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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. 


  1. Maloy (1987), Microbial Genetics, 2nd edition, Jones and Bartlett Publishers.

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