A
retrovirus is an
RNA virusAn RNA virus is a virus that has RNA as its genetic material. This nucleic acid is usually single-stranded RNA but may be double-stranded RNA...
that is duplicated in a host cell using the
reverse transcriptaseIn the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
enzyme to produce DNA from its RNA genome. The DNA is then incorporated into the host's genome by an
integraseRetroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
enzyme. The virus thereafter replicates as part of the host cell's DNA. Retroviruses are enveloped viruses that belong to the viral family
Retroviridae.
A special variant of retroviruses are
endogenous retrovirusEndogenous retroviruses are sequences in the genome thought to be derived from ancient viral infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome....
es which are integrated into the genome of the host and inherited across generations.
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
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
it targets as an
obligate parasiteAn obligate parasite is a parasitic organism that cannot complete its life cycle without dependence on its host.-See also:*Obligate intracellular parasite*Parasitism*Parasitic plant*Facultative parasite...
, and constitutes the
infectionAn infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
. Once in the host's cell, the RNA strands undergo reverse transcription in the
cytoplasmThe cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
and are integrated into the host's genome, at which point the retroviral DNA is referred to as a
provirusA provirus is a virus genome that is integrated into the DNA of a host cell.This state can be a stage of virus replication, or a state that persists over longer periods of time as either inactive viral infections or an endogenous retrovirus. In inactive viral infections the virus will not replicate...
. It is difficult to detect the virus until it has infected the host.
In most viruses, DNA is
transcribedTranscription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
into RNA, and then RNA is
translatedIn molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
into
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
. However, retroviruses function differently - 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. So, the information contained in a retroviral gene is used to generate the corresponding protein via the sequence: RNA → DNA → RNA → protein. This extends the fundamental process identified by
Francis CrickFrancis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...
and
James WatsonJames Dewey Watson is an American molecular biologist, geneticist, and zoologist, best known as one of the co-discoverers of the structure of DNA in 1953 with Francis Crick...
, in which the sequence is: DNA → RNA → protein.
Retroviruses are proving to be valuable research tools in molecular biology and have been used successfully in gene delivery systems.
Structure
Virions of retroviruses consist of enveloped particles about 100 nm in diameter. The virions also contain two identical single-stranded
RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
molecules 7-10 kilobases (kb) 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
Many viruses have viral envelopes covering their protein capsids. The envelopes typically are derived from portions of the host cell membranes , but include some viral glycoproteins. Functionally, viral envelopes are used to help viruses enter host cells...
: composed of lipids obtained from the host plasma membrane during budding process as well as glycoproteins encoded by the env gene.
- RNA
Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
: consists of a dimer RNA. It has a cap at 5' end and polyadenyle at 3' end. The RNA genome also has terminal noncoding regions, which are important in replication, and internal regions that encode virion proteins for gene expressionGene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
. The 5' end includes four regions, which are R, U5, PBS, and L. R region is a short repeated sequence at each end of the genome during the reverse transcription in order to ensure correct end-to-end transfer in 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 signal for packaging of genome RNA. The 3' end includes 3 regions, which are PPT (polypurine tract), U3, and R. PPT is primer for plus-strand DNA synthesis during reverse transcription. U3 is a sequence between PPT and R, which has signal that provirus can use in transcriptionTranscription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
. R is the terminal repeated sequence at 3' end. The Nucleotide sequence at the 5'-reminus of the avian sarcoma virus genome was initially sequences by J. Shine and A. P. Czernilofsky et al. (Proc. Natl. Acad. Sci. USA, Vol 75, pp 1473-1477, 1977) and the nucleotide sequence of an untranslated but conserved domain at the 3'-end of the avian sarcoma virus genome was initially published by A.P. Czernilofsky et al. (Nucleic Acuds Research, Vol. 8, pp 2967-2984, 1980).
- Proteins: consisting of gag proteins, protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
(PR), pol proteins and env proteins. Gag proteins are major components of the viral capsidA capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
, 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. Finally, env proteins play a role in association and entry of virion into the host cell.. Possessing a functional copy of an env gene is what makes retroviruses distinct from retroelements. The env gene serves three distinct functions: enabling the retrovirus to enter/exit host cells through endosomal membrane trafficking, protection from the extracellular environment via the lipid bilayer is confers, and the ability to enter cells. 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, while the ability of the retrovirus to enter the cell via membrane fusion is imparted by the membrane-anchored trans-membrane component (TM). Thus the env protein is what enables the retrovirus to be infectious.
