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Post  sudheerkher on Fri Nov 14, 2008 2:49 pm

M. Burhan Khan

Definition - Plasmids or episomes are extra-chromosomal genetic material characteristic of prokaryote cells (i.e. bacteria) and capable of autonomous replication.

Description - These are usually circular, double-stranded DNA molecules that are not essential for the normal and vital functioning of the host bacterium but grant it properties such as toxigenicity and drug resistance.

Transfer - Plasmids may be transferred from one bacteria to another either by binary fission, conjugation, or bacteriophages.

Classification - Plasmids can be classified according to whether they are self transmissible (conjugating) or non self transmissible (non-conjugating), on the property encoded (sex, drug resistance, etc), or by restriction endonuclease fingerprinting amongst other methods.

Functionally plasmids are classified into five main classes:

1. Fertility F-plasmids: capable of self transmission, conjugation.
2. Resistance R-plasmids: contain genes that are drug/poison resistant.
3. Col-plasmids: coding for bacteriocins, which kill other bacteria.
4. Degradative plasmids: enable the digestion of unusual substances
5. Virulence plasmids: which turn the bacteria into a pathogen.
Uses –
1. Plasmids are being used to test antibiotic resistance of bacteria.
2. They are also used as a cheap and easy way of mass producing proteins such as insulin used in treatment today.


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