Biochemical Techniques and Instrumentation

Isolation of Plasmid DNA by centrifugation

A plasmid is an extra chromosomal DNA molecule that can replicate independently. They are double stranded and circular. Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms. Their size varies from 1 to 1,000 kilo base pairs(kbp). The number of identical plasmids within a single cell can range from one to even thousands. Plasmids are "naked" DNA and do not encode genes necessary to encase the genetic material for transfer to a new host. Plasmids may carry genes that provide resistance to naturally occurring antibiotics in a competitive environmental niche, or alternatively the proteins produced may act as toxins under similar circumstances. Plasmids also provide bacteria with an ability to fix elemental nitrogen or to degrade recalcitrant organic compounds which provide an advantage under conditions of nutrient deprivation.