom Plantae) and the kingdoms Fungi, Protista, and Monera in the five-kingdom system of classification.
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). In a recently proposed system they are called the eukarya and classified as an overarching group (domain) above the kingdom level.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a single- and multiple-celled plant and animal organism in which the body of the cell, in contrast to the cells of a prokaryote, is differentiated into the cytoplasm and the nucleus enclosed by a membrane. The most recent system of the organic kingdom gives the eukaryotes the rank of a superkingdom (including the animal, mushroom, and plant kingdoms) and juxtaposes them to the superkingdom of the prokaryotes.

The genetic material of the nucleus of eukaryotes is organized into chromosomes that are capable of duplication and distribution through mitosis between daughter cells. The molecular basis of the chromosomes is deoxyribonucleic acid (DNA), which is closely associated with histones and other proteins. In most eukaryotes there is a typical sexual process, with the fusion of cell nuclei during fertilization and reduction division during meiosis. The cytoplasm of the cells of eukaryotes, in contrast to that of prokaryote cells, has a complex system of membranes that form an endoplasmotic network, the Golgi apparatus, the mitochondria, and other organoids.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A cell with a definitive nucleus. Also spelled eucaryote.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
(24) Alsmark et al., "Horizontal Gene Transfer in Eukaryotic Parasites; DaLage, Feller, and Janecek, "Horizontal Gene Transfer from Eukarya to Bacteria and Domain Shuffling: The Alpha-amylase Model"; Jackson et al., "A Horizontal Gene Transfer Supported the Evolution of an Early Metazoan Biomineralization Strategy"; Boto, "Horizontal Gene Transfer in Evolution: Facts and Challenges."
From volcanic origins of chemoautotrophic life to Bacteria, Archaea and Eukarya. Philosophical Transactions of the Royal Society B: Biological Sciences.
Interestingly, one of the characteristics of many archaeal proteins led to visual support that these "simpler" organisms may be more related to eukarya at certain levels than their bacterial counterparts.
Eukarya: The biological domain whose members are characterized by being composed of cells having internal membrane bound structures.
The three domains include diverse life forms such as the Eukarya (organisms, including humans, yeast, and plants, whose cells have a DNA-containing nucleus) as well as Bacteria and Archaea (two distinct groups of unicellular microorganisms whose DNA floats around in the cell instead of in a nucleus).
From the Eukarya evolved three sorts of multi-celled organisms: Plants, Fungi and Animals.
This GOGAT/GS pathway operates widely across all three taxa: bacteria, archaea, and eukarya. Glutamine synthase incorporates [NH.sub.3] into glutamate to produce glutamine, which in a coupled reaction, catalyzed by glutamate synthase, combines with 2-oxo-glutarate to generate two molecules of glutamate.
Later, however, comprehensive sequence comparisons of the nucleic acids and proteins of these microorganisms showed fundamental differences to bacteria, so that evolutionary biologists reclassified the five kingdoms of life to three domains: archaea, single-celled organisms lacking nuclei; bacteria, which also lack nuclei; and eukarya, organisms with nuclei (single-celled protists, single- and multi-celled algae, single- and multi-celled fungi, plants, animals, and humans).
While there are a multitude of PCR primers described in the literature that target large phylogenetic groups (e.g., Bacteria, Archaea, Eukarya), specific microbial groups/species (e.g., [beta]-proteobacteria, cyanobacteria, etc.) or functional genes (e.g., genes necessary for nitrogen fixation, ammonia oxidation, etc.) ; the selection of the appropriate PCR primers for a given study may not be trivial.
Coverage encompasses methods for microbes representing Bacteria, Archaea, and Eukarya, but aside from a chapter on bacteriophages, the manual does not cover viruses, algae, or protozoa.
The third lineage, Eukarya, not only contains unicellular organisms but also myriad multicellular organisms.