Illustration of a bacterium showing chromosomal DNA and plasmids (Not to scale)

Another way to classify plasmids is by function. There are five main classes:

Another major use of plasmids is to make large amounts of proteins. In this case, researchers grow bacteria containing a plasmid harboring the gene of interest. Just as the bacterium produces proteins to confer its antibiotic resistance, it can also be induced to produce large amounts of proteins from the inserted gene. This is a cheap and easy way of mass-producing the protein the gene codes for, for example, insulin.

In contrast, plasmids used in biotechnology, such as pUC18, pBR322 and derived vectors, hardly ever contain toxin-antitoxin addiction systems, and therefore need to be kept under antibiotic pressure to avoid plasmid loss.

Other types of plasmids are often related to yeast cloning vectors that include:

Plasmids are often used to purify a specific sequence, since they can easily be purified away from the rest of the genome. For their use as vectors, and for molecular cloning, plasmids often need to be isolated.

In the latter, much larger volumes of bacterial suspension are grown from which a maxi-prep can be performed. In essence, this is a scaled-up miniprep followed by additional purification. This results in relatively large amounts (several hundred micrograms) of very pure plasmid DNA.

Many commercial kits have been created to perform plasmid extraction at various scales, purity, and levels of automation.

Plasmid DNA may appear in one of five conformations, which (for a given size) run at different speeds in a gel during electrophoresis. The conformations are listed below in order of electrophoretic mobility (speed for a given applied voltage) from slowest to fastest:

The rate of migration for small linear fragments is directly proportional to the voltage applied at low voltages. At higher voltages, larger fragments migrate at continuously increasing yet different rates. Thus, the resolution of a gel decreases with increased voltage.

Because of its tight conformation, supercoiled DNA migrates faster through a gel than linear or open-circular DNA.

Many plasmids have been created over the years and researchers have given out plasmids to plasmid databases such as the non-profit organisations and . One can find and request plasmids from those databases for research. Researchers also often upload plasmid sequences to the , from which sequences of specific plasmids can be retrieved.