Science Fair Project Encyclopedia
Spumavirus 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. The retroviral DNA, when integrated into the host's genome, is termed a provirus.
While transcription was classically thought to only occur from DNA to RNA, reverse transcriptase transcribes RNA into DNA. The term "retro" in retrovirus refers to this reversal of the Central dogma of molecular biology. Reverse transcriptase activity outside of retroviruses has been found in almost all eukaryotes, enabling the generation and insertion of new copies of retrotransposons into the host genome.
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, and 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 (group-specific antigen) codes for core and structural proteins of the virus;
- pol (polymerase) codes for reverse transcriptase, protease and integrase; and,
- env (envelope) codes for the retroviral coat proteins.
Four identified human retroviruses (HTLV 1&2, HIV 1&2) attack CD4 cells. Another feature common to all retroviruses is a lipid coating surrounding their capsid. It is essential for their propagation. This explains why retroviruses can be made inactive by just washing hands.
Studies of retroviruses led to the first demonstrated 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 the increased chemical stability of DNA.
The following genera are included here:
- Genus Alpharetrovirus ; type species: Avian leucosis virus
- Genus Betaretrovirus ; type species: Mouse mammary tumour virus
- Genus Gammaretrovirus ; type species: Murine leukeamia virus ; others include Feline leukemia virus
- Genus Deltaretrovirus ; type species: Bovine leukaemia virus ; others include Human T-lymphotropic virus
- Genus Epsilonretrovirus ; type species: Walleye dermal sarcoma virus
- Genus Lentivirus ; type species: Human immunodeficiency virus 1; others include Simian, Feline immunodeficiency viruses
- Genus Spumavirus ; type species: Chimpanzee foamy virus
These were previously divided into three subfamilies (Oncovirinae, Lentivirinae, and Spumavirinae), but with our current knowledge of retroviruses, this is no longer appropriate.
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