Difference between revisions of "Retrovirus" - New World Encyclopedia
Jyoti Sharma (talk | contribs) |
Jyoti Sharma (talk | contribs) |
||
| Line 21: | Line 21: | ||
A virus (Latin, poison) is a submicroscopic particle that can infect the [[cell]]s of a biological organism. A virus stores its nucleic acid genome and serves as a means of delivery of that genome into cells it targets to serve as obligate parasites and constitutes an infection. An obligate parasite is an organism that cannot live independently of its host. | A virus (Latin, poison) is a submicroscopic particle that can infect the [[cell]]s of a biological organism. A virus stores its nucleic acid genome and serves as a means of delivery of that genome into cells it targets to serve as obligate parasites and constitutes an infection. An obligate parasite is an organism that cannot live independently of its host. | ||
| − | Once in the host cell, the RNA strands of a retrovirus undergo [[reverse transcription]] in the cytosol (internal fluid of the cell) and are integrated into the host's genome, at which point the retroviral DNA is referred to as a ''provirus''. Reverse transcription refers to the opposite of the normal transcription process which involves the synthesis of RNA from DNA. Hence, reverse transcription is the synthesis of double stranded DNA from RNA. Retroviruses rely on the enzyme ''reverse transcriptase'' to perform this reverse transcription of its genome, which is then integrated into the host's genome with the use of an integrase enzyme. The viral genes become part of the cell for its entire lifetime. Therefore, infections caused by viruses are usually lifelong. | + | Once in the host cell, the RNA strands of a retrovirus undergo [[reverse transcription]] in the cytosol (internal fluid of the cell) and are integrated into the host's genome, at which point the retroviral DNA is referred to as a ''provirus''. Reverse transcription refers to the opposite of the normal transcription process which involves the synthesis of RNA from DNA. Hence, reverse transcription is the synthesis of double stranded DNA from RNA. Retroviruses rely on the enzyme ''reverse transcriptase'' to perform this reverse transcription of its genome, which is then integrated into the host's genome with the use of an integrase enzyme. The viral genes become part of the cell for its entire lifetime and reproduce as the host cell reproduces. Therefore, infections caused by viruses are usually lifelong. |
Retroviruses are enveloped viruses, meaning they have an outer layering derived from the membrane of its host cell. Characteristics of the outer envelope have a major effect on what types of cells the virus can attack. Antibodies, which are created to prevent and fight viruses, are produced against the envelope and are found in vaccines. Retroviruses have a diameter of approximately 100 nanometers and are formed by budding off of cell membranes. | Retroviruses are enveloped viruses, meaning they have an outer layering derived from the membrane of its host cell. Characteristics of the outer envelope have a major effect on what types of cells the virus can attack. Antibodies, which are created to prevent and fight viruses, are produced against the envelope and are found in vaccines. Retroviruses have a diameter of approximately 100 nanometers and are formed by budding off of cell membranes. | ||
Revision as of 18:57, 12 July 2006
| Retroviruses | ||||
|---|---|---|---|---|
| Virus classification | ||||
| ||||
| Genera | ||||
|
Alpharetrovirus |
A retrovirus is a type of RNA virus that binds to receptors on host cells and integrates its genome into the host cell's nucleus to affect protein transcription. Retroviruses rely on the enzyme reverse transcriptase to perform the reverse transcription of their genome from RNA into DNA. A common retrovirus is the Human Immunodeficiency Virus, commonly known as HIV and formerly known as HTLV-III.
The genome of a retrovirus consists of two RNA molecules, which may or may not be identical. The genome of an organism is defined as the hereditary information of that organism which is encoded in the DNA or RNA, which serves as the template for translation of genes into proteins.
Virus Description
A virus (Latin, poison) is a submicroscopic particle that can infect the cells of a biological organism. A virus stores its nucleic acid genome and serves as a means of delivery of that genome into cells it targets to serve as obligate parasites and constitutes an infection. An obligate parasite is an organism that cannot live independently of its host.
Once in the host cell, the RNA strands of a retrovirus undergo reverse transcription in the cytosol (internal fluid of the cell) and are integrated into the host's genome, at which point the retroviral DNA is referred to as a provirus. Reverse transcription refers to the opposite of the normal transcription process which involves the synthesis of RNA from DNA. Hence, reverse transcription is the synthesis of double stranded DNA from RNA. Retroviruses rely on the enzyme reverse transcriptase to perform this reverse transcription of its genome, which is then integrated into the host's genome with the use of an integrase enzyme. The viral genes become part of the cell for its entire lifetime and reproduce as the host cell reproduces. Therefore, infections caused by viruses are usually lifelong.
Retroviruses are enveloped viruses, meaning they have an outer layering derived from the membrane of its host cell. Characteristics of the outer envelope have a major effect on what types of cells the virus can attack. Antibodies, which are created to prevent and fight viruses, are produced against the envelope and are found in vaccines. Retroviruses have a diameter of approximately 100 nanometers and are formed by budding off of cell membranes.
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 the enzymes reverse transcriptase, protease and integrase
- envelope (env) codes for the retroviral coat proteins
Several genera of retroviruses exist, as listed below.
- Genus Alpharetrovirus; type species: Avian leucosis 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 Human T-lymphotropic virus
- Genus Epsilonretrovirus; type species: Walleye dermal sarcoma virus
- Genus Lentivirus; type species: Human immunodeficiency virus 1, others include Simian and 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.
Medical Pathologies and Research
Retroviruses can attack several types of cells, including somatic cells (body cells) and germ line cells. When retroviruses have integrated their genome into the germ line, their genome is passed on to the 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 (body does not recognize self and consequently attacks itself) 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 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, which states that the order of transcription is DNA into RNA into protein. 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.
Thus far, four human retroviruses — HTLV 1 and 2, in conjunction with HIV 1 and 2 — have been found to attack helper T cells, which "help" the immune response.
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.
See also
External links
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
