The plasmid is a self-replicating piece of DNA separate from the chromosome. Usually, it has a circular shape and double helix. It is generally found in bacteria, sometimes also in eukaryotes. Plasmids range in size from 1 to 400 kilobase pairs. A cell can contain one copy of a large plasmid and hundreds of copies of smaller plasmids. There may even be thousands of copies of some artificial plasmids (such as those in the pUC plasmid series) that have been selected to have a high copy number.

Plasmid DNA Recovery

Plasmids are often used for the purification of certain DNA sequences because they can be easily separated from the rest of the genome. In order to use plasmids as vectors or for molecular cloning, they must be isolated first.

There are several methods of obtaining plasmid DNA from bacteria. These methods are called miniprep, midiprep and maxiprep. The first is often used to check that if the plasmid has the desired property. Although the DNA obtained is not very pure, it has enough quality for restriction digestion and some simple cloning techniques.

Large volume cell suspension is used in the maxiprep technique. The process is essentially a large-scale miniprep technique, with additional purification steps followed by a relatively large amount (several mg) of very pure plasmid DNA.