Multiplication
When retroviruses have integrated their own genome into the
germ lineIn biology and genetics, the germline of a mature or developing individual is the line of germ cells that have genetic material that may be passed to a child.For example, gametes such as the sperm or the egg, are part of the germline...
, their genome is
passed onInheritance is the practice of passing on property, titles, debts, rights and obligations upon the death of an individual. It has long played an important role in human societies...
to a following generation. These
endogenous retrovirusEndogenous retroviruses are sequences in the genome thought to be derived from ancient viral infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome....
es (ERVs), contrasted with
exogenousExogenous DNA refers to any deoxyribonucleic acid that originates outside of the organism of concern or study.The introduction of exogenous DNA into a cell is called...
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
placentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
l development in the course of the
germinationGermination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
of an
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, and resistance to exogenous retroviral infection. Endogenous retroviruses have also received special attention in the research of
immunologyImmunology is a broad branch of biomedical science that covers the study of all aspects of the immune system in all organisms. It deals with the physiological functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders ; the...
-related pathologies, such as
autoimmune diseaseAutoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...
s like
multiple sclerosisMultiple sclerosis is an inflammatory disease in which the fatty myelin sheaths around the axons of the brain and spinal cord are damaged, leading to demyelination and scarring as well as a broad spectrum of signs and symptoms...
, 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 transcriptaseIn the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
transcribes RNA into DNA. The term "retro" in retrovirus refers to this reversal (making DNA from RNA) of the
central dogma of molecular biologyThe central dogma of molecular biology was first articulated by Francis Crick in 1958 and re-stated in a Nature paper published in 1970:In other words, the process of producing proteins is irreversible: a protein cannot be used to create DNA....
. Reverse transcriptase activity outside of retroviruses has been found in almost all
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s, enabling the generation and insertion of new copies of
retrotransposonRetrotransposons are genetic elements that can amplify themselves in a genome and are ubiquitous components of the DNA of many eukaryotic organisms. They are a subclass of transposon. They are particularly abundant in plants, where they are often a principal component of nuclear DNA...
s 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 transcribed 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
capsidA capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
, 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 inhibitorReverse-transcriptase inhibitors are a class of antiretroviral drug used to treat HIV infection, tumors, and cancer. RTIs inhibit activity of reverse transcriptase, a viral DNA polymerase enzyme that retroviruses need to reproduce.-Mechanism:...
s can quickly be proved ineffective because the gene sequences that code for the protease and the reverse transcriptase can undergo many substitutions. These substitutions of nitrogenous bases, which make up the DNA strand, can make either the protease or the reverse transcriptase difficult to attack. The amino acid substitution enables the enzymes to evade the drug regimens because mutations in the gene sequences can cause physical or chemical change, which makes them harder to detect by the drug. When the drugs that are supposed to attack enzymes, such as protease, are designed, the manufacturers target specific sites on the enzyme. One way to attack these targets can be through hydrolysis of molecular bonds, which means that the drug will add molecules of H
2O (water) to specific bonds. By adding molecules of water at a site on the virus, the drug breaks the previous bonds that were linked to each other. If several of these breaks occur, the result can lead to lysis, the death of the virus.
Because reverse transcription lacks the usual
proofreadingThe term proofreading is used in genetics to refer to the error-correcting processes, first proposed by John Hopfield and Jacques Ninio, involved in DNA replication, immune system specificity, enzyme-substrate recognition among many other processes that require enhanced specificity...
of DNA replication, a retrovirus mutates very often. This enables the virus to grow
resistantImmunology is a broad branch of biomedical science that covers the study of all aspects of the immune system in all organisms. It deals with the physiological functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders ; the...
to antiviral pharmaceuticals quickly, and impedes the development of effective vaccines and inhibitors for the retrovirus.
One drawback of 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. There is concern that insertional mutagenesis due to integration into the host genome might lead to cancer or leukemia. This is unlike
LentiviridaeLentivirus is a genus of slow viruses of the Retroviridae family, characterized by a long incubation period...
, a subclass of Retroviridae which are able to integrate their RNA into the genome of non-dividing host cells.
Genes
Retrovirus genomes commonly contain these three open reading frames that encode for proteins that can be found in the mature virus:
- group-specific antigen
Group-specific antigen is the genetic material that codes for the core structural proteins of a retrovirus.It comprises part of the gag-onc fusion protein.-Gag in HIV:...
