Everything about Retrovirus totally explained
A
retrovirus is any
virus belonging to the viral family
Retroviridae. They are enveloped viruses possessing an
RNA genome, and replicate via a
DNA intermediate. Retroviruses rely 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 virus then replicates as part of the cell's DNA.
Description of virus
The virus itself stores its nucleic acid, in the form of a +mRNA (including the 5'cap and 3'PolyA inside the virion) genome and serves as a means of delivery of that genome into
cells it targets as an
obligate parasite, and constitutes 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.
Multiplication
When retroviruses have integrated their own genome into the
germ line, their genome is
passed on to a following generation. These
endogenous retroviruses, contrasted with
exogenous ones, now make up 5-8% 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 like
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 2005 peer-reviewed article.
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 (making DNA from RNA) 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. It is important to note that a retrovirus must "bring" its own reverse transcriptase in its
capsid, otherwise it's unable to utilize the infected cell's enzymes to carry out the task, due to the unusual nature of producing DNA from RNA.
Because reverse transcription lacks the usual
proofreading of DNA replication, this kind of virus
mutates very often. This enables the virus to grow
resistant to antiviral pharmaceuticals quickly, and impedes the development of effective
vaccines and
inhibitors for the retrovirus.
Genes
Retrovirus genomes commonly contain these three
open reading frames that encode for proteins that can be found in the mature virus:
Provirus
This DNA can be incoporated into host genome as a provirus that can be passed on to progeny cells. In this way some retroviruses can convert normal cells into cancer cells.
Development of retroviruses
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, the
RNA world hypothesis, cellular organisms adopted the more chemically stable DNA when retroviruses evolved to create
DNA from the
RNA templates.
Classification
Exogenous
The following genera are included here:
Genus Alpharetrovirus; type species: Avian leukosis virus
Genus Betaretrovirus; type species: Mouse mammary tumour virus
Genus Gammaretrovirus; type species: Murine leukemia virus; others include Feline leukemia virus
Genus Deltaretrovirus; type species: Bovine leukemia virus; others include the cancer-causing 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 current knowledge of retroviruses, this is no longer appropriate. (The term oncovirus is still commonly used, though.)
Endogenous
Endogenous retroviruses are not formally included in this classification system, and are broadly classified into three classes, on the basis of relatedness to exogenous genera:
Class I are most similar to the gammaretroviruses
Class II are most similar to the betaretroviruses and alpharetroviruses
Class III are most similar to the spumaviruses
Treatment
Antiretroviral drugs are medications for the treatment of infection by retroviruses, primarily HIV. Different classes of antiretroviral drugs act at different stages of the HIV life cycle. Combination of several (typically three or four) antiretroviral drugs is known as highly active anti-retroviral therapy (HAART).
Genetic barrier
The genetic barrier is loosely defined as the difficulty for retroviruses to evade antiretroviral drugs by mutating into resistant types.
Further Information
Get more info on 'Retrovirus'.
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