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75-449: Totiviridae is a family of double-stranded RNA viruses . Giardia lamblia , leishmania , trichomonas vaginalis , and fungi serve as natural hosts. The name of the group derives from Latin toti which means undivided or whole. There are 28 species in this family, assigned to 5 genera. Viruses in the family Totiviridae are non-enveloped, double-stranded RNA viruses with icosahedral geometries, and T=2 symmetry. The virion consists of

150-564: A DNA strand from the ssRNA strand, and the RNA strand is degraded and replaced with a DNA strand to create a dsDNA genome. The genome is then integrated into the DNA of the host cell, where it is now called a provirus . The host cell's RNA polymerase II then transcribes RNA in the nucleus from the proviral DNA. Some of this RNA may become mRNA whereas other strands will become copies of the viral genome for replication. ssRNA-RT viruses are all included in

225-417: A Gag-Pol fusion protein (180 kDa) formed by a -1 ribosomal frameshift. L-A can support the replication and encapsidation in separate viral particles of any of several satellite dsRNAs, called M dsRNAs, each of which encodes a secreted protein toxin (the killer toxin) and immunity to that toxin. L-A and M are transmitted from cell to cell by the cytoplasmic mixing that occurs in the process of mating. Neither

300-524: A delay in lysis which may be described as a "carrier state". Since cells do not produce double-stranded RNA during normal nucleic acid metabolism , natural selection has favored the evolution of enzymes that destroy dsRNA on contact. The best known class of this type of enzymes is Dicer . It is hoped that broad-spectrum anti-virals could be synthesized that take advantage of this vulnerability of double-stranded RNA viruses. Baltimore classification Baltimore classification also closely corresponds to

375-417: A few exceptions and peculiarities exist. The family Anelloviridae is the only ssDNA family whose members have negative sense genomes, which are circular. Parvoviruses, as previously mentioned, may package either the positive or negative sense strand into virions. Lastly, bidnaviruses package both the positive and negative linear strands. In any case, the sense of ssDNA viruses, unlike for ssRNA viruses,

450-439: A gap in one strand, which is repaired to create a complete dsDNA genome prior to transcription. dsDNA-RT viruses are transcribed in the same manner as dsDNA viruses, but make use of reverse transcription to replicate their circular genome while it is still in the capsid. The host cell's RNA polymerase II transcribes RNA strands from the genome in the cytoplasm, and the genome is replicated from these RNA strands. The dsDNA genome

525-404: A highly conserved helix -pair/β-sheet/helix-pair sandwich fold but lacks the β-barrel flap present in orthoreovirus λ2 . The stacking of turret protein functional domains and the presence of constrictions and A spikes along the mRNA release pathway indicate a mechanism that uses pores and channels to regulate the highly coordinated steps of RNA transcription, processing, and release. Rotavirus

600-409: A loop around the circular genome is also common. Some dsDNA viruses use a strand displacement method whereby one strand is synthesized from a template strand, and a complementary strand is then synthesized from the prior synthesized strand, forming a dsDNA genome. Lastly, some dsDNA viruses are replicated as part of a process called replicative transposition whereby a viral genome in a host cell's DNA

675-405: A negative sense, single-stranded RNA (-ssRNA) genome. mRNA, which is positive sense, is transcribed directly from the negative sense genome. The first process for -ssRNA transcription involves RdRp binding to a leader sequence on the 3′ end of the genome, transcribing a 5′ triphosphate-leader RNA that is capped, then stopping and restarting on a transcription signal which is capped , continuing until

750-441: A nucleotide not included in the template strand. Editing of a genomic template would impair gene expression, so RNA editing is only done during and after transcription. For ebola viruses , RNA editing improves the ability to adapt to their hosts. Alternative splicing differs from RNA editing in that alternative splicing does not change the mRNA sequence like RNA editing but instead changes the coding capacity of an mRNA sequence as

825-443: A portion of their genome transcribed. Typically, subgenomic RNA (sgRNA) strands are used for translation of structural and movement proteins needed during intermediate and late stages of infection. sgRNA transcription may occur by commencing RNA synthesis within the genome rather than from the 5′-end, by stopping RNA synthesis at specific sequences in the genome, or by, as a part of both prior methods, synthesizing leader sequences from

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900-419: A realm. The first Baltimore group contains viruses that have a double-stranded DNA (dsDNA) genome. All dsDNA viruses have their mRNA synthesized in a three-step process. First, a transcription preinitiation complex binds to the DNA upstream of the site where transcription begins, allowing for the recruitment of a host RNA polymerase . Second, once the RNA polymerase is recruited, it uses the negative strand as

975-454: A result of alternative splicing sites. The two mechanisms otherwise have the same result: multiple proteins are expressed from a single gene. Translation is the process by which proteins are synthesized from mRNA by ribosomes . Baltimore groups do not directly pertain to the translation of viral proteins, but various atypical types of translation used by viruses are usually found within specific Baltimore groups: Baltimore classification