(gag) codes for core and structural proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s of the virus;
- polymerase
A polymerase is an enzyme whose central function is associated with polymers of nucleic acids such as RNA and DNA.The primary function of a polymerase is the polymerization of new DNA or RNA against an existing DNA or RNA template in the processes of replication and transcription...
(pol) codes for reverse transcriptaseIn the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
, proteaseA protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
and integraseRetroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
; and,
- envelope
Many viruses have viral envelopes covering their protein capsids. The envelopes typically are derived from portions of the host cell membranes , but include some viral glycoproteins. Functionally, viral envelopes are used to help viruses enter host cells...
(env) codes for the retroviral coat proteins.
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
oncogeneAn oncogene is a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels.An oncogene is a gene found in the chromosomes of tumor cells whose activation is associated with the initial and continuing conversion of normal cells into cancer...
s. 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
eukaryoteA eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
s and
prokaryoteThe prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s. 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 hypothesisThe RNA world hypothesis proposes that life based on ribonucleic acid pre-dates the current world of life based on deoxyribonucleic acid , RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme protein...
, cellular organisms adopted the more chemically stable DNA when retroviruses evolved to create
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
from the
RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
templates.
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.
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.
Exogenous
The following genera are included here:
- Genus Alpharetrovirus
Alpharetrovirus is a genus of the retroviridae family. It has type C morphology. Members can cause sarcomas, other tumors, and anaemia of wild and domestic birds and also affect rats....
; type species: Avian leukosis virus; others include Rous sarcoma virusRous sarcoma virus is a retrovirus and is the first oncovirus to have been described: it causes sarcoma in chickens.As with all retroviruses, it reverse transcribes its RNA genome into cDNA before integration into the host DNA.-History:...
- Genus Betaretrovirus
A betaretrovirus is a genus of the retroviridae family. It has type B or type D morphology. The type B is common for a few exogenous, vertically transmitted and endogenous viruses of mice; some primate and sheep viruses are the type D....
; type species: Mouse mammary tumour virus
- Genus Gammaretrovirus
A gammaretrovirus is a genus of the retroviridae family. Example species are the murine leukemia virus and the feline leukemia virus.Many endogenous retroviruses, closely related to exogenous gammaretroviruses, are present in the DNA of mammals , birds, reptiles and amphibians.Many of the...
; type species: Murine leukemia virusThe murine leukemia viruses are retroviruses named for their ability to cause cancer in murine hosts. Some MLVs may infect other vertebrates. MLVs include both exogenous and endogenous viruses...
; others include Feline leukemia virusFeline leukemia virus is a retrovirus that infects cats. FeLV can be transmitted between infected cats when the transfer of saliva or nasal secretions is involved. If not defeated by the animal’s immune system, the virus can be lethal...
- Genus Deltaretrovirus
A deltaretrovirus is a genus of the retroviridae family. It consists of exogenous horizontally-transmitted viruses found in several groups of mammals.Examples are the bovine leukemia virus and the Human T-lymphotropic virus.-External links:*...
; type species: Bovine leukemia virusBovine leukemia virus is a bovine virus closely related to HTLV-I, a human tumour virus.BLV is a retrovirus which integrates a DNA intermediate as a provirus into the DNA of B-lymphocytes of blood and milk. It contains an oncogene coding for a protein called Tax. Nevertheless in its natural host...
; others include the cancer-causing Human T-lymphotropic virusHuman T-cell Lymphotropic Virus Type 1 , also called the Adult T-cell lymphoma virus type 1, a virus that has been seriously implicated in several kinds of diseases including HTLV-I-associated myelopathy, Strongyloides stercoralis hyper-infection, and a virus cancer link for leukemia...
- Genus Epsilonretrovirus
An epsilonretrovirus is a genus of the retroviridae family. It infects fish. The species include the Walleye dermal sarcoma virus, and Walleye epidermal hyperplasia virus 1 and 2.-External links:*...
; type species: Walleye dermal sarcoma virus
- Genus Lentivirus
Lentivirus is a genus of slow viruses of the Retroviridae family, characterized by a long incubation period...
; type species: Human immunodeficiency virus 1Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
; others include SimianSimian immunodeficiency virus , also known as African Green Monkey virus and also as Monkey AIDS is a retrovirus able to infect at least 33 species of African primates...
