|Virus classification |
|Group: ||Group VI (ssRNA-RT) |
|Family: ||Retroviridae |
A retrovirus is a virus which has a genome consisting of two identical plus sense RNA molecules. It relies on reverse transcriptase to perform a kind of reverse transcription of its genome from RNA into DNA for insertion by integrase into the host's genome. The virus itself is just a storage form for its nucleic acid genome; the reverse transcription takes place in the host's cytosol. Any virus' genome integrated into the host's genome is called a provirus.
While transcription usually works from DNA to RNA, reverse transcriptase transcribes RNA into DNA. The term "retro" in retrovirus refers to this reversal of the central dogma of genetics.
Because reverse transcription is missing the usual "proofreading" of DNA transcription, this kind of virus mutates very often. This enables the virus to grow resistant to antiviral pharmaceuticals quickly. That is one of the main reasons why an effective vaccine for HIV has not been developed yet.
Among others, retrovirus genomes commonly contain three genes, that encode proteins that can be found back in the mature virus:
- gag codes for core and structural proteins of the virus.
- pol codes for reverse transcriptase, protease and integrase.
- env codes for the virus coat proteins.
Four identified human retroviruses (HTLV 1&2, HIV 1&2) attack CD4 cells. Another feature common to all retroviruses is a lipid envelope surrounding their capsid. It is essential for their function. This explains why retroviruses can be killed by just washing hands.
Studies of these viruses are what first demonstrated the synthesis of DNA from RNA templates, a fundamental mode for transferring genetic material that occurs in both eukaryotes and prokaryotes. Certain groups speculate that the processes followed by retroviruses (that is, RNA>DNA>RNA>Protein) may be the key to the evolution of DNA; thus, that in the "primordial soup," retroviruses evolved to create DNA from the RNA templates, and it was subsequently adopted by cellular organisms due to its increased stability.
Note: In view of current knowledge of retroviruses, the classification into three sub-families (Oncovirinae, Lentivirinae, and Spumavirinae) is no longer appropriate.
Note: Reverse transcriptase activity outside of retroviruses has been found in almost all eukaryotes, enabling the generation and insertion of new copies of retroposons into the genome.
- Retroviruses (http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/Retroviruses.html)
- NCBI retrovirus book online (http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=rv.TOC)