Monday 13 March 2006
Definition: A retrovirus is a virus which has a genome consisting of two RNA molecules, which may or may not be identical. It relies on the enzyme reverse transcriptase to perform the reverse transcription of its genome from RNA into DNA, which can then be integrated into the host’s genome with an integrase enzyme.
The retrovirus itself stores its nucleic acid genome and serves as a means of delivery of that genome into cells it targets as an obligate parasite, and constitute the infection. Once in the host’s cell, the RNA strands undergo reverse transcription in the cytosol and are integrated into the host’s genome, at which point the retroviral DNA is referred to as a provirus.
When retroviruses have integrated their genome into the germ line, their genome is passed on to a following generation. These endogenous retroviruses, contrasted with exogenous ones, now make up 8% of the human genome. Most insertions have no known function and are often referred to as "junk DNA". However, many endogenous retroviruses play important roles in host biology, such as control of gene transcription, cell fusion during placental development in the course of the germination of an embryo, and resistance to exogenous retroviral infection. Endogenous retroviruses have also received special attention in the research of immunology-related pathologies, such as autoimmune diseases such as multiple sclerosis, although endogenous retroviruses have not yet been proven to play any causal role in this class of disease. The role of endogenous retroviruses in human gene evolution is explored in a recent peer-reviewed article. abstract.
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 lacks 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 impedes, for example, the development of an effective vaccine against HIV.
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.
It has been speculated that the RNA to DNA transcription processes used by retroviruses may have first caused DNA to be used as genetic material. In this model, cellular organisms adopted the more chemically stable DNA when retroviruses evolved to create DNA from the RNA templates.
Retrovirus genomes commonly contain these three genes, among others, that encode for proteins that can be found in the mature virus:
group-specific antigen (gag) codes for core and structural proteins of the virus;
polymerase (pol) codes for reverse transcriptase, protease and integrase; and,
envelope (env) codes for the retroviral coat proteins.
Thus far, four human retroviruses, HTLV-1, HTLV-2, HIV-1, HIV-2, have been found to attack CD4+ helper T cells.
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human endogenous retroviruses (HERVs)