, FelineFeline immunodeficiency virus is a lentivirus that affects domesticated housecats worldwide and is the causative agent of feline AIDS. From 2.5% up to 4.4% of cats worldwide are infected with FIV...
immunodeficiency viruses
- Genus Spumavirus
A spumavirus or foamyvirus is a genus of the retroviridae family. Spumaviruses are exogenous viruses that have specific morphology with prominent surface spikes. The virions contain significant amounts of double-stranded full-length DNA, and assembly is rather unusual in these viruses...
; type species: Simian foamy virusThe simian foamy virus is a spumavirus closely related to the Human Immunodeficiency Virus, HIV, the virus that can lead to AIDS. Its discovery in primates has led to some speculation that HIV may have been spread to the human species in Africa through contact with blood from apes, monkeys, and...
These were previously divided into three subfamilies (
Oncovirinae,
Lentivirinae, and
Spumavirinae), but with current knowledge of retroviruses, this is no longer appropriate. (The term
oncovirusAn oncovirus is a virus that can cause cancer. This term originated from studies of acutely-transforming retroviruses in the 1950–60s, often called oncornaviruses to denote their RNA virus origin. It now refers to any virus with a DNA or RNA genome causing cancer and is synonymous with "tumor...
is still commonly used, though.)
Retroviruses were in 2 groups of the Virus classification#Baltimore classification.
Group VI viruses
All members of Group VI use virally encoded
reverse transcriptaseIn the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
, 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
pseudovirusesThe Pseudoviridae are a family of viruses, including the following genera:*Genus Pseudovirus; type species: Saccharomyces cerevisiae Ty1 virus*Genus Hemivirus; type species: Drosophila melanogaster copia virus...
, where it is replicated and
transcribedTranscription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
by the host.
Group VI includes:
- Family Metaviridae
Metaviridae are a family of viruses which exist as retrotransposons in a eukaryotic host’s genome. They are very closely related to retroviruses: Metaviridae share many genomic elements with retroviruses, including length, organization, and genes themselves. This includes genes that encode reverse...
- Family Pseudoviridae
The Pseudoviridae are a family of viruses, including the following genera:*Genus Pseudovirus; type species: Saccharomyces cerevisiae Ty1 virus*Genus Hemivirus; type species: Drosophila melanogaster copia virus...
- Family Retroviridae - Retroviruses, e.g. HIV
Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
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 transcriptaseIn the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
uses the pre-genomic RNA as a template for the creation of genomic DNA.
Group VII includes:
- Family Hepadnaviridae
Hepadnaviruses are a family of viruses which can cause liver infections in humans and animals. There are two recognized genera:*Genus Orthohepadnavirus; type species: Hepatitis B virus...
- e.g. Hepatitis B virus
- Family Caulimoviridae
-General overview:The Caulimoviridae are a family of viruses, including the following genera:*Genus Badnavirus; type species: Commelina yellow mottle virus*Genus Caulimovirus; type species: Cauliflower mosaic virus...
- e.g. Cauliflower mosaic virusCauliflower mosaic virus is the type member of the caulimoviruses, one of the six genera in the Caulimoviridae family, pararetroviruses that infect plants...
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
HIVHuman immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
. Different classes of antiretroviral drugs act on different stages of the
HIVHuman immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
life cycleA life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
. Combination of several (typically three or four) antiretroviral drugs is known as highly active anti-retroviral therapy (HAART).
Treatment of veterinary retroviruses
Feline Leukemia VirusFeline leukemia virus is a retrovirus that infects cats. FeLV can be transmitted between infected cats when the transfer of saliva or nasal secretions is involved. If not defeated by the animal’s immune system, the virus can be lethal...
and
Feline immunodeficiency virusFeline immunodeficiency virus is a lentivirus that affects domesticated housecats worldwide and is the causative agent of feline AIDS. From 2.5% up to 4.4% of cats worldwide are infected with FIV...
infections are treated with biologics, including
Lymphocyte T-Cell Immune ModulatorLymphocyte T-Cell Immune Modulator is an immune regulating polypeptide manufactured by T-Cyte Therapeutics and distributed by . Lymphocyte T-Cell Immune Modulator is a potent regulator of CD-4 lymphocyte production and function. It has been shown to increase lymphocyte numbers and Interleukin-2 ...
(LTCI) marketed by
IMULAN BioTherapeutics, LLC-About:IMULAN BioTherapeutics, LLC, is a veterinary immunobiology company developing a range of immune based therapies for veterinary medicine. IMULAN BioTherapeutics, LLC has several divisions related to immuno-biology and comparative medicine....
.
External links