Plasmids are small, circular DNA molecules that may reproduce independently of the organism's chromosomal DNA or self-replicating molecules. Because of this, plasmids are sometimes known as extrachromosomal DNA.
Plasmids play a vital role in genetic engineering because they help with gene cloning and gene therapy.
The five types of plasmids are:
Resistance Plasmids - Resistance or R plasmids contain genes that help a bacterial cell defend against environmental factors such as poisons or antibiotics.
Virulence Plasmids - When a virulence plasmid is inside a bacterium, it turns that bacterium into a pathogen, which is an agent of disease.
Degradative Plasmids - Degradative plasmids contain special genes which help the host bacterium digest complex compounds like camphor, xylene, toluene, and salicylic acid.
Col Plasmids - Col plasmids contain genes that make proteins that kill other bacteria called bacteriocins, and thus defend the host bacterium.
Fertility F Plasmids - Fertility plasmids, also known as F-plasmids, contain transfer genes that allow genes to be transferred from one bacteria to another through conjugation.