Retrovirus - wikipedia Retrovirus `` Retrovirology '' redirects here. For the journal, see Retrovirology ( journal ). Retroviruses HIV retrovirus schematic of cell infection, virus production and virus structure Virus classification Group : Group VI ( ssRNA - RT ) Order : Ortervirales Family : Retroviridae Genera Subfamily : Orthoretrovirinae Alpharetrovirus Betaretrovirus Deltaretrovirus Epsilonretrovirus Gammaretrovirus Lentivirus Subfamily : Spumaretrovirinae Spumavirus A retrovirus is a single - stranded positive - sense RNA virus with a DNA intermediate and, as an obligate parasite, targets a host cell. Once inside the host cell 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. It is difficult to detect the virus until it has infected the host. At that point, the infection will persist indefinitely. In most viruses, DNA is transcribed into RNA, and then RNA is translated into protein. However, retroviruses function differently, as their RNA is reverse - transcribed into DNA, which is integrated into the host cell 's genome ( when it becomes a provirus ), and then undergoes the usual transcription and translational processes to express the genes carried by the virus. The information contained in a retroviral gene is thus used to generate the corresponding protein via the sequence : RNA → DNA → RNA → polypeptide. This extends the fundamental process identified by Francis Crick ( one gene - one peptide ) in which the sequence is DNA → RNA → peptide ( proteins are made of one or more polypeptide chains ; for example, haemoglobin is a four - chain peptide ). Retroviruses are valuable research tools in molecular biology, and they have been used successfully in gene delivery systems. Contents 1 Structure 2 Multiplication 3 Transmission 4 Provirus 5 Early evolution 6 Gene therapy 7 Cancer 8 Classification 8.1 Exogenous 8.1. 1 Group VI viruses 8.1. 2 Group VII viruses 8.2 Endogenous 9 Treatment 10 Treatment of veterinary retroviruses 11 References 12 External links Structure Virions of retroviruses consist of enveloped particles about 100 nm in diameter. The virions also contain two identical single - stranded RNA molecules 7 -- 10 kilobases in length. Although virions of different retroviruses do not have the same morphology or biology, all the virion components are very similar. The main virion components are : Envelope : composed of lipids ( obtained from the host plasma membrane during the budding process ) as well as glycoprotein encoded by the env gene. The retroviral envelope serves three distinct functions : protection from the extracellular environment via the lipid bilayer, enabling the retrovirus to enter / exit host cells through endosomal membrane trafficking, and the ability to directly enter cells by fusing with their membranes. RNA : consists of a dimer RNA. It has a cap at the 5 ' end and a poly ( A ) tail at the 3 ' end. The RNA genome also has terminal noncoding regions, which are important in replication, and internal regions that encode virion proteins for gene expression. The 5 ' end includes four regions, which are R, U5, PBS, and L. The R region is a short repeated sequence at each end of the genome used during the reverse transcription to ensure correct end - to - end transfer in the growing chain. U5, on the other hand, is a short unique sequence between R and PBS. PBS ( primer binding site ) consists of 18 bases complementary to 3 ' end of tRNA primer. L region is an untranslated leader region that gives the signal for packaging of the genome RNA. The 3 ' end includes 3 regions, which are PPT ( polypurine tract ), U3, and R. The PPT is a primer for plus - strand DNA synthesis during reverse transcription. U3 is a sequence between PPT and R, which serves as a signal that the provirus can use in transcription. R is the terminal repeated sequence at 3 ' end. Proteins : consisting of gag proteins, protease ( PR ), pol proteins, and env proteins. Group - specific antigen ( gag ) proteins are major components of the viral capsid, which are about 2000 -- 4000 copies per virion. Protease is expressed differently in different viruses. It functions in proteolytic cleavages during virion maturation to make mature gag and pol proteins. Pol proteins are responsible for synthesis of viral DNA and integration into host DNA after infection. Env proteins play a role in association