Retroviruses

Biology: IB Diploma Higher Level, topic 1.4.2

Retroviruses subvert the Central Dogma using the enzyme reverse transcriptase, which allows the conversion of RNA to DNA.

Retroviruses (also called RNA viruses) use the enzyme reverse transcriptase to transcribe DNA from RNA.
Human immunodeficiency virus (HIV) is an example of a retrovirus.

HIV has an RNA genome covered by a protective layer of proteins, called a capsid.
The capsid is made up of protein subunits called capsomeres.
HIV is an enveloped virus, meaning it is surrounded by a phospholipid bilayer.
Glycoproteins embedded in the viral envelope are used to attach to host cells.
Other envelope proteins are the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions.

A retrovirus gains entry to a cell by binding to a receptor on the host cell.
Once inside the cell, the viral RNA genome is converted into a single-stranded piece of DNA using the enzyme reverse transcriptase.
A double-stranded copy of viral DNA is then made using DNA polymerase.
This double-stranded DNA is called cDNA.
cDNA is inserted into the host cell's DNA using the enzyme intergrase, which is packaged within the virus.
Host DNA is unwound by helicase prior to cDNA insertion and topoisomerases prevent supercoiling.

Once the reversibly transcribed viral DNA has been incorporated in the host cell genome, the virus hijacks the cell's machinery.
The viruses use the cell's machinery, such as RNA polymerase, to replicate its genome creating viral mRNA.
The viral mRNA then makes use of the cell's protein synthesis equipment to great new virions.
The last stage of viral replication is the release of the new virions produced in the host organism, which can then infect adjacent cells.

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