Plasmids are closed circles of double-stranded DNA,
ranging in size from 1 to 200 kilobases, found in many bacteria and in a few eukaryotic cells. They frequently carry genes
conferring antibiotic resistance; infective drug resistance, originally discovered in Shigella, is due to plasmids. Plasmids are
widely used as carriers of cloned genes, for example the E.
coli plasmid pBR322, and numbers of such plasmids may be
amplified by treating the donor cells with chloramphenicol,
which stops host cell DNA synthesis but not that of the plas_x0002_mid. Some bacteriophages may exist either as integrated or
free copies in the cell, and either case can be termed plasmids.
Yeast cells have been found to harbour some plasmids, and the
Ti plasmid of Agrobacterium can survive in either bacterial or
plant cells.