and entry of virions into the host cell. Possessing a functional copy of an env gene is what makes retroviruses distinct from retroelements. The ability of the retrovirus to bind to its target host cell using specific cell - surface receptors is given by the surface component ( SU ) of the Env protein, while the ability of the retrovirus to enter the cell via membrane fusion is imparted by the membrane - anchored trans - membrane component ( TM ). Thus it is the Env protein that enables the retrovirus to be infectious. Multiplication A retrovirus has a membrane containing glycoproteins, which are able to bind to a receptor protein on a host cell. There are two strands of RNA within the cell that have three enzymes : protease, reverse transcriptase, and integrase ( 1 ). The first step of replication is the binding of the glycoprotein to the receptor protein ( 2 ). Once these have been bound, the cell membrane degrades, becoming part of the host cell, and the RNA strands and enzymes enter the cell ( 3 ). Within the cell, reverse transcriptase creates a complementary strand of DNA from the retrovirus RNA and the RNA is degraded ; this strand of DNA is known as cDNA ( 4 ). The cDNA is then replicated, and the two strands form a weak bond and enter the nucleus ( 5 ). Once in the nucleus, the DNA is integrated into the host cell 's DNA with the help of integrase ( 6 ). This cell can either stay dormant, or RNA may be synthesized from the DNA and used to create the proteins for a new retrovirus ( 7 ). Ribosome units are used to transcribe the mRNA of the virus into the amino acid sequences which can be made into proteins in the rough endoplasmic reticulum. This step will also make viral enzymes and capsid proteins ( 8 ). Viral RNA will be made in the nucleus. These pieces are then gathered together and are pinched off of the cell membrane as a new retrovirus ( 9 ). When retroviruses have integrated their own genome into the germ line, their genome is passed on to a following generation. These endogenous retroviruses ( ERVs ), 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. While transcription was classically thought to occur only 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. These inserts are transcribed by enzymes of the host into new RNA molecules that enter the cytosol. Next, some of these RNA molecules are translated into viral proteins. For example, the gag gene is translated into molecules of the capsid protein, the pol gene is translated into molecules of reverse transcriptase, and the env gene is translated into molecules of the envelope protein. It is important to note that a retrovirus must `` bring '' its own reverse transcriptase in its capsid, otherwise it is unable to utilize the enzymes of the infected cell to carry out the task, due to the unusual nature of producing DNA from RNA. Industrial drugs that are designed as protease and reverse transcriptase inhibitors are made such that they target specific sites and sequences within their respective enzymes. However these drugs can quickly become ineffective due to the fact that the gene sequences that code for the protease and the reverse transcriptase quickly mutate. These changes in bases cause specific codons and sites with the enzymes to change and thereby avoid drug targeting by losing the sites that the drug actually targets. Because reverse transcription lacks the usual proofreading of DNA replication, a retrovirus 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. One difficulty faced with some retroviruses, such as the Moloney retrovirus, involves the requirement for cells to be actively dividing for transduction. As a result, cells such as neurons are very resistant to infection and transduction by retroviruses. This gives rise to a concern that insertional mutagenesis due to integration into the host genome might lead to cancer or leukemia. This is unlike Lentivirus, a genus of Retroviridae, which are able to integrate their RNA into the genome of non-dividing host cells. Transmission Cell - to - cell Fluids Airborne, like the Jaagsiekte sheep retrovirus. Provirus This DNA can be incorporated into host genome as a provirus that can be passed on to progeny cells. The retrovirus DNA is inserted at random into the host genome. Because of this, it can be inserted into oncogenes. In this way some retroviruses can