Editing Cell (biology) (section)

=== Prokaryotic cells ===

{{main|Prokaryote}}

[[File:Prokaryote cell.svg|thumb|upright=1.25|Structure of a typical [[prokaryotic]] cell]]

[[Prokaryote]]s include [[bacteria]] and [[archaea]], two of the [[three domain system|three]] [[Domain (biology)|domains of life]]. Prokaryotic cells were the first form of [[life]] on Earth, characterized by having vital [[biological process]]es including [[cell signaling]]. They are simpler and smaller than eukaryotic cells, and lack a [[cell nucleus|nucleus]], and other membrane-bound [[organelle]]s. The [[DNA]] of a prokaryotic cell consists of a single [[Circular prokaryote chromosome|circular chromosome]] that is in direct contact with the [[cytoplasm]]. The nuclear region in the cytoplasm is called the [[nucleoid]]. Most prokaryotes are the smallest of all organisms, ranging from 0.5 to 2.0&nbsp;μm in diameter.<ref name="Black 2004 p. ">{{cite book |last=Black |first=Jacquelyn G. |title=Microbiology |publisher=Wiley |publication-place=New York Chichester |date=2004 |isbn=978-0-471-42084-2 |page=}}</ref>{{rp|78}}

A prokaryotic cell has three regions:

* Enclosing the cell is the [[cell envelope]], generally consisting of a [[plasma membrane]] covered by a [[cell wall]] which, for some bacteria, may be further covered by a third layer called a [[bacterial capsule|capsule]]. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as ''[[Mycoplasma]]'' (bacteria) and ''[[Thermoplasma]]'' (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of [[peptidoglycan]] in bacteria and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting ([[cytolysis]]) from [[osmotic pressure]] due to a [[hypotonic]] environment. Some eukaryotic cells ([[plant cell]]s and [[fungal]] cells) also have a cell wall.
* Inside the cell is the [[cytoplasm|cytoplasmic region]] that contains the [[genome]] (DNA), ribosomes and various sorts of inclusions.<ref name="NCBI">{{NCBI-scienceprimer |article=What Is a Cell? |url=https://web.archive.org/web/20130503014839/http://www.ncbi.nlm.nih.gov/About/primer/genetics_cell.html |access-date=3 May 2013 |date=30 March 2004}}</ref> The genetic material is freely found in the cytoplasm. Prokaryotes can carry [[extrachromosomal DNA]] elements called [[plasmid]]s, which are usually circular. Linear bacterial plasmids have been identified in several species of [[spirochete]] bacteria, including members of the genus ''[[Borrelia]]'' notably ''[[Borrelia burgdorferi]]'', which causes Lyme disease.<ref>European Bioinformatics Institute, [http://www.ebi.ac.uk/2can/genomes/bacteria/Borrelia_burgdorferi.html Karyn's Genomes: Borrelia burgdorferi] {{Webarchive|url=https://web.archive.org/web/20130506040937/http://www.ebi.ac.uk/2can/genomes/bacteria/Borrelia_burgdorferi.html |date=2013-05-06 }}, part of 2can on the EBI-EMBL database. Retrieved 5 August 2012</ref> Though not forming a nucleus, the [[DNA]] is condensed in a [[nucleoid]]. Plasmids encode additional genes, such as [[antibiotic resistance]] genes.
* On the outside, some prokaryotes have [[flagella]] and [[Pilus|pili]] that project from the cell's surface. These are structures made of proteins that facilitate movement and communication between cells.