Structure of plasmids

A plasmid is defined as an entire molecule of DNA that is replicated independently of the host chromosome. This alone does not differentiate them from viruses, so in addition a virus is defined as having extracellular structures for protection outside the host. A plasmid should carry no genes that are essential for the existence of the cell. This helps us to distinguish between very large plasmids (over 100 kbp or more, known as megaplasmids) and the smaller chromosomes. Plasmids often carry genes that help the cell to survive environmental challenges, such as a sudden rise in the concentration of mercury, but do not carry any of the genes for the central metabolic pathways. This can be tested experi- mentally by curing the cell, which is removing the plasmid. If the cell can still grow when cured, the DNA is a plasmid and not a chromosome. As there are more than 300 known types of naturally occurring plasmids for E. coli alone, this section will be restricted to a discussion of Bacterial plasmids, specifically those from E. coli. These have also been adapted to be the most useful, with applications in recombinant DNA technology.