ABSTRACT
It is known that many prokaryote cells contain, in addition to the basic chromosome, small extrachromosomal DNAs that are called “plasmids.” A plasmid is an independent, stable, circular (or linear) piece of DNA in a bacterial cell that is not a part of the normal cell genome and that never becomes integrated into the host chromosome, capable of autonomous replicating (independently from the basic chromosome). Though many plasmids provide signi„cant selective replication advantages for the host cells (tolerance to antibiotics, heavy metals, etc.), most of them are cryptic, that is, do not appear in the cells’ phenotype. Plasmids that may contain from several thousands to hundreds of thousands of base pairs, and produce from one to several hundreds of replicas per cell, are stably inherited in many cell generations.
