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Plasmids are supercoiled DNA molecules present in most species of bacteria. These are not integrated into the host chromosome and are much smaller in length.

Plasmids are not necessary for the survival of a bacteria but can contain genes that are advantageous in changing environmental conditions, an example would be antibiotic resistance genes[1].

Plasmids have no replication machinery of their own and are reliant upon the host for duplication.

Plasmids are very useful as vectors and in recombinant DNA techniques. Desired genes can be inserted in and amplified up.

Examples of plasmids include the puC18, or the F plasmid. Note that the F plasmids are unusually large. This property allows large-scale genetic exchange between bacteria.

Plasmid Conformation

Plasmids exist in three forms: Open circle, supercoiled and linear. Supercoiled is when the plasmid has is tightly wound around itself in order to fit inside the cell. Open circle is when there is a nick in the outer ring of the plasmid and linear is when the plasmid is cut with a restriction enzyme. Concatamers also form which is where there are several repeats of plasmid DNA produced and not snipped. This forms one long chain of DNA which runs very slowly down the agarose gel[2].

When run on an agarose gel, supercoiled runs the fastest down the gel, than linear open circle.


  1. Maloy (1987), Microbial Genetics, 2nd edition, Jones and Bartlett Publishers.
  2. Biological Sciences - Microbiology: Ronen Borenstein and Niza Frenkel Cloning human herpesvirus 6A genome into bacterial artificial chromosomes and study of DNA replication intermediates PNAS 2009 106 (45) 19138-19143; published ahead of print October 26, 2009, doi:10.1073/pnas.090850410
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