ABSTRACT
Plasmid is a circle of self-replicating DNA, which is distinct from the chromosomal DNA and which contains genes that are generally not essential to the growth or survival of the cell. However, some plasmids can integrate into the host genome. The term “plasmid” was fi rst introduced by Joshua Lederberg in 1952 and defi ned as the generic term for any extra-chromosomal genetic particle that is separate from the chromosomal DNA with a capacity of autonomous (self) replication [1]. Later, Jacob and Wollmann introduced the concept of episome to refer to accessory genetic elements transmissible from cell to cell and present either free in the cytoplasm or integrated into the bacterial chromosome [2]. The term “episome,” which persisted in the literature for more than a decade, was somewhat confusing since it referred to both plasmid and phage genetic elements. Indeed, it included the fertility factor F, a large plasmid present either free in the cytoplasm of Escherichia coli (strains designated F+) or integrated at specifi c sites of the bacterial chromosome (strains designated Hfr for high frequency of homologous recombination), and the temperate bacteriophage lambda, which during its life cycle can be either integrated in the chromosome or extra-chromosome.
