A plasmid is a small, circular, double-stranded DNA molecule that is distinct from a cell’s chromosomal DNA. Plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes.
plasmids can be classified according to function. If we are to classify plasmids based on function then there are 5 major classes of plasmids.
Fertility F plasmids :
Fertility plasmids contain tra
genes. They are capable of conjugation and
result in the expression of sex pili.
Resistance plasmids :
From the name its easier to deduce that they are plasmids which contain
genes that provide resistance against
antibiotics or poisons . That’s to say that resistance is their major function. Also known
Col plasmids :
Col plasmids contain genes that
code for bacteriocins, proteins that can kill
Degradative plasmids, which enable the
digestion of unusual substances,
Virulence plasmids :
The function of virulence plamid is to turn the
bacterium into a pathogen .
Plasmids can belong to more than one of
these functional groups.e.g salicylic acid.