Introduction to Plasmid DNA
What is Plasmid DNA?
A plasmid is a closed circular molecule of double-stranded DNA that range in size from 1 to >200 kb. Plasmids are found in various bacterial species, where they behave as additional genetic units inherited and replicated independently of the bacterial chromosome. However, they rely upon enzymes and proteins provided by the host for their successful transcription and replication.
Bacterial plasmids often contain genes that code for enzymes that can benefit the host cell. The encoded enzymes may be involved in resistance to, or production of, antibiotics, resistance to toxins found in the environment (e.g., complex organic compounds), or the production of toxins by the bacteria itself. For biotechnology applications, genes for a protein of interest can be inserted into the plasmid, and the resulting protein is then purified from the bacterial culture.
Diagram of a bacterium showing its bacterial chromosome and plasmids 
What is the main function of plasmid DNA in biotechnology?
Plasmids play a crucial role in biotechnology, particularly in genetic engineering. The purified DNA from plasmids finds diverse applications in downstream processes such as sequencing, PCR, transfection, protein expression, gene editing, gene therapy, and the production of RNA therapeutics and vaccines.
A specific site, or multiple sites, on the plasmid DNA is cleaved using restriction endonucleases, enabling the insertion of a foreign DNA element into the plasmid. The resulting circular recombinant DNA molecule, equipped with a selectable marker typically represented by an antibiotic resistance gene, is then introduced into bacterial cells through transformation.
The bacterial cells undergo selection in a media containing antibiotics, acting as a filter to isolate only the transformed cells. Afterward, the selected bacteria are cultivated in large quantities, and the plasmid of interest can be subsequently purified using various methods of plasmid purification.
A schematic diagram of a plasmid