<P> 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 . </P> <P> In addition to DNA found outside of the nucleus in cells, infection of viral genomes also provides an example of extrachromosomal DNA . </P> <P> 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 . </P> <P> Naturally occurring circular plasmids can be modified to contain multiple resistance genes and several unique restriction sites, making them valuable tools as cloning vectors in biotechnology applications . Circular bacterial plasmids are also the basis for the production of DNA vaccines . Plasmid DNA vaccines are genetically engineered to contain a gene which encodes for an antigen or a protein produced by a pathogenic virus, bacterium or other parasite . Once delivered into the host, the products of the plasmid genes will then stimulate both the innate immune response and the adaptive immune response of the host . The plasmids are often coated with some type of adjuvant prior to delivery to enhance the immune response from the host . </P>

Extra-chromosomal dna within a bacterial cell is known as