A plasmid is double-stranded DNA not native to the regular chromosomal DNA of the organism. They are hoop-shaped, which is largely why the cell does not eliminate them. Most of the time, plasmids are found in bacteria. They are fairly short, from 1 to 400 kilobase pairs, and may be found in one copy or hundreds of copies in any given cell. Plasmid DNA is replicated separate from nuclear DNA, and is usually passed on to daughter cells.
Plasmids can confer several advantages onto their hosts. For instance, bacteria may take up plasmids from other bacteria or from a host that confers antibiotic resistance.
Episomes are a type of plasmid that integrates into the chromosomal DNA of its host. This creates a new modified bacteria that can consistently pass down traits like antibiotic resistance to its daughter cells.
Often, plasmids are used by genetic engineers as vectors to transfer genes from one organism to another. They can also be used to make many copies of desired genes for later use, or for the creation of large quantities of desired proteins.
Plasmids are found primarily in prokaryotes like bacteria, but are sometimes found in eukaryotic organisms; they are double-stranded DNA molecules separate from the cell's chromosomal DNA, and are typically circular in shape. Plasmids can be made up of from one to four hundred kilobase pairs. You'll find them alone, or you may find hundreds of copies of the same plasmid in a single cell. When the cell divides, the plasmids are duplicated to both daughter cells.
Plasmids provide a variety of advantages for the bacteria. The most commonly studied is antibiotic resistance; in addition, a resistant plasmid, once developed, can be passed to other bacteria as well. Some related plasmids are incompatible, and one must be eliminated from the cell line.
Episomes are a type of plasmid that can integrate itself into the chromosomal DNA of the host organism. Plasmids that are used in genetic engineering are called vectors; they transfer genes into the bacteria, and generally contain a genetic marker that can be selected for or against in the final resulting organism. Once genes have been transferred into a bacteria, genetic tests can be performed, or the bacteria can be used in different ways such as the mass-production of a specific protein.
Functions of plasmids include fertility, resistance, colicines (or proteins that kill other bacteria), degrative, or unusual digestive, properties, and virulence. Any given plasmid may belong to more than one of these groups.
Web Resources On Plasmid
Recombinant DNA and Gene Cloning
Book Resources On Plasmid
Plasmid Biology by Funnell & Phillips
The Biology of Plasmids by Summers