What Are Plasmids?

Plasmids are one of the most important factors in the success of biotechnology. 

The word plasmid was introduced Joshua Lederberg in 1952 as the American molecular biologist who was awarded the Nobel Prize in Physiology /Medicine in 1958. He was just 33 years old but he discovered that bacteria could exchange genes when they mated. That prize was shared with Edward L. Tatum and George Beadle for their work on genetics.

A Plasmid: What Is It ?

A plasmid is a simple piece of DNA that is separate from chromosomal DNA. It also replicates independently from chromosomal DNA. It is described as an autonomously replicating piece of DNA.

Plasmids occur naturally in bacteria but are sometimes found in eukaryotic organisms. Consider the 2-micrometer-ring in the brewing yeast, Sachharomyces cerevisiae. It is thought that they evolved in bacteria/prokaryotes and were part of their make-up. They are best known for expressing  products from antibiotic resistance genes which also means they develop resistance to antibiotics. 

The value of a plasmid is it can be modified by carrying a piece of inserted DNA which is inserted artificially. These vehicles are critical in synthetic biology. It will also replicate in E.coli bacteria which means the bacteria now becomes a plasmid DNA factory. It is possible to insert what is desired as a gene into the plasmid and have it replicate sufficient amounts of a desired protein without destroying the insert during the process. For the biotechnologist, they can now express proteins from all sorts of sources.

Plasmid Sizes

A bacteria has at least one circular chromosome which is often of 4.3 million base pairs (b.p.) with a length of 1.3mm. A plasmid is only 5,000 base pairs but there can be many of them residing in a cell. They only carry a few genes.

Multiplication Of Plasmids

They have a single origin of replication.

They are replicated by exactly the same machinery as for bacterial chromosomes. Some plasmids are copied at the same rate as a typical chromosome so a single cell often only has a single copy of a plasmid. Other plasmids are copied at a higher rate and can carry over 50 or more of them.

Plasmids can enter a bacteri with considerable ease. It probably accounts for there ability to confer antibiotic resistance in bacteria. That property also makes bacteria ideal as host cells because they can be transformed so easily with a recombinant piece of DNA.

Plasmids Can be Classified On the Basis Of Function

Some plasmids are fertility plasmids, usually called F Plasmid. These carry fertility genes (tra-genes) which are needed for conjugation and allow the transfer of genetic information between two cells.

We mentioned the Resistance plasmids (R Plasmid) that contains genes for building machinery for dealing with poisons and antibiotics.

The Col Plasmids are those that encode the antibacterial polypeptides called bacteriocins which is a protein that kills other strains of bacteria. The col proteins of E. coli are encoded by proteins such as Col E1.

First uses In Biotechnology

Plasmids are the first type of cloning vector where they are used to clone DNA in bacteria. These engineered plasmids are used to clone genetic material of up to 10,000 base pairs.

Two good examples are the pBR322 plasmid and the pUC18 plasmid. 

The pBR322 plasmid has what are now classic features. It is 4,261 base pairs with a single origin of replication (ori). It contains two genes that confer resistance to two different antibiotics – for ampicillin (amp) and for tetracyclin (tet). There are some unique recognition sequences called EcoR1 and BamH1. There are also insertion points for genes inside the resistance sequences for genes.

The pUC18 plasmid is much more complex 



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