<P> The term plasmid was introduced in 1952 by the American molecular biologist Joshua Lederberg to refer to "any extrachromosomal hereditary determinant ." The term's early usage included any bacterial genetic material that exists extrachromosomally for at least part of its replication cycle, but because that description includes bacterial viruses, the notion of plasmid was refined over time to comprise genetic elements that reproduce autonomously . Later in 1968, it was decided that the term plasmid should be adopted as the term for extrachromosomal genetic element, and to distinguish it from viruses, the definition was narrowed to genetic elements that exist exclusively or predominantly outside of the chromosome and can replicate autonomously . </P> <P> In order for plasmids to replicate independently within a cell, they must possess a stretch of DNA that can act as an origin of replication . The self - replicating unit, in this case the plasmid, is called a replicon . A typical bacterial replicon may consist of a number of elements, such as the gene for plasmid - specific replication initiation protein (Rep), repeating units called iterons, DnaA boxes, and an adjacent AT - rich region . Smaller plasmids make use of the host replicative enzymes to make copies of themselves, while larger plasmids may carry genes specific for the replication of those plasmids . A few types of plasmids can also insert into the host chromosome, and these integrative plasmids are sometimes referred to as episomes in prokaryotes . </P> <P> Plasmids almost always carry at least one gene . Many of the genes carried by a plasmid are beneficial for the host cells, for example: enabling the host cell to survive in an environment that would otherwise be lethal or restrictive for growth . Some of these genes encode traits for antibiotic resistance or resistance to heavy metal, while others may produce virulence factors that enable a bacterium to colonize a host and overcome its defences, or have specific metabolic functions that allow the bacterium to utilize a particular nutrient, including the ability to degrade recalcitrant or toxic organic compounds . Plasmids can also provide bacteria with the ability to fix nitrogen . Some plasmids, however, have no observable effect on the phenotype of the host cell or its benefit to the host cells cannot be determined, and these plasmids are called cryptic plasmids . </P> <P> Naturally occurring plasmids vary greatly in their physical properties . Their size can range from very small mini-plasmids of less than a 1 kilobase pairs (Kbp), to very large megaplasmids of several megabase pairs (Mbp). At the upper end, little can differentiate between a megaplasmid and a minichromosome . Plasmids are generally circular, however examples of linear plasmids are also known . These linear plasmids require specialized mechanisms to replicate their ends . </P>

When plasmids are isolated from bacterial cells they may exist in a number of forms