Editing Chromosome (section)

=== 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]].