Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transmission
DNA: Structure of Chromosomes and Genes
Some bacteria contain DNA in the form of plasmids

In addition to a very large circular chromosome located in the nuclear region, most Bacteria contain one or small multiple circular DNA molecules that exist freely in the Cell Cytoplasm. These extrachromosomal elements are called Plasmids (Fig. 27-20). For the most part, plasmids are very small and carry only a few genes—far fewer than the bacterial chromosome, which contains thousands. In some Cells, however, plasmids can be quite large. Plasmids carry Genetic information and replicate to form daughter plasmids that are partitioned into daughter cells upon Cell Division. Over many cycles of cell division, plasmids lead an independent existence, separate from the chromosomal DNA. However, they can occasionally integrate into the chromosomal DNA and subsequently excise from it just as readily.

Some plasmids carry genes that confer resistance in the host bacterium to Antibiotics such as tetracycline and streptomycin. Bacteria harboring such plasmids are resistant to these antibiotics. When treating a bacterial infection with an antibiotic, these cells can survive within The Human Body because the antibiotic kills only susceptible cells. The antibiotic-resistant cells can then proliferate and cause an infection that cannot be controlled by the antibiotic. For this reason, antibiotics should not be used indiscriminately to treat infections: it is always essential to ensure that the CAUSATIVE AGENT OF the disease is susceptible to the antibiotic being used. Plasmids can be transferred from antibiotic-resistant cells to susceptible Cells of the same or different species, thereby rendering those cells resistant as well.

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Fig. 27-20. Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF plasmids isolated from two bacterial species.

A. Plasmid pSC101 from E. coli, which confers tetracycline resistance to the cells. B. Plasmids from Neisseria gonorrhoeae, the bacterium that causes Gonorrhea. Most of the molecules are in the relaxed state. The twisted supercoiled plasmids (one of which is in the center of the micrograph) illustrate how remarkably efficient the compact packing of circular DNA can be achieved through negative supercoiling.

Another important feature of plasmids is that they can be readily isolated from bacterial cells. New genes from other species can be inserted into an isolated plasmid, and this modified plasmid can then be introduced back into its normal host cell. Such a plasmid, carrying a foreign Gene, will replicate and transcribe, and can prompt the host cell to synthesize Proteins encoded by the artificially inserted gene, even though it is not part of The Cell's normal genome. Later we will see how such recombinant DNAs are produced, transcribed, and translated to yield potentially useful products.