convert normal cells into cancer cells. Some provirus remains latent in the cell for a long period of time before it is activated by the change in cell environment. Early evolution 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. An estimate of the date of evolution of the foamy - like endogenous retroviruses placed the time of the most recent common ancestor at > 450 million years ago. Gene therapy Gammaretroviral and lentiviral vectors for gene therapy have been developed that mediate stable genetic modification of treated cells by chromosomal integration of the transferred vector genomes. This technology is of use, not only for research purposes, but also for clinical gene therapy aiming at the long - term correction of genetic defects, e.g., in stem and progenitor cells. Retroviral vector particles with tropism for various target cells have been designed. Gammaretroviral and lentiviral vectors have so far been used in more than 300 clinical trials, addressing treatment options for various diseases. Retroviral mutations can be developed to make transgenic mouse models to study various cancers and their metastatic models. Cancer Retroviruses that cause tumor growth include Rous sarcoma virus and Mouse mammary tumor virus. Cancer can be triggered by proto - oncogenes that were mistakenly incorporated into proviral DNA or by the disruption of cellular proto - oncogenes. Rous sarcoma virus contains the src gene that triggers tumor formation. Later it was found that a similar gene in cells is involved in cell signaling, which was most likely excised with the proviral DNA. Nontransforming viruses can randomly insert their DNA into proto - oncogenes, disrupting the expression of proteins that regulate the cell cycle. The promoter of the provirus DNA can also cause over expression of regulatory genes. Classification Phylogeny of Retroviruses Exogenous These are infectious RNA - or DNA - containing viruses which are transmitted from individual to individual. Reverse - transcribing viruses fall into 2 groups of the Baltimore classification. Group VI viruses All members of Group VI use virally encoded reverse transcriptase, an RNA - dependent DNA polymerase, to produce DNA from the initial virion RNA genome. This DNA is often integrated into the host genome, as in the case of retroviruses and pseudoviruses, where it is replicated and transcribed by the host. Group VI includes : Family Belpaoviridae Family Metaviridae Family Pseudoviridae Family Retroviridae -- Retroviruses, e.g. HIV The family Retroviridae includes the following genera : Genus Alpharetrovirus ; type species : Avian leukosis virus ; others include Rous sarcoma 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 : Simian foamy virus These were previously divided into three subfamilies ( Oncovirinae, Lentivirinae, and Spumavirinae ), but are now divided into two : Orthoretrovirinae and Spumaretrovirinae. The term oncovirus is now commonly used to describe a cancer - causing virus. Group VII viruses Both families in Group VII have DNA genomes contained within the invading virus particles. The DNA genome is transcribed into both mRNA, for use as a transcript in protein synthesis, and pre-genomic RNA, for use as the template during genome replication. Virally encoded reverse transcriptase uses the pre-genomic RNA as a template for the creation of genomic DNA. Group VII includes : Family Hepadnaviridae -- e.g. Hepatitis B virus Family Caulimoviridae -- e.g. Cauliflower mosaic virus Families Belpaoviridae, Metaviridae, Pseudoviridae, Retroviridae, and Caulimoviridae constitute the order Ortervirales. Endogenous Main article : Endogenous retrovirus 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 on 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 ). Treatment of veterinary retroviruses Feline leukemia virus and Feline immunodeficiency virus infections are treated with biologics, including the only immunomodulator currently licensed for sale in the United States, Lymphocyte T - Cell Immune Modulator ( LTCI ). References ^ Jump up to : Kurth, Reinhard ; Bannert, Norbert, eds. ( 2010 ). Retroviruses : Molecular Biology, Genomics and Pathogenesis. Horizon Scientific. ISBN 978 - 1 - 904455 - 55 - 4. Jump up ^ Coffin, John M. ( 1992 ). `` Structure and Classification of Retroviruses ''. In Levy, Jay A. The Retroviridae. 