Prokaryotes are unicellular organisms that lack membrane-bound organelles or other internal membrane-bound structures. Their chromosome—usually single—consists of a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall outside the plasma membrane. The cell wall functions as a protective layer, and it is responsible for the organism’s shape. Some bacterial species have a capsule outside the cell wall. The capsule enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes pathogens more resistant to our immune responses. Some species also have flagella (singular, flagellum) used for locomotion, and pili (singular, pilus) used for attachment to surfaces including the surfaces of other cells. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.
Prokaryotes are divided into two different domains, Bacteria and Archaea, which together with Eukarya, comprise the three domains of life.
Major bacterial phyla include the Proteobacteria, the Chlamydias, the Spirochaetes, the photosynthetic Cyanobacteria, and the Gram-positive bacteria. The Proteobacteria are in turn subdivided into several classes, from the Alpha- to the Epsilon proteobacteria. Eukaryotic mitochondria are thought to be the descendants of alphaproteobacteria, while eukaryotic chloroplasts are derived from cyanobacteria.
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