1050-462: A single capsid protein and is about 40 nanometers in diameter. The genome is composed of a monopartite, linear double-stranded RNA molecule of 4.6–6.7 kilobases. It contains two overlapping open reading frames (ORF) – gag and pol – which respectively encode the capsid protein and the RNA-dependent RNA polymerase . Some totiviruses contain a third small potential ORF. Viral replication

1125-496: A single pre-mRNA strand or for other specific purposes. For certain viruses, including the families Orthomyxoviridae and Papillomaviridae , alternative splicing acts as a way to regulate early and late gene expression during different stages of infection. Herpesviruses use it as a potential anti-host defense mechanism to prevent synthesis of specific antiviral proteins. Furthermore, in addition to alternative splicing, because cellular unspliced RNA cannot be transported out of

1200-498: A single virion so that the whole genome is in one virus particle, and the separate segments contain different genes. Monopartite viruses are found in all Baltimore groups, whereas multipartite viruses are usually RNA viruses. This is because most multipartite viruses infect plants or fungi, which are eukaryotes, and most eukaryotic viruses are RNA viruses. The family Pleolipoviridae varies as some viruses are monopartite ssDNA while others are bipartite with one segment being ssDNA and

1275-422: A single, long open reading frame (ORF), or translatable portion, and a site-specific nick in the 5′ region of the positive strand. dsRNA viruses are classified into two phyla within the kingdom Orthornavirae of the realm Riboviria : The fourth Baltimore group contains viruses that have a positive sense single-stranded RNA (+ssRNA) genome. For +ssRNA viruses, the genome functions as mRNA, so no transcription

1350-410: A stop signal is reached. The second manner is similar but instead of synthesizing a cap, RdRp may make use of cap snatching , whereby a short sequence of host cell mRNA is taken and used as the 5′ cap of the viral mRNA. Genomic -ssRNA is replicated from the positive sense antigenome in a similar manner as transcription, except in reverse using the antigenome as a template for the genome. RdRp moves from

1425-480: A template for synthesizing mRNA strands. Third, the RNA polymerase terminates transcription upon reaching a specific signal, such as a polyadenylation site. dsDNA viruses make use of several mechanisms to replicate their genome. Bidirectional replication, in which two replication forks are established at a replication origin site and move in opposite directions of each other, is widely used. A rolling circle mechanism that produces linear strands while progressing in

1500-465: Is ligated into a circular loop. The new ssDNA may be packaged into virions or replicated by a DNA polymerase to form a double-stranded form for transcription or continuation of the replication cycle. Parvoviruses contain linear ssDNA genomes that are replicated via rolling hairpin replication (RHR), which is similar to RCR. Parvovirus genomes have hairpin loops at each end of the genome that repeatedly unfold and refold during replication to change

1575-481: Is a double-stranded RNA non-enveloped virus. The members of genus Orbivirus within the Reoviridae family are arthropod borne viruses and are responsible for high morbidity and mortality in ruminants . Bluetongue virus (BTV) which causes disease in livestock ( sheep , goat , cattle ) has been in the forefront of molecular studies for the last three decades and now represents the best understood orbivirus at

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1650-458: Is a member of the Cystoviridae family. It infects Pseudomonas bacteria (typically plant-pathogenic P. syringae ). It has a three-part, segmented, double-stranded RNA genome, totalling ~13.5 kb in length. Φ6 and its relatives have a lipid membrane around their nucleocapsid, a rare trait among bacteriophages . It is a lytic phage, though under certain circumstances has been observed to display

1725-422: Is chiefly based on the transcription of the viral genome, and viruses within each group typically share the manners by which the mRNA synthesis occurs. While not the direct focus of Baltimore classification, groups are organized in such a manner that viruses in each group also typically have the same mechanisms of replicating the viral genome. Because of this, Baltimore classification provides insights into both

1800-447: Is cytoplasmic. Replication follows the double-stranded RNA virus replication model. Double-stranded RNA virus transcription is the method of transcription. Translation takes place by -1 ribosomal frameshifting, +1 ribosomal frameshifting, viral initiation, and RNA termination-reinitiation. The virus exits the host cell by cell-to-cell movement. Giardia lamblia protozoa, leishmania protozoa, protozoan trichomonas vaginalis, and fungi serve as

1875-525: Is divided into two subphyla: Haploviricotina , whose members synthesize a cap structure on viral mRNA required for protein synthesis, and Polyploviricotina , whose members instead obtain caps on mRNA via cap snatching. Reverse transcribing (RT) viruses have genomes made of either DNA or RNA and replicate via reverse transcription. Two groups of reverse transcribing viruses exist: single-stranded RNA-RT (ssRNA-RT) viruses, and double-stranded DNA-RT (dsDNA-RT) viruses. Reverse transcribing viruses are classified in

1950-400: Is initiated by an endonuclease that bonds to and cleaves the positive strand, allowing a DNA polymerase to use the negative strand as a template for replication. Replication progresses in a loop around the genome by means of extending the 3′-end of the positive strand, displacing the prior positive strand, and the endonuclease cleaves the positive strand again to create a standalone genome that

