PlasmidPlasmids constitute secondary genetic material in bacteria, which can be used if the need arises (see also Genome above). Plasmids can be present in one or several copies in a cell, as they replicate independently of the chromosome. A bacterium may carry multiple different plasmids.
Function
Plasmids may contain genetic information, which is necessary for:
- Antibiotic resistance.
- Synthesis of antibiotics such as streptomycin in Streptomyces spp.
- Synthesis of bacteriocins that are toxic to the strains of the same bacterial species, which do not contain the same plasmid. Example: colicines of E. coli.
- Conjugation [e.g. F (fertility) plasmid in E. coli].
- Synthesis of enzymes that degrade organic substances.
- Virulence genes (see Virulence factors), such as those of the genera Shigella, Salmonella and Yersinia. Virulence genes can also be found in so-called prophages, which is another type of secondary genetic material.
Practical use
Plasmids have been of great use in molecular biology including cloning of genes. Genetically modified plasmids, which contain only a few genes and a suitable cloning site, are then used.Updated: 2013-03-06.
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