1 ( 1st ed. ). New York : Plenum. p. 20. ISBN 0 - 306 - 44074 - 1. Jump up ^ Coffin 1992, pp. 26 -- 34 Jump up ^ Kim FJ, Battini JL, Manel N, Sitbon M ( January 2004 ). `` Emergence of vertebrate retroviruses and envelope capture ''. Virology. 318 ( 1 ) : 183 -- 91. doi : 10.1016 / j. virol. 2003.09. 026. PMID 14972546. Jump up ^ Robert Belshaw ; Pereira V ; Katzourakis A ; Talbot G ; Paces J ; Burt A ; Tristem M. ( April 2004 ). `` Long - term reinfection of the human genome by endogenous retroviruses ''. Proc Natl Acad Sci USA. 101 ( 14 ) : 4894 -- 9. doi : 10.1073 / pnas. 0307800101. PMC 387345. PMID 15044706. Jump up ^ Medstrand P, van de Lagemaat L, Dunn C, Landry J, Svenback D, Mager D ( 2005 ). `` Impact of transposable elements on the evolution of mammalian gene regulation ''. Cytogenet Genome Res. 110 ( 1 - 4 ) : 342 -- 52. doi : 10.1159 / 000084966. PMID 16093686. Jump up ^ Svarovskaia ES ; Cheslock SR ; Zhang WH ; Hu WS ; Pathak VK. ( January 2003 ). `` Retroviral mutation rates and reverse transcriptase fidelity ''. Front. Biosci. 8 ( 1 - 3 ) : d117 -- 34. doi : 10.2741 / 957. PMID 12456349. Jump up ^ Jolly C ( March 2011 ). `` Cell - to - cell transmission of retroviruses : Innate immunity and interferon - induced restriction factors ''. Virology. 411 ( 2 ) : 251 -- 9. doi : 10.1016 / j. virol. 2010.12. 031. PMC 3053447. PMID 21247613. Jump up ^ Aiewsakun P, Katzourakis A ( 2017 ) Marine origin of retroviruses in the early Palaeozoic Era. Nat Commun 8 : 13954 Jump up ^ Desport, M, ed. ( 2010 ). Lentiviruses and Macrophages : Molecular and Cellular Interactions. Caister Academic. ISBN 978 - 1 - 904455 - 60 - 8. Jump up ^ Krupovic, M ; Blomberg, J ; Coffin, JM ; Dasgupta, I ; Fan, H ; Geering, AD ; Gifford, R ; Harrach, B ; Hull, R ; Johnson, W ; Kreuze, JF ; Lindemann, D ; Llorens, C ; Lockhart, B ; Mayer, J ; Muller, E ; Olszewski, N ; Pappu, HR ; Pooggin, M ; Richert - Pöggeler, KR ; Sabanadzovic, S ; Sanfaçon, H ; Schoelz, JE ; Seal, S ; Stavolone, L ; Stoye, JP ; Teycheney, PY ; Tristem, M ; Koonin, EV ; Kuhn, JH ( 4 April 2018 ). `` Ortervirales : A new viral order unifying five families of reverse - transcribing viruses ''. Journal of virology. doi : 10.1128 / JVI. 00515 - 18. PMID 29618642. Jump up ^ Rutherford, G.W. ; Sangani, P.R. ; Kennedy, G.E. ( 2003 ). `` Three - or four - versus two - drug antiretroviral maintenance regimens for HIV infection ''. The Cochrane Database of Systematic Reviews ( 4 ) : CD002037. doi : 10.1002 / 14651858. CD002037. ISSN 1469 - 493X. PMID 14583945. Jump up ^ Gingerich DA ( 2008 ). `` Lymphocyte T - cell immunomodulator ( LTCI ) : Review of the immunopharmacology of a new biologic '' ( PDF ). Intern J Appl Res Vet Med. 6 ( 2 ) : 61 -- 8. External links Wikimedia Commons has media related to Retroviruses. ViralZone A Swiss Institute of Bioinformatics resource for all viral families, providing general molecular and epidemiological information ( follow links for `` Retro - transcribing viruses '' ) Retrovirus Animation ( Flash Required ) Retrovirology Scientific journal Retrovirus life cycle chapter From Kimball 's Biology ( online biology textbook pages ) Coffin, John M ; Hughes, Stephen H ; Varmus, Harold E, eds. ( 1997 ). Retroviruses. Cold Spring Harbor Laboratory. ISBN 0 - 87969 - 571 - 4. NBK19376. Specter, Michael ( 3 December 2007 ). `` Annals of Science : Darwin 's Surprise ''. The New Yorker. Microbiology : Virus Components Viral envelope Capsid Viral protein Viral life cycle Viral entry Viral replication Viral shedding Virus latency Viroplasm Genetics Reassortment Antigenic shift Antigenic drift Phenotype mixing By host Bacteriophage Virophage Mycovirus Plant virus ( Plant to Human ) Animal virus Human virome Archea virus Amoeba virus Other Viral disease Helper virus Laboratory diagnosis of viral infections Viral load Virus - like particle Viral quantification Antiviral drug Neurotropic virus Oncovirus Social history of viruses Satellite virus Giant virus Virus : Retroviruses ssRNA - RT virus Retroviridae Alpharetrovirus Avian sarcoma leukosis virus Rous sarcoma virus Betaretrovirus Mouse mammary tumor virus Jaagsiekte sheep retrovirus Deltaretrovirus Human T - lymphotropic virus ( HTLV - 1 HTLV - 2, 3, 4 ) Simian - T - lymphotropic virus ( types 1 - 4 ) Bovine leukemia virus Epsilonretrovirus Walleye epidermal hyperplasia virus Gammaretrovirus