Plasmids

A plasmid is a circular extrachromosomal circular DNA containing from 1000 to 200,000 bp, and reproducing independent from the chromosomal DNA. Plasmids are inherited by daughter cells after cell division. Plasmid replication can be inhibited by curing the cells with compounds such as ethidium bromide. Some of the plasmids may exist in a limited number (1–3) of copies (stringent plasmids) or relatively large number (10–220) of copies (relaxed plasmids).

Relaxed plasmids are most useful as cloning vectors. Some plasmids cannot coexist, making them incompatible with other plasmids in the same cell.

There are several categories of plasmid :

. Conjugative plasmids, which carry genes that code for their own transfer to other cells. F factors or sex factors are conjugative plasmids that can become integrated into the chromosomes. E. coli strains that possess the chromosome-integrated F factors are called Hfr (High frequency of recombination).

. Resistance transfer factors (R factors) are plasmids that confer upon the host cell resistance to antibiotics (e.g., tetracycline, chloramphenicol, streptomycin) and heavy metals (e.g., mercury, nickel, cadmium). There is a great concern over these plasmids by the medical profession. The widespread use of antibiotics in medicine and agriculture results in the selection of multiple drug-resistant bacteria with R factors.

. Col factors are plasmids that code for production of colicins, which are proteinaceous bacterial inhibiting substances.

. Catabolic plasmids code for enzymes that drive the degradation of unusual molecules such as camphor, naphthalene, and other xenobiotics found in environmental samples. They are important in the field of pollution control. Plasmids can be engineered to contain desired genes and can be replicated by introduction into an appropriate host.