RETROVIRUS

A retrovirus is a type of virus that inserts a copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backwards). The new DNA is then incorporated into the host cell genome by an integrase enzyme, at which point the retroviral DNA is referred to as a provirus. The host cell then treats the viral DNA as part of its own genome, transcribing and translating the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus.

Retroviruses comprise a large and diverse family of enveloped RNA viruses defined by common taxonomic denominators that include structure, composition, and replicative properties. The virions are 80–100 nm in diameter, and their outer lipid envelope incorporates and displays the viral glycoproteins. The shape and location of the internal protein core are characteristic for various genera of the family. The virion RNA is 7–12 kb in size, and it is linear, single-stranded, non-segmented, and of positive polarity. The hallmark of the family is its replicative strategy which includes as essential steps reverse transcription of the virion RNA into linear double-stranded DNA and the subsequent integration of this DNA into the genome of the cell.

Human Endogenous and Exogenous Retroviruses

Similar to other vertebrate animals, humans possess retroviruses that exist in two forms: as normal genetic elements in their chromosomal DNA (endogenous retroviruses) and as horizontally-transmitted infectious RNA-containing viruses which are transmitted from human-to-human (exogenous retroviruses, e.g. HIV and human T cell leukemia virus, HTLV). Endogenous retroviruses in animals and humans probably evolved from transposable elements, some of them gaining the ability to package themselves in a virion structure, leave the cell and infect another cell.

Some retroviruses contain oncogenes. They are so called because their expression in virus-infected cells is associated with tumour development. Retroviral oncogenes are derived from cellular genes picked up during viral integration into host DNA, way back in evolution: Most oncogenes code for proteins with growth promoting properties. Their expression can lead to uncontrolled proliferation of the infected cell. This may contribute to tumour development. None of the retroviruses known to infect humans have oncogenes.

Many animal retroviruses are defective. Defective viruses are viruses that have lost a gene essential for replication and can therefore only undergo productive infection if the cell that is harbouring the virus is super-infected with a helper virus, which can supply the function of the lost gene.

Thanks and Regards

Jessica

Managing Editor

Journal of HIV and Retrovirus

Email ID: Retrovirus@eurorendezvous.org