Prokaryote

Prokaryote are organisms whose cells lack a nucleus and other organelles. Most of them are small, single-celled organisms that have a relatively simple structure. A plasma membrane surrounds the prokaryotic cells, however, within the cytoplasm, there are no internal membrane-bound organelles. Prokaryotes are distinct from eukaryotes in the sense that they have no nucleus and no membrane-bound organelles. There are two distinct classes of prokaryotes: bacteria and archaea. The genetic material of most prokaryotes is a single molecule of circular DNA. The dna is found within a nucleoid in prokaryotes. The nucleoid is a central area of the cell which does not have any surrounding nuclear membrane. There are plasmid DNA molecules other than chromosomal DNA in many prokaryotes which are small and circular and provide both genetic and survival advantages in adverse environments. Some prokaryotes can also form large colonies, for example, cyanobacteria. Some others have life cycles that consist of multicellular stages. The cytoskeleton of prokaryotes is much more primitive than that of eukaryotes. Besides homologs of acting and tubulin, flagellin is another major cytoskeletal protein of bacteria. It is the building block of the flagellum and provides the structural background of chemotaxis which is the basic physiological response of bacteria.

Primitive organelle-like intracellular structures can be found in some prokaryotes. In some groups of prokaryotes, membranous organelles like vacuoles are common as well as intracellular membranes like systems that mediate photosynthesis. Some species also have micro compartments which have many proteins that play specific physiological roles. [Tahirah Yasmin]