Retrovirus
A retrovirus is a type of virus that inserts a copy of its RNA genome[a] into the DNA of a host cell that it invades, thus changing the genome of that cell.[3] Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backwards). The new DNA is then incorporated into the host cell genome by an integrase enzyme, at which point the retroviral DNA is referred to as a provirus. The host cell then treats the viral DNA as part of its own genome, transcribing and translating the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus.
Although retroviruses have different subfamilies, they have three basic groups: the oncoretroviruses (oncogenic retroviruses), the lentiviruses (slow retroviruses) and the spumaviruses (foamy viruses).[4] The oncoretroviruses are able to cause cancer in some species, the lentiviruses are able to cause severe immunodeficiency and death in humans and other animals, and the spumaviruses are benign and not linked to any disease in humans or animals.[4]
Many retroviruses cause serious diseases in humans, other mammals, and birds.[5] Human retroviruses include HIV-1 and HIV-2, the cause of the disease AIDS. Also, human T-lymphotropic virus (HTLV) causes disease in humans. The murine leukemia viruses (MLVs) cause cancer in mouse hosts.[6] Retroviruses are valuable research tools in molecular biology, and they have been used successfully in gene delivery systems.[7]
Virions, viruses in the form of independent particles of retroviruses, consist of enveloped particles about 100 nm in diameter. The outer lipid envelope consists of glycoprotein.[8] The virions also contain two identical single-stranded RNA molecules 7–10 kilobases in length. The two molecules are present as a dimer, formed by base pairing between complementary sequences. Interaction sites between the two RNA molecules have been identified as a "kissing stem-loop".[5] Although virions of different retroviruses do not have the same morphology or biology, all the virion components are very similar.[9]
The retroviral genome is packaged as viral particles. These viral particles are dimers of single-stranded, positive-sense, linear RNA molecules.[10]
Retroviruses (and orterviruses in general) follow a layout of 5'–gag–pro–pol–env–3' in the RNA genome. gag and pol encode polyproteins, each managing the capsid and replication. The pol region encodes enzymes necessary for viral replication, such as reverse transcriptase, protease and integrase.[19]Depending on the virus, the genes may overlap or fuse into larger polyprotein chains. Some viruses contain additional genes. The lentivirus genus, the spumavirus genus, the HTLV / bovine leukemia virus (BLV) genus, and a newly introduced fish virus genus are retroviruses classified as complex. These viruses have genes called accessory genes, in addition to gag, pro, pol and env genes. Accessory genes are located between pol and env, downstream from the env, including the U3 region of LTR, or in the env and overlapping portions. While accessory genes have auxiliary roles, they also coordinate and regulate viral gene expression. In addition, some retroviruses may carry genes called oncogenes or onc genes from another class.Retroviruses with these genes (also called transforming viruses) are known for their ability to quickly cause tumors in animals and transform cells in culture into an oncogenic state.[20]