Murine leukemia virus Abelson murine leukemia virus Friend virus Feline leukemia virus Koala retrovirus ( KIDS ) Xenotropic murine leukemia virus - related virus Lentivirus HIV Simian immunodeficiency virus ( SIV ) Feline immunodeficiency virus ( FIV ) Puma lentivirus Equine infectious anemia Bovine immunodeficiency virus ( BIV ) Caprine arthritis encephalitis virus Visna virus Spumavirus Simian foamy virus Human foamy virus Other Metaviridae Pseudoviridae dsDNA - RT virus Hepadnaviridae ( Hepatitis B virus ) Caulimoviridae Endogenous ERVWE1 HCP5 Human teratocarcinoma - derived virus Baltimore ( virus classification ) DNA I : dsDNA viruses Caudovirales Myoviridae Podoviridae Siphoviridae Herpesvirales Alloherpesviridae Herpesviridae Malacoherpesviridae Ligamenvirales Lipothrixviridae Rudiviridae Unassigned NLCDV : Ascoviridae Asfarviridae Iridoviridae Marseilleviridae Mimiviridae Phycodnaviridae Poxviridae genera : Dinodnavirus nonenveloped : Adenoviridae Papillomaviridae Papovaviridae ( obsolete ) Polyomaviridae genera : Rhizidiovirus Ampullaviridae Baculoviridae Bicaudaviridae Clavaviridae Corticoviridae Fuselloviridae Globuloviridae Guttaviridae Hytrosaviridae Nimaviridae Nudiviridae Plasmaviridae Polydnaviridae Sphaerolipoviridae Tectiviridae Turriviridae genera : Salterprovirus II : ssDNA viruses Anelloviridae Bidnaviridae Circoviridae Geminiviridae Inoviridae Microviridae Nanoviridae Parvoviridae Spiraviridae genera : Bacilladnavirus RNA III : dsRNA viruses Amalgaviridae Birnaviridae Chrysoviridae Cystoviridae Endornaviridae Hypoviridae Megabirnaviridae Partitiviridae Picobirnaviridae Quadriviridae Reoviridae Totiviridae IV : ( + ) ssRNA viruses ( primarily icosahedral ) Nidovirales Arteriviridae Coronaviridae Mesoniviridae Roniviridae Picornavirales Comoviridae ( obsolete ) Dicistroviridae Iflaviridae Marnaviridae Picornaviridae Secoviridae Sequiviridae ( obsolete ) genera : Bacillarnavirus Labyrnavirus Tymovirales Alphaflexiviridae Betaflexiviridae Gammaflexiviridae Tymoviridae Unassigned Alphatetraviridae Alvernaviridae Astroviridae Barnaviridae Benyviridae Bromoviridae Caliciviridae Carmotetraviridae Closteroviridae Flaviviridae Flexiviridae ( obsolete ) Hepeviridae Leviviridae Luteoviridae Narnaviridae Nodaviridae Permutotetraviridae Potyviridae Tetraviridae ( obsolete ) Togaviridae Tombusviridae Virgaviridae genera : Cilevirus Higrevirus Idaeovirus Ourmiavirus Polemovirus Sobemovirus V : ( -- ) ssRNA viruses ( primarily helical ) Mononegavirales Bornaviridae Filoviridae Nyamiviridae Paramyxoviridae Rhabdoviridae Bunyavirales Hantaviridae Nairoviridae Peribunyaviridae Phenuiviridae Tospoviridae Feraviridae Fimoviridae Jonviridae Phasmaviridae Unassigned Arenaviridae Ophioviridae Orthomyxoviridae genera : Deltavirus RT VI : ssRNA - RT viruses Metaviridae Pseudoviridae Retroviridae VII : dsDNA - RT viruses Caulimoviridae Hepadnaviridae Viruses portal Taxon identifiers Wd : Q189308 EoL : 8053 GBIF : 1940 NCBI : 11632 LCCN : sh85113381 GND : 4121587 - 4 BNF : cb12066909q ( data ) Retrieved from `` https://en.wikipedia.org/w/index.php?title=Retrovirus&oldid=835267776 '' Categories : Molecular biology Retroviruses Virotherapy Hidden categories : Use dmy dates from April 2017 Articles with ' species ' microformats Wikipedia articles needing clarification from April 2013 All articles with unsourced statements Articles with unsourced statements from April 2013 Commons category with local link different than on Wikidata Wikipedia articles with LCCN identifiers Wikipedia articles with GND identifiers Wikipedia articles with BNF identifiers Talk Contents About Wikipedia Wikispecies Wikiquote Afrikaans Български Bosanski Català Čeština Dansk Deutsch Eesti Español Euskara فارسی Français Gaeilge Galego 한국어 Հայերեն हिन्दी Bahasa Indonesia Italiano עברית ქართული Қазақша Latina Latviešu Luganda Magyar Монгол Nederlands 日本 語 Norsk Norsk nynorsk Occitan Polski Português Română Русский Scots Simple English Slovenščina Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Türkçe Українська Tiếng Việt 粵語 中文 40 more Edit links This page was last edited on 7 April 2018, at 16 : 47. Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia ® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. About Wikipedia

Question: viral dna that is incorporated into the genome of a host cell is called a
Answer: retrovirus