Editing Chromosome (section)

== Prokaryotes ==
{{Main|Nucleoid}}

The [[prokaryote]]s&nbsp;– [[bacteria]] and [[archaea]]&nbsp;– typically have a single [[circular chromosome]].<ref>{{cite journal | vauthors = Thanbichler M, Shapiro L | title = Chromosome organization and segregation in bacteria | journal = Journal of Structural Biology | volume = 156 | issue = 2 | pages = 292–303 | date = November 2006 | pmid = 16860572 | doi = 10.1016/j.jsb.2006.05.007 }}</ref> The chromosomes of most bacteria (also called [[genophore]]s), can range in size from only 130,000 [[base pair]]s in the [[endosymbiotic]] bacteria ''[[Candidatus Hodgkinia cicadicola]]''<ref name="VanLeuven-2014">{{cite journal | vauthors = Van Leuven JT, Meister RC, Simon C, McCutcheon JP | title = Sympatric speciation in a bacterial endosymbiont results in two genomes with the functionality of one | journal = Cell | volume = 158 | issue = 6 | pages = 1270–1280 | date = September 2014 | pmid = 25175626 | doi = 10.1016/j.cell.2014.07.047 | s2cid = 11839535 | doi-access = free }}</ref> and ''[[Tremblaya princeps|Candidatus Tremblaya princeps]]'',<ref>{{cite journal | vauthors = McCutcheon JP, von Dohlen CD | title = An interdependent metabolic patchwork in the nested symbiosis of mealybugs | journal = Current Biology | volume = 21 | issue = 16 | pages = 1366–72 | date = August 2011 | pmid = 21835622 | pmc = 3169327 | doi = 10.1016/j.cub.2011.06.051 | bibcode = 2011CBio...21.1366M }}</ref> to more than 14,000,000 base pairs in the soil-dwelling bacterium ''[[Sorangium cellulosum]]''.<ref>{{cite journal | vauthors = Han K, Li ZF, Peng R, Zhu LP, Zhou T, Wang LG, Li SG, Zhang XB, Hu W, Wu ZH, Qin N, Li YZ | title = Extraordinary expansion of a Sorangium cellulosum genome from an alkaline milieu | journal = Scientific Reports | volume = 3 | pages = 2101 | year = 2013 | pmid = 23812535 | pmc = 3696898 | doi = 10.1038/srep02101 | bibcode = 2013NatSR...3.2101H }}</ref>

Some bacteria have more than one chromosome. For instance, [[Spirochaete]]s such as ''[[Borrelia burgdorferi]]'' (causing [[Lyme disease]]), contain a single ''linear'' chromosome.<ref>{{cite journal | vauthors = Hinnebusch J, Tilly K | title = Linear plasmids and chromosomes in bacteria | journal = Molecular Microbiology | volume = 10 | issue = 5 | pages = 917–22 | date = December 1993 | pmid = 7934868 | doi = 10.1111/j.1365-2958.1993.tb00963.x | s2cid = 23852021 | url = https://zenodo.org/record/1230611 }}</ref> ''[[Vibrio]]s'' typically carry two chromosomes of very different size. Genomes of the genus ''[[Burkholderia]]'' carry one, two, or three chromosomes.<ref>{{Cite journal |last1=Touchon |first1=Marie |last2=Rocha |first2=Eduardo P.C. |date=January 2016 |title=Coevolution of the Organization and Structure of Prokaryotic Genomes |journal=Cold Spring Harbor Perspectives in Biology |language=en |volume=8 |issue=1 |pages=a018168 |doi=10.1101/cshperspect.a018168 |issn=1943-0264 |pmc=4691797 |pmid=26729648}}</ref>

=== Structure in sequences ===
Prokaryotic chromosomes have less sequence-based structure than eukaryotes. Bacteria typically have a one-point (the [[origin of replication]]) from which replication starts, whereas some archaea contain multiple replication origins.<ref>{{cite journal | vauthors = Kelman LM, Kelman Z | title = Multiple origins of replication in archaea | journal = Trends in Microbiology | volume = 12 | issue = 9 | pages = 399–401 | date = September 2004 | pmid = 15337158 | doi = 10.1016/j.tim.2004.07.001 }}</ref> The genes in prokaryotes are often organized in [[operon]]s and do not usually contain [[intron]]s, unlike eukaryotes.

