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A prokaryote is a single-celled organism that lacks a membrane-bound nucleus (karyon), mitochondria, or any other membrane-bound organelle.〔(【引用サイトリンク】title=Prokaryotes: Single-celled Organisms )〕 The word ''prokaryote'' comes from the Greek πρό- (''pro''-) "before" and καρυόν (''karyon'') "nut or kernel".〔Campbell, N. "Biology:Concepts & Connections". Pearson Education. San Francisco: 2003.〕〔(【引用サイトリンク】url=http://www.etymonline.com/index.php?term=prokaryote&allowed_in_frame=0 )〕 Prokaryotes can be divided into two domains, Archaea and Bacteria, with the remainder of species, called eukaryotes, in the third domain Eukaryota. In the prokaryotes all the intracellular water-soluble components (proteins, DNA and metabolites) are located together in the cytoplasm enclosed by the cell membrane, rather than in separate cellular compartments. Bacteria, however, do possess protein-based bacterial microcompartments, which are thought to act as primitive organelles enclosed in protein shells. Some prokaryotes, such as cyanobacteria may form large colonies. Others, such as myxobacteria, have multicellular stages in their life cycles. Molecular studies have provided insight into the evolution and interrelationships of the three domains of biological species. Eukaryotes are organisms, including humans, whose cells have a well defined membrane-bound nucleus (containing chromosomal DNA) and organelles. The division between prokaryotes and eukaryotes reflects the existence of two very different levels of cellular organization. Distinctive types of prokaryotes include extremophiles and methanogens; these are common in some extreme environments.〔 ==Structure== Prokaryotes have a prokaryotic cytoskeleton, albeit more primitive than that of the eukaryotes. Besides homologues of actin and tubulin (MreB and FtsZ), the helically arranged building-block of the flagellum, flagellin, is one of the most significant cytoskeletal proteins of bacteria, as it provides structural backgrounds of chemotaxis, the basic cell physiological response of bacteria. At least some prokaryotes also contain intracellular structures that can be seen as primitive organelles. Membranous organelles (or intracellular membranes) are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties, such as photosynthesis or chemolithotrophy. In addition, some species also contain protein-enclosed microcompartments, which have distinct physiological roles (e.g. carboxysomes or gas vacuoles). Most prokaryotes are between 1 µm and 10 µm, but they can vary in size from 0.2 µm to 750 µm (''Thiomargarita namibiensis''). . 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Prokaryote」の詳細全文を読む スポンサード リンク
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