<P> Artificially constructed plasmids may be used as vectors in genetic engineering . These plasmids serve as important tools in genetics and biotechnology labs, where they are commonly used to clone and amplify (make many copies of) or express particular genes . A wide variety of plasmids are commercially available for such uses . The gene to be replicated is normally inserted into a plasmid that typically contains a number of features for their use . These include a gene that confers resistance to particular antibiotics (ampicillin is most frequently used for bacterial strains), an origin of replication to allow the bacterial cells to replicate the plasmid DNA, and a suitable site for cloning (referred to as a multiple cloning site). </P> <P> Plasmids are the most - commonly used bacterial cloning vectors . These cloning vectors contain a site that allows DNA fragments to be inserted, for example a multiple cloning site or polylinker which has several commonly used restriction sites to which DNA fragments may be ligated . After the gene of interest is inserted, the plasmids are introduced into bacteria by a process called transformation . These plasmids contain a selectable marker, usually an antibiotic resistance gene, which confer on the bacteria an ability to survive and proliferate in a selective growth medium containing the particular antibiotics . The cells after transformation are exposed to the selective media, and only cells containing the plasmid may survive . In this way, the antibiotics act as a filter to select only the bacteria containing the plasmid DNA . The vector may also contain other marker genes or reporter genes to facilitate selection of plasmid with cloned insert . Bacteria containing the plasmid can then be grown in large amounts, harvested, and the plasmid of interest may then be isolated using various methods of plasmid preparation . </P> <P> A plasmid cloning vector is typically used to clone DNA fragments of up to 15 kbp . To clone longer lengths of DNA, lambda phage with lysogeny genes deleted, cosmids, bacterial artificial chromosomes, or yeast artificial chromosomes are used . </P> <P> Another major use of plasmids is to make large amounts of proteins . In this case, researchers grow bacteria containing a plasmid harboring the gene of interest . Just as the bacterium produces proteins to confer its antibiotic resistance, it can also be induced to produce large amounts of proteins from the inserted gene . This is a cheap and easy way of mass - producing the protein the gene codes for, for example, insulin . </P>

When do plasmids replicate as compared to genomic dna