Difference Between Retrovirus and Bacteriophage

Main Difference – Retrovirus vs Bacteriophage

Retrovirus and bacteriophage are two types of viruses. Viruses are simple organisms which consist of a core of genetic material, DNA or RNA, surrounded by a protein capsid. They require a host in order to complete their life cycle. Therefore, viruses are considered as intracellular parasites. Usually, retroviruses infect plant and animal cells and bacteriophages infect bacteria. But, retroviruses called prophages infect bacteria in rare occasions. The main difference between retrovirus and bacteriophage is that retroviruses consist of a single-stranded, positive-sense RNA molecule as their genetic material whereas bacteriophages consist of either single-stranded or double-stranded DNA or RNA genomes.

This article looks at,

1. What is Retrovirus
      – Definition, Characteristics, Features, Examples
2. What is Bacteriophage
      – Definition, Characteristics, Features, Examples
3. What is the difference between Retrovirus and Bacteriophage

What is a Retrovirus

Any virus that belongs to the family retroviridae, which carries the genetic blueprint in the form of RNA, is identified as a retrovirus. Retroviruses consist of their own reverse transcriptase enzyme in order to produce DNA from its single-stranded, positive-sense RNA genome. Reverse transcription is the reversal process to the cellular transcription that usually transcribes DNA into RNA. However, reverse transcriptase makes the RNA genome of the retrovirus available in the form of DNA that can be permanently incorporated into the host genome. The incorporation of the retroviral DNA (provirus) into the host genome is conducted by integrase. Genes of the retrovirus are expressed using the gene expression mechanisms of the host. Due to this successful gene delivery mechanism of retroviruses, they are used as a powerful gene transfer tool in Molecular Biology.

Human T-cell lymphotropic virus type 1 (HTLV-1) forms a cancer called adult T-cell leukemia (ALT) in humans. It is the first human virus which was discovered in humans. Human immunodeficiency virus (HIV) is another type of retrovirus which causes acquired immunodeficiency syndrome (AIDS) in humans. Human endogenous retrovirus (HERVs) gas become a fossil virus in the human genome over the cause of evolution. It implicates several diseases like multiple sclerosis. The structure of the HIV is shown in figure 1.

Figure 1: HIV

What is a Bacteriophage

Any group of viruses that infect bacteria is called bacteriophages or bacterial viruses. Bacteriophages are also capable of infecting archaea. The genetic material of phages can be either DNA or RNA and single-stranded or double-stranded. Phages are classified under several families like Inoviridae, Microviridae, Rudiviridae, and Techtiviridae. Three basic structural forms are present: icosahedral head with a tail, icosahedral without a tail, and filamentous. During phage therapy, bacteriophages are used to target multi-drug-resistant strains of disease-causing bacteria. Bacteriophages like lambda, M13, and MU are used in recombinant DNA technology. The structure of a typical tailed bacteriophage is shown in figure 2.

Figure 2: Typical tailed bacteriophage

Once a phage infects a bacterium, one of the two life cycles, either lytic or lysogenic, is followed by the phage. Lytic phages take over cellular machinery to produce phage components. The release of the new phages lyses the infected cell. But, lysogenic phages incorporate their nucleic acids to the bacterial genome and replicate along with the bacterium. Under certain conditions, lysogenic phages are induced to become lytic phages. Other life cycles of the phage include pseudolysogeny and chronic infection. Phages are preserved by psedolysogeny in order to undergo unfavorable growth conditions of the host. In a chronic infection, New phage particles are continuously produced over a considerable period of time without destroying the infected host cells.  

Difference Between Retrovirus and Bacteriophage

Definition

Retrovirus: Any virus that belongs to the group retroviridae which carries the genetic blueprint in the form of RNA is identified as a retrovirus.

Bacteriophage: Any group of viruses that infect bacteria is called bacteriophages.

Genome

Retrovirus: Retroviral genome is made up of single-stranded, positive sense RNA.

Bacteriophage: This is made up of either single-stranded or double-stranded DNA or RNA genomes.

Infection Mechanism

Retrovirus: RNA genome of the retrovirus is reverse transcribed into DNA which permanently attaches to the host genome. Retroviral genes are expressed along with the host genes.

Bacteriophage: Bacteriophages exhibit either lytic or lysogenic life cycles.

Reverse Transcriptase

Retrovirus: Retrovirus contains their own reverse transcriptase enzyme.

Bacteriophage: Bacteriophages do not contain a reverse transcriptase.

Attachment

Retrovirus: Retroviruses attach to plasma membrane proteins and glycoproteins.

Bacteriophage: Tail fibers of the bacteriophage attach to cell wall proteins of the bacteria.

Penetration

Retrovirus: Retroviral capsid enters the cell via endocytosis or fusion.

Bacteriophage: Viral DNA is injected into the bacterium by bacteriophages.

Uncoating

Retrovirus: Capsid proteins of the retrovirus are removed by enzymatic reactions after penetration.

Bacteriophage: Since viral genome is directly injected, no capsid uncoating is required inside the host.

Biosynthesis (Eclipse)

Retrovirus: Retroviral biosynthesis occurs in the nucleus in DNA viruses and in the cytoplasm in RNA viruses.

Bacteriophage: Bacteriophage synthesis occurs in the cytoplasm.

Release

Retrovirus: Enveloped retroviruses are bud out and non-enveloped viruses rupture the plasma membrane.

Bacteriophage: Bacteriophages are released from the lysis of the host cell.

Conclusion

Retroviruses and bacteriophages are two types of infectious bacteria. Retroviruses infect plants and animals while bacteriophages infect bacteria and archaea. Retroviruses consist of RNA in their genome.  bacteriophages consist of either DNA or RNA in their genome. The life cycle of the bacteriophage can be either lytic or lysogenic. However, the main difference between retrovirus and bacteriophage is in their hosts that are infected by each of the bacteria.

Reference:
1. “Retrovirus.” Encyclopædia Britannica. Encyclopædia Britannica, inc., n.d. Web. 26 May 2017. <https://www.britannica.com/science/retrovirus>.
2.”Bacteriophage.” Encyclopædia Britannica. Encyclopædia Britannica, inc., n.d. Web. 26 May 2017. <https://www.britannica.com/science/bacteriophage>.

Image Courtesy:
1. “HI-virion-structure en” By Thomas Splettstoesser (www.scistyle.com) – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Tevenphage” By Adenosine (original); en:User:Pbroks13 (redraw) http://commons.wikimedia.org/wiki/Image:Tevenphage.png (CC BY-SA 2.5) via Commons Wikimedia