Difference between Retrovirus and Bacteriophage

Viruses are the most abundant creature to be living on the planet. A virus is a small infectious agent that has either DNA or RNA as their genetic material. They infect animals, plants as well as bacteria. Depending on the organism they infect and the type of genetic material they have, viruses are divided into different types. Two of those types are retrovirus and bacteriophage. Let us discuss these viruses in detail.


A virus that is composed of single-stranded RNA as their genetic material are referred to as retroviruses. The RNA is mostly of a positive polarity and is 7-10 kb in size. They belong to the family Retroviridae. They have an outer protective envelope that is made up of lipids and glycoproteins.

The hallmark of a retrovirus is the ability to synthesise reverse transcriptase enzymes. Because their genetic material is RNA, they need to transcribe back into double-stranded DNA. This is made possible by the process of reverse transcription. The virus incorporates itself into the host’s genome and the host machinery is compelled to replicate and transcribe the viral DNA.

Retroviruses are of two types, based on the number of polyproteins they encode. Simple retroviruses that encode pol, gag and env proteins, and complex retroviruses that code six more accessory proteins in addition to the three polyproteins.

The infection of retrovirus is seen commonly in plants and animals. The glycoproteins on the envelope bind to receptor proteins on the cell surface on the host. The protective coat of the virus degrades and becomes a part of the host cell. The RNA and reverse transcriptase enzyme enters the host cell and starts replication. It is then integrated into the host genome. The host genome then replicates capsid proteins and viral RNA, and new retroviruses are gathered together which then attack the host cell.

The most common examples of retrovirus as seen in humans are Human Immunodeficiency Virus (HIV) and Human T-Lymphotropic Virus (HTLV).

Structural difference between retrovirus and bacteriophage.


A virus that infects bacteria and archaea are referred to as bacteriophages. They have a proteinaceous capsule on the outside. Their genetic material is either DNA or RNA, that is single or double-stranded. They are also known as a phage, informally.

The bacteriophage was discovered by Frederick W. Twort in Great Britain (1915) and Félix d’Hérelle in France (1917), separately. This particular class of virus is ubiquitous on this planet and can be found everywhere.

The phages are specific to a certain or a group of bacteria that it can infect. It usually binds to protein receptors on the surface of bacteria to infect them. After infection, the phage undergoes two types of life cycles. The first bacteriophage life cycle is lytic cycle, where the host cell is lysed as soon as the viral genetic material is replicated to aid in the release of new viral particles. Another is the lysogenic cycle, where the host cell is not immediately lysed but the genetic material is incorporated into the host genome and the phage may enter the lytic cycle at some later stages.

Examples of phage include T4, T2, T6, M13 and ƛphage. These were the very first phages to be studied. The T2 phage was used in the Hershey and Chase experiment to demonstrate that only the nucleic acids of the virus are injected into the host cell for replication and not protein molecules.

Retrovirus vs Bacteriophage




A virus that contains single-stranded positive sense RNA as their genetic material is referred to as retrovirus.

A virus that infects bacteria and archaea is referred to as bacteriophage.

Genetic Material

Always RNA.




Can be single-stranded or double-stranded.


It infects mostly plant and animal cells.

It infects only bacteria and archaea.


It contains reverse transcriptase enzymes and is a hallmark of the viruses.

It does not contain the reverse transcriptase enzyme.

Release of Viral Particle

The viral particle is released by pinching off the host’s cell membrane.

The viral particle is released by cell lysis, budding, or extrusion.

Life Cycle

The RNA is released in the host cell where it is reverse transcribed to form a DNA intermediate. The intermediate then gets incorporated into the host’s genome for replication.

The phage either undergoes lytic cycle where the viral cells take over the host cell’s machinery for replication, or lysogenic cycle where the genetic material is incorporated into the host’s genome.


HIV virus that infects human beings and causes AIDS.

ƛ phage that infects E.coli mainly.

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Frequently Asked Questions

What are the characteristics of retroviruses?

Retroviruses are characterised by single-stranded, positive sense RNA as their genetic material. They synthesise reverse transcriptase enzymes to form a double stranded DNA to incorporate themselves into the host genome.

Where does a retrovirus come from?

According to a study, retroviruses are believed to have originated from the marine environment.

Is Covid an RNA virus?

Yes, SARS-CoV-2 is a RNA virus.

Who discovered the human retroviruses?

Robert Gallo and his coworkers reported the first human retrovirus (HTLV-I) in 1980.

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