Pierce, Marcia M. Department of Biological Sciences, Eastern Kentucky University, Richmond, Kentucky.
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- Bacteria and Archaea
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Any of the unicellular microorganisms belonging to the Bacteria and Archaea domains that lack a true nucleus or any other membrane-bound organelle. Taxonomists divide prokaryotic organisms into two domains: Bacteria and Archaea. In the past, all prokaryotes were collected into a single taxonomic grouping termed the Prokaryotae, but this term is no longer in use. Prokaryotic cells (see illustration) are usually microscopic, but some are visible to the naked eye. Instead of a nucleus, prokaryotic cells contain a nucleoid, which is a deoxyribonucleic acid (DNA)–dense region of the cell in which DNA replication and transcription occur. The chromosome of these cells is circular and, in contrast to eukaryotic cells, continuously codes for cellular proteins and other molecules. Some cells also contain a plasmid, which is a smaller separate piece of circular DNA. The ribosomes of prokaryotes are found floating loose in the cytoplasm (in eukaryotic cells, the ribosomes are found in or on the rough endoplasmic reticulum), and these cell organelles convert stored genetic information into protein molecules. Most prokaryotes are found as single-cell microorganisms; however, some species form filamentous, mycelial, or colonial structures. See also: Archaea; Bacteria; Biological classification; Cell organization; Chromosome; Deoxyribonucleic acid (DNA); Eukaryota; Plasmid; Ribosomes
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