=== DNA packaging ===
[[Prokaryote]]s do not possess nuclei. Instead, their DNA is organized into a structure called the [[nucleoid]].<ref>{{cite journal | vauthors = Thanbichler M, Wang SC, Shapiro L | title = The bacterial nucleoid: a highly organized and dynamic structure | journal = Journal of Cellular Biochemistry | volume = 96 | issue = 3 | pages = 506–21 | date = October 2005 | pmid = 15988757 | doi = 10.1002/jcb.20519 | s2cid = 25355087 | doi-access = free }}</ref><ref name="Le-2013">{{cite journal | vauthors = Le TB, Imakaev MV, Mirny LA, Laub MT | title = High-resolution mapping of the spatial organization of a bacterial chromosome | journal = Science | volume = 342 | issue = 6159 | pages = 731–4 | date = November 2013 | pmid = 24158908 | pmc = 3927313 | doi = 10.1126/science.1242059 | bibcode = 2013Sci...342..731L }}</ref> The nucleoid is a distinct structure and occupies a defined region of the bacterial cell. This structure is, however, dynamic and is maintained and remodeled by the actions of a range of histone-like proteins, which associate with the bacterial chromosome.<ref>{{cite journal | vauthors = Sandman K, Pereira SL, Reeve JN | title = Diversity of prokaryotic chromosomal proteins and the origin of the nucleosome | journal = Cellular and Molecular Life Sciences | volume = 54 | issue = 12 | pages = 1350–64 | date = December 1998 | pmid = 9893710 | doi = 10.1007/s000180050259 | s2cid = 21101836 | pmc = 11147202 }}</ref> In [[archaea]], the DNA in chromosomes is even more organized, with the DNA packaged within structures similar to eukaryotic nucleosomes.<ref>{{cite journal | vauthors = Sandman K, Reeve JN | title = Structure and functional relationships of archaeal and eukaryal histones and nucleosomes | journal = Archives of Microbiology | volume = 173 | issue = 3 | pages = 165–9 | date = March 2000 | pmid = 10763747 | doi = 10.1007/s002039900122 | bibcode = 2000ArMic.173..165S | s2cid = 28946064 }}</ref><ref>{{cite journal | vauthors = Pereira SL, Grayling RA, Lurz R, Reeve JN | title = Archaeal nucleosomes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 94 | issue = 23 | pages = 12633–7 | date = November 1997 | pmid = 9356501 | pmc = 25063 | doi = 10.1073/pnas.94.23.12633 | bibcode = 1997PNAS...9412633P | doi-access = free }}</ref>

Certain bacteria also contain [[plasmid]]s or other [[extrachromosomal DNA]]. These are circular structures in the [[cytoplasm]] that contain cellular DNA and play a role in [[horizontal gene transfer]].<ref name="Schleyden-1847" /> In prokaryotes and viruses,<ref name="Johnson-2000">{{cite journal | vauthors = Johnson JE, Chiu W | title = Structures of virus and virus-like particles | journal = Current Opinion in Structural Biology | volume = 10 | issue = 2 | pages = 229–35 | date = April 2000 | pmid = 10753814 | doi = 10.1016/S0959-440X(00)00073-7 }}</ref> the DNA is often densely packed and organized; in the case of archaea, by homology to eukaryotic histones, and in the case of bacteria, by [[Histone-like nucleoid-structuring protein|histone-like]] proteins.

Bacterial chromosomes tend to be tethered to the [[plasma membrane]] of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of the membranes (and the attached DNA).

Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally [[supercoiled]]. The DNA must first be released into its relaxed state for access for [[Transcription (genetics)|transcription]], regulation, and [[DNA replication|replication]].