Plasmids first hit the scientific headlines when it was discovered that they are the means by which bacteria are able to transfer properties, such as resistance to an antibiotic, from one cell to another. It is now known that plasmids are circular, double-stranded molecules of non-chromosomal DNA that contain their own origin of replication, and can therefore replicate independently within the bacterial cell. They are considerably smaller than the chromosomal DNA, but can contain genes, not only for antibiotic resistance, but also for antibiotic synthesis, toxin production, nitrogen fixation, production of degradative enzymes, and conjugation. The plasmids can be transferred from one cell to another, and therefore act as carriers or “vectors” of the extrachromosomal genes they contain.
