Plasmid and its types

A plasmid is an extrachromosomal, circular molecule inside a cell. Plasmids are most commonly found in bacterial cells. A plasmid is not a part of chromosomal DNA and can replicate autonomously. Plasmids often carry genes that may improve the chances of survival of the organism. For example, in bacteria, certain plasmids carry genes that give the bacterium the property of antibiotic resistance.

Plasmids are replicons. A replicon is a unit of DNA capable of replicating autonomously within a suitable host. Plasmids may be transferred from one bacterium to another. Inter-species transfers may also take place.

Three mechanisms regulate this transfer: transformation, transduction, and conjugation.

The host-to-host transfer of genetic material is called horizontal gene transfer. Plasmids are "naked" DNA; they do not carry  genes necessary to provide the encasing machinery for transfer to a new host. Some plasmids that encode the conjugative "sex" pilus necessary for their own transfer are probably the only exception to this phenomenon.

A plasmid can be anywhere between 1 to over 1,000 kbp in size. In some cells, there can be 1-1000 identical plasmids.

 

The relationship between microbes and plasmid DNA is neither parasitic nor mutualistic, as parasitism and mutualism imply that there are debilitating/exacting or symbiotic/commensal relations with the host organism. Plasmids, however, provide a mechanism for horizontal gene transfer within a population of microorganisms and typically provide a selective advantage given a certain environmental state.

 

Plasmids may elicit an array of advantageous conditions for the bacterium carrying them from resistance to naturally occurring antimicrobials in a particular niche, to the production of proteins that are toxic under similar circumstances . Plasmids can also allow the uptake and use of unconventional organic compounds that would facilitate survival when nutrients are scarce.

 

In order for the plasmids to replicate independently within a cell, they must possess a stretch of DNA that can act as an origin of replication. The self-replicating unit, in this case the plasmid, is called a replicon.

 

There are two ways using which a plasmid integrates itself into a host bacterium: Non-integrating plasmids replicate without becoming incorporated into the host genome, whereas episomes, the example shown below, can integrate into the host chromosome.

 

Plasmids are broadly classified into conjugative plasmids and non-conjugative plasmids.

Conjugative plasmids contain a set of transfer or tra genes which promote sexual conjugation between different cells. In the complex process of conjugation, plasmids may be transferred from one bacterium to another via sex pili encoded by the aforementioned tra genes (see figure). Non-conjugative plasmids are incapable of initiating conjugation, hence they can be transferred only with the assistance of conjugative plasmids.




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