Retrovirus

Retroviruses are viruses with RNA as genetic material. They belong to the family Retroviridae of Retroviruses. Once it has infected a cell, it converts its RNA into DNA by reverse transcription. This viral DNA is then inserted into the DNA of the host cell where it starts replicating. For eg., Human Immunodeficiency Virus (HIV).

Retrovirus Structure

It consists of two concentric outer circles of the lipid bilayer that contains the envelope protein complex embedded in it. The capsid proteins are hexagonal. The copies of the RNA genome are in the form of a loop bound by nucleoproteins.

The main virion components are:

Envelope

The retroviral envelope is composed of lipids and glycoproteins encoded by env genes. It serves three important functions:

  • the lipid bilayer protects the retrovirus from the extracellular environment.

  • helps the retrovirus to enter or exit the host cell through endosomal membrane trafficking.

  • direct entry into the cells by fusing with the membranes.

RNA

It consists of a dimer RNA with a 5′ cap and a 3′ poly A tail. It has non-coding regions at the terminal which help in replication, whereas, the internal regions encode virion proteins for gene expression.

Proteins

It consists of gag proteins, pol proteins, protease, env proteins.

Retrovirus Replication

Retrovirus

                                            ↓ Virus infects normal cell

Viral RNA introduced in the host cell

                                          ↓ Reverse Transcription

Viral DNA produced by Reverse Transcriptase

  ↓

Viral genetic material enters the nucleus of host cell

  ↓

Viral DNA integrates into the host genome

  ↓

Viral genes are transcribed and translated

  ↓

New virus particles assemble and come out of host cell

New viruses can infect other cells

Retroviral Vectors

Retroviral vectors are genetically engineered retroviruses. They have been used for the transfer of genes into the mammalian host cells for over 20 years.

The retroviral vectors derived from the Moloney murine leukaemia virus are the most common retrovirus.

They can efficiently integrate and replicate inside the genome of the host cells. This is the reason why they have been used as vectors in various gene therapies.

The retroviral vectors are created by replacing the gag, pol and env genes by therapeutic genes.

One of the limitations of using retroviral vectors in gene therapy is that the integrase enzyme can insert the genetic material in the middle of the original genes of the host cell. This causes the disruption of the gene.

Difference Between Retrovirus And Virus

Retrovirus Virus
These have RNA as genetic material. These have RNA and DNA as the genetic material.
They do not destroy the host cell. They destroy the host cell once they start replicating.
They undergo reverse transcription to convert the RNA into DNA. They do not undergo any such process.

Also read: Central Dogma

Retrovirus Examples

There are three types of retrovirus affecting humans:

HIV

HIV attacks the immune system of a person. It destroys the CD4 T-cells which are responsible for fighting against infections. Thus the immune system of a person becomes weaker and weaker. It can be transmitted through needles, body fluids, or from mother to a child during childbirth or breastfeeding. It cannot be cured through medications and treatment. AIDS is the last stage of HIV infection ad a person does not survive.

Human T-cell Lymphotropic Virus (HTLV) types I and II

The type I and II are closely related to each other. A person infected with HTLV I virus develops acute T-cell leukaemia. It affects the spinal cord and causes a neurological disorder. It was the first human retrovirus to be discovered and can be transmitted through blood transfusion, needle sharing, breastmilk, or sexual contact. HTLV II is also associated with neurological disorders and some types of blood cancers. However, very less is known about this type of virus.

Endogenous Retrovirus

The endogenous retroviruses consist of the genetic material from those extinct viruses whose genetic composition is similar to that of the extant retroviruses. These viruses have distributed among humans with the evolution over the years. They contribute 5% of the human genome and are passed on from one generation to another.

Also read: Virus

For more information on Retrovirus and related topics, visit BYJU’S website or go to BYJU’S app for further reference.

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Practise This Question

Given below is a sample of portion of DNA strand giving the base sequence on the opposite strands.  What is so special shown in it?
5'-GAATTC-3'
3'-CTTAAG-5'