Extra Chromosomal Genetic Element (Plasmid)

In addition to the normal DNA some bacteria also have extra chromosomal DNA (i.e. DNA other then present in chromosomes) it is known as plasmid. Plasmid lye freely in the cytoplasm of bacterial cell and is capable of replicating itself, independently from chromosomal DNA.

As plasmid is a circular piece of DNA, it contain some additional genes. Some plasmids can integrate to the bacterial DNA chromosomes and are called episomes.

Presence of this extra chromosomal DNA, provide some special features to the bacteria. Some plasmid called R plasmids which confer resistance to a number of antibiotics. Some of the R plasmids can transfer from one cell to other cell by conjunction.


Most bacteriophages, the virulent phages, undergo a rapid lytic growth cycle in their host cells. They inject nuclear DNA into the bacterium, which is replicated rapidly and also direct the synthesis of new phage proteins. Within 10 – 20 minutes, depending upon the phage, the new DNA combines the proteins to make whole phage particle, which are released by destruction of cell wall and lysis of cell.

However, some bacteria viruses, the temperate phages, which ordinary do not lysis the cell wall, carry the DNA in their chromosomal DNA and are called episomes. Like other episomes, such as the F factor these viral DNA get integrated into the bacterial genome; they are then known as prophases. Bacteria that carry prophases can be induced with UV light and other agents to make the prophases start to replicate rapidly and go through a lytic life cycle. Resulting in the lysis of cell and release of new phase particles.

Here phase particles build, will have part of cell DNA rather then complete phage DNA. When these phage attack on other bacteria, they introduce cell DNA as well together the phage DNA. This is how phage particles transfer cell DNA to other bacterial cells.

This we can define bacterial transduction as the transfer by a bacteriophage, serving as vector, of a portion of DNA from one bacterium (donor) to other (receiver). 

Generalized Transduction

If all fragments of bacterial DNA have a chance to enter a transducing phage, the process is called generalized Transduction. In this process phage start a lytic cycle, viral enzymes digest the cell DNA and break it into many small pieces. Any part of the bacterial chromosome may be incorporated into phage, during phage assembly and it usually not associated with viral DNA.

Specialized transduction

Bacterial gene can also be transduced by bacteriophages in another process known as specialized transduction in which certain temperate phage strains can transfer only a few restriction genes of the bacterial chromosomes. More specifically, the phages transduce only those bacterial genes adjacent to the prophages in the bacterial chromosomes. Thus the process is also called restricted transduction.

It occur when the phage DNA after incorporating into bacterial DNA (as in prophase ), again becomes free, the set of small DNA of bacterial chromosome bind to the phage DNA. In this way, when these phage infect another bacteria, genes of previous bacteria can also be introduce  to the new bacteria.

Bacterial Transformation

In 1928, an English health officer named Griffith injected mice with a mixture consisting of a few rough pneumococci and a large number of heat – killed smooth (capsulated and pathogenic) cells. (Living smooth pneumococci cause pneumonia in humans and other animals.) 

The mice subsequently died of pneumonia, and live smooth cells are isolated from their blood. Apparently, some factor responsible for the pathogenicity of the smooth bacteria had been transferred to the living rough bacteria and had transformed them into pathogenic smooth ones.

Thus transformation is the process whereby cell-free or naked DNA contain a limited amount of genetic information is transferred from one bacterial cell to another. The DNA is obtained from the donor cell by natural cell lysis or by chemical extraction. Once the DNA is taken up by the recipient cell, recombination occur. 

Bacteria that have inherited markers from the donor cells are said to be transformed.

Thus certain bacteria, when grown in the presence of dead cells, culture filtrates, or cell extracts of a closely related strain, will acquire and subsequently transmit a characteristic of the related strain.

Reference: Microbiology Pelczar

Gaurav Singh

Editor in Chief Medical Microbiology & RDT Labs - RDT Labs Magazine | BSc Medical Microbiology | MSc Microbiology

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