Episomes, plasmids, insertion sequences, and transposons

Episomes, plasmids , insertion sequences, and transposons are elements of DNA (deoxyribonucleic acid ) that can exist independent of the main, or genomic, DNA.

An episome is a non-essential genetic element. In addition to its independent existence, an episome can also exist as an integrated part of the host genome of bacteria . It originates outside the host, in a virus or another bacterium. When integrated, a new copy of the episome will be made as the host chromosome undergoes replication. As an autonomous unit, the viral episome genetic material destroys the host cell as it utilizes the cellular replication machinery to make new copies of itself. But, when integrated into the bacterial chromosome they multiply in cell division and are transferred to the daughter cells. Another type of episome is called the F factor. The F factor is the best studied of the incompatibility groups that have the property of conjugation (the transfer of genetic material from one bacterial cell to another). The F factor can exist in three states. F+ is the autonomous, extrachromosomal state. Hfr (or high frequency recombination ) refers to a factor, which has integrated into the host chromosome. Finally, F, or F prime, state refers to the factor when it exists outside the chromosome, but with a section of chromosomal DNA attached to it. An episome is distinguished from other pieces of extrachromosomal DNA, such as plasmids, on the basis of their size. Episomes are large, having a molecular weight of at least 62 kilobases.

In contrast to episomes, a plasmid exists only as an independent piece of DNA. It is not capable of integration with the chromosomal DNA; it carries all the information necessary for its own replication. In order to maintain itself, a plasmid must divide at the same rate as the host bacterium. A plasmid is typically smaller than an episome, and exists as a closed circular piece of double stranded DNA. A plasmid can be readily distinguished from the chromosomal DNA by the techniques of gel electrophoresis or cesium chloride buoyant density gradient centrifugation. In addition to the information necessary for their replication, a plasmid can carry virtually any other gene . While not necessary for bacterial survival, plasmids can convey a selective advantage on the host bacterium. For example, some plasmids carry genes encoding resistance to certain antibiotics . Such plasmids are termed resistance or R factors. Other traits carried on plasmids include degradation of complex macromolecules, production of bacteriocins (molecules that inhibit bacterial growth or kill the bacteria), resistance to various heavy metals, or diseasecausing factors necessary for infection of animal or plant hosts. Such traits can then be passed on to other bacteria, as some (but not all) plasmids also have the ability to promote transfer of their genetic material, in a process called conjugation. Conjugation is a one-way event—the DNA is transferred from one bacterium (the donor) to another bacterium (the recipient). All plasmids belong to one of the 30 or more incompatibility groups. The groups determine which plasmids can co-exist in a bacterial cell and help ensure that the optimum number of copies of each plasmid is maintained.

Plasmids have been exploited in molecular biology research. The incorporation of genes into plasmids, which maintain large numbers of copies in a cell (so-called multicopy plasmids), allows higher levels of the gene product to be expressed. Such plasmids are also a good source of DNA for cloning .

Transposons and insertion sequences are known as mobile genetic elements. While they can also exist outside of the chromosome, they prefer and are designed to integrate into the chromosome following their movement from one cell to another. The are of interest to researchers for the insight they provide into basic molecular biology and evolution , as well as for their use as basic genetic tools. Transposons contain genes unrelated to the transposition of the genetic material from one cell to another. For example, Class 1 transposons encode drug resistance genes. In contrast, insertion sequences encode only the functions involved in their insertion into chromosomal DNA. Both transposons and insertion sequences can induce changes in chromosomal DNA upon their exiting and insertions, and so can generate mutations .

See also Bacteria; DNA (deoxyribonucleic acid); Electro-phoresis; Microbial genetics