Extrachromosomal DNA

Extrachromosomal DNA is any DNA that is found outside of the nucleus of a cell. It is also referred to as extranuclear DNA or cytoplasmic DNA. Most DNA in an individual genome is found in chromosomes but DNA found outside of the nucleus also serves important biological functions.

In prokaryotes, nonviral extrachromosomal DNA is primarily found in plasmids whereas in eukaryotes extrachromosomal DNA is primarily found in organelles. Mitochondrial DNA is a main source of this extrachromosomal DNA in eukaryotes. Extrachromosomal DNA is often used in research of replication because it is easy to identify and isolate.

Extrachromosomal DNA was found to be structurally different from nuclear DNA. Cytoplasmic DNA is less methylated than DNA found within the nucleus. It was also confirmed that the sequences of cytoplasmic DNA was different from nuclear DNA in the same organism, showing that cytoplasmic DNAs are not simply fragments of nuclear DNA.

In addition to DNA found outside of the nucleus in cells, infection of viral genomes also provides an example of extrachromosomal DNA.

Although prokaryotic organisms do not possess a membrane bound nucleus like the eukaryotes, they do contain a nucleoid region in which the main chromosome is found. Extrachromosomal DNA exists in prokaryotes outside of the nucleoid region as circular or linear plasmids. Bacterial plasmids are typically short sequences, consisting of 1 kilobase (kb) to a few hundred kb segments, and contain an origin of replication which allows the plasmid to replicate independently of the bacterial chromosome. The total number of a particular plasmid within a cell is referred to as the copy number and can range from as few as two copies per cell to as many as several hundred copies per cell. Circular bacterial plasmids are classified according to the special functions that the genes encoded on the plasmid provide. Fertility plasmids, or f plasmids,allow for conjugation to occur whereas resistance plasmids, or r plasmids, contain genes that convey resistance to a variety of different antibiotics such as ampicillin and tetracycline. There also exists virulence plasmids that contain the genetic elements necessary for bacteria to become pathogenic as well as degradative plasmids that harbor the genes that allow bacteria to degrade a variety of substances such as aromatic compounds and xenobiotics. Bacterial plasmids can also function in pigment production, nitrogen fixation and the resistance to heavy metals in those bacteria that possess them.

...
Wikipedia