Replication of plasmids

The plasmids of E. coli are complete, closed, circular (ccc) double-stranded DNA mol- ecules. This means that they are circular, joined at both ends, and are supercoiled within the cytoplasm. Typically the plasmids are about 5% of the genome size, and the mini- mum size is only around 2000 bp. The plasmid is replicated by the host cell machinery, using DNA polymerase III and all the other apparatus associated with it. Some plasmids (the episomes) integrate themselves into the chromosome and are replicated along with it, much like the lysogenic bacteriophages (Section F10). The number of plasmids per cell is known as the copy number, and this can vary between very low (stringent plasmids) with only one or two copies per cell, and very high (relaxed plasmids) with hundreds of copies per cell. The relaxed plasmids in particular place a considerable energetic burden on the cell, since replicating each base pair requires one molecule of ATP. The cell will try to relieve itself of the burden of carrying the plasmid if there is no reason to keep it, so to maintain a plasmid in a strain, some form of selection must be applied. In order to be replicated by the host machinery, a plasmid requires an origin of replication and a region of site-specific recombination much like a chromosome. However, some plasmids are replicated unidirectionally.