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eukaryote(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.
..... Click the link for more information. , 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.
..... Click the link for more information. 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).
..... Click the link for more information. ). 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
..... Click the link for more information. , the eukaryotes have comprised the taxonomic kingdoms ProtistaProtista
, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms.
..... Click the link for more information. , 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).
..... Click the link for more information. , 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.
..... Click the link for more information. ), 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.
..... Click the link for more information. ). In a recently proposed system they are called the eukarya and classified as an overarching group (domain) above the kingdom level.
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.