Difference Between Generalized and Specialized Transduction

Key Difference – Generalized vs Specialized Transduction
 

Transduction is a mechanism which transfers DNA from one bacterium to another bacterium by a bacteriophage. Bacteriophage is a virus that infects and replicates within a bacterium. It is capable of attaching to the bacterial cell wall and injecting its DNA to the bacterium. Inside the bacterium, viral DNA replicates and creates necessary components and enzymes to make new many bacteriophages. During this process, bacterial DNA degrades into pieces and integrates with viral genome or, viral DNA directly integrates with bacterial DNA. New bacteriophages bear the bacterial DNA inside them. When these bacteriophages infect another bacteria, mixing of bacterial DNA occurs. Transduction can happen either by lytic cycle or lysogenic cycle based on the type of the bacteriophage. Hence, there are two types of transduction namely generalized transduction and specialized transduction. The key difference between generalized and specialized transduction is that generalized transduction is done by virulent bacteriophages in which bacterial cell is lysed when new bacteriophages are released while specialized transduction is done by temperate bacteriophages in which bacterial cell is not lysed, and viral DNA integrates with bacterial DNA and survives in prophage stage within the bacteria for several generations.

CONTENTS
1. Overview and Key Difference
2. What is Generalized Transduction
3. What is Specialized Transduction
4. Side by Side Comparison – Generalized vs Specialized Transduction
5. Summary

What is Generalized Transduction?

There are two types of bacteriophages: virulent and temperate. Virulent bacteriophage is capable of killing the host bacterium. They always undergo lytic life cycle which causes the death of host bacteria. Infection of a bacterium by a virulent bacteriophage and transferring bacterial DNA to another bacterium during the second infection is known as generalized transduction. Hence, generalized transduction can be defined as the transfer of bacterial DNA from one bacterium to another bacterium by a virulent bacteriophage during the lytic cycle of the bacteriophage. Bacterial DNA transfer happens due to errors of genetic material packaging in the new phages. The packaging of newly replicated viral DNA into new phages shows low fidelity. Hence, during the genetic material packaging, small pieces of bacterial DNA or recombined bacterial DNA with viral DNA can be included into phages wrongly. If bacterial DNA is inserted inside the viral capsid by chance, the second infection introduces this DNA into another bacterium. Hence, the transduction completes between two bacteria successfully.

After the infection, virulent phages are capable of controlling bacterial cell machinery to replicate its own DNA. The virus also becomes capable of degrading bacterial chromosome into small pieces and causes sudden disruption of bacterial cell wall for the release of assembled phages causing cell death.

Generalized Transduction Process

Generalized transduction is a fast process in which bacteria die within a short time period. Bacteriophage is capable of breaking bacterial DNA into pieces, destroying the bacterial cell. The steps of generalized transduction can be summarized as follows.

1. A virulent (lytic) bacteriophage infects a bacterium.
2. The phage genome enters the bacterial cell.
3. Virus governs the bacterial metabolic mechanisms to make its own DNA and other necessary components and enzymes.
4. Bacterial DNA hydrolyses into small pieces.
5. Genetic material packs inside the new phages. Occasionally bacterial DNA fragments pack in new phage capsids
6. Bacterial cell lyses and releases the new phages.
7. When transduced phage infects another bacterium, the previous bacterial DNA incorporates into a new.

What is Specialized Transduction?

Temperate bacteriophages show lysogenic life cycles. They are involved with specialized transduction process in which a fragment of bacterial DNA is transferred from one bacterium to another bacterium due to an error. Hence, specialized transduction can be defined as the transfer of donor bacterial DNA to another bacterium by the temperate bacteriophages. When temperate phages infect bacteria, they are able to integrate viral DNA into bacterial chromosome and remain in the prophage stage for several bacterial generations without releasing it from the bacterial genome. During the bacterial genome replication, viral DNA is subject to replication and enters into new bacterial cells and survives. However, when the prophages are induced by the certain factors, viral DNA detaches from the bacterial chromosome. Sometimes during this detachment, fragments of bacterial chromosomes detach and remain attached to prophage DNA. Due to the induction, phages undergo lytic cycle afterward. Viral genome replicates with attached bacterial DNA and packs inside new capsids and makes new phages. New phages release the bacterial cell by lysis. When a new phage infects another bacterium, bacterial DNA transfers to it.

Specialized Transduction Process

The steps of specialized transduction can be summarized as follows.

1. A temperate bacteriophage infects a bacterium.
2. Viral DNA integrates into bacterial chromosome and becomes the prophage stage
3. Viral DNA remains within bacteria for several generations
4. Upon a spontaneous induction, viral DNA detaches the bacterial chromosomal DNA.
5. Fragments of bacterial DNA detach from the bacterial chromosome with viral DNA.
6. Viral DNA replicates together with bacterial genes and package inside new capsids and make new phages.
7. Bacterial cell lyses and releases the new phages.
8. New phages infect new bacteria.
9. Bacterial DNA mixes with new bacteria during the infection.

   Figure 02: Specialized transduction shown by lambda phage

What is the difference between Generalized and Specialized Transduction?

Summary – Generalized vs Specialized Transduction

Transduction is the process of transferring bacterial DNA from one bacterium to another by a virus. It is a natural process that happens through lytic or lysogenic cycles. Virulent phages show generalized transduction. Temperate phages show specialized transduction. During generalized transduction, the virus destroys the bacterial cell. In specialized transduction, bacterial cells are not quickly destroyed unless there is an induction. This is the main difference between generalized and specialized transduction. Viral DNA integrates into the bacterial chromosome in specialized transduction and integration does not occur in generalized transduction.

Reference:
1. Boundless. “Bacterial Transduction – Boundless Open Textbook.” Boundless. Boundless, 08 Aug. 2016. Web. 26 Apr. 2017
2. “Transduction – Generalized Transduction.” Chromosome, Genetic, Viral, and Dna – JRank Articles. N.p., n.d. Web. 26 Apr. 2017
“Transduction (prokaryotes).” Nature News. Nature Publishing Group, n.d. Web. 26 Apr. 2017

Image Courtesy:
1. “Lytic Cycle” by xxoverflowed (CC BY 2.0) via Flickr
2. “Lambda Phage” By Tinastella – Own work (Public Domain) via Commons Wikimedia