Plasmids

Most plasmids are double-stranded circular DNA molecules. Plasmids are extremely diverse, can vary in size from ~2 to several hundred kilobase pairs, replicate using different molecular mechanisms, and exist at different number of copies per cell, and their host range can be restricted to a few genera or include a large number of them. Although plasmids do not code for functions that are essential under all growth conditions, they can include genes specifying specialized functions such as virulence factors, antibiotic resistance, or metabolic pathways, which can be essential when the host cell is growing under certain conditions. The study of plasmids provided an extraordinary contribution to the understanding of a large number of biological processes and to the development of recombinant DNA technology.