2025-414: Is naturally released from the cell or enters cells by other mechanisms, but the high frequency of yeast mating in nature results in the wide distribution of these viruses in natural isolates. Moreover, the structural and functional similarities with dsRNA viruses of mammals has made it useful to consider these entities as viruses. Infectious bursal disease virus (IBDV) is the best-characterized member of

2100-557: Is not sufficient to separate ssDNA viruses into two groups since all ssDNA viral genomes are converted to dsDNA forms prior to transcription and replication. ssDNA viruses are classified into one of the four realms and include several families that are unassigned to a realm: RNA viruses have genomes made of ribonucleic acid (RNA) and comprise three groups: double-stranded RNA (dsRNA) viruses, positive sense single-stranded RNA (+ssRNA) viruses, and negative sense single-stranded RNA (-ssRNA) viruses. The majority of RNA viruses are classified in

2175-442: Is produced from pregenomic RNA strands via the same general mechanism as ssRNA-RT viruses, but with replication occurring in a loop around the circular genome. After replication, the dsDNA genome may be packed or sent to the nucleus for further rounds of transcription. dsDNA-RT viruses are, like ssRNA-RT, all included in the class Revtraviricetes . Two families of dsDNA-RT viruses are recognized: Caulimoviridae , which belongs to

2250-464: Is replicated to another part of a host genome. dsDNA viruses can be subdivided between those that replicate in the nucleus, and as such are relatively dependent on host cell machinery for transcription and replication, and those that replicate in the cytoplasm, in which case they have evolved or acquired their own means of executing transcription and replication. dsDNA viruses are also commonly divided between tailed dsDNA viruses, referring to members of

2325-434: Is required for translation. +ssRNA viruses will also, however, produce positive sense copies of the genome from negative sense strands of an intermediate dsRNA genome. This acts as both a transcription and a replication process since the replicated RNA is also mRNA. The 5′-end may be naked, capped, or covalently bound to a viral protein, and the 3′-end may be naked or polyadenylated. Many +ssRNA viruses are able to have only

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2400-482: Is the largest and most diverse dsRNA virus family in terms of host range. Two clades of dsRNA viruses exist: the phylum Duplornaviricota and the class Duplopiviricetes , which is in the phylum Pisuviricota . Both are included in the kingdom Orthornavirae in the realm Riboviria . Based on phylogenetic analysis of RdRp, the two clades do not share a common dsRNA ancestor but are instead separately descended from different positive-sense, single-stranded RNA viruses. In

2475-428: Is the most common cause of acute gastroenteritis in infants and young children worldwide. This virus contains a dsRNA genome and is a member of the Reoviridae family. The genome of rotavirus consists of eleven segments of dsRNA. Each genome segment codes for one protein with the exception of segment 11, which codes for two proteins. Among the twelve proteins, six are structural and six are non-structural proteins. It

2550-476: Is used as a template by the viral RNA-dependent RNA polymerase (RdRp) to transcribe a positive-strand RNA functioning as messenger RNA (mRNA) for the host cell's ribosomes , which translate it into viral proteins. The positive-strand RNA can also be replicated by the RdRp to create a new double-stranded viral genome. A distinguishing feature of the dsRNA viruses is their ability to carry out transcription of

2625-563: The Baltimore classification system, dsRNA viruses belong to Group III. Virus group members vary widely in host range ( animals , plants , fungi , and bacteria ), genome segment number (one to twelve), and virion organization ( T-number , capsid layers, or turrets). Double-stranded RNA viruses include the rotaviruses , known globally as a common cause of gastroenteritis in young children, and bluetongue virus , an economically significant pathogen of cattle and sheep. The family Reoviridae

2700-519: The Baltimore classification system, which groups viruses together based on their manner of mRNA synthesis, dsRNA viruses are group III. Duplornaviricota contains most dsRNA viruses, including reoviruses , which infect a diverse range of eukaryotes, and cystoviruses , which are the only dsRNA viruses known to infect prokaryotes. Apart from RdRp, viruses in Duplornaviricota also share icosahedral capsids that contain 60 homo- or heterodimers of

2775-537: The electrophoretic migration profiles of their genome segments. Cypovirus has only a single capsid shell, which is similar to the orthoreovirus inner core. CPV exhibits striking capsid stability and is fully capable of endogenous RNA transcription and processing. The overall folds of CPV proteins are similar to those of other reoviruses. However, CPV proteins have insertional domains and unique structures that contribute to their extensive intermolecular interactions. The CPV turret protein contains two methylase domains with

2850-486: The 1990s to the 2010s, virus taxonomy used a 5-rank system ranging from order to species with Baltimore classification used in conjunction. Outside of the ICTV's official framework, various supergroups of viruses joining different families and orders were created over time based on increasing evidence of deeper evolutionary relations. Consequently, in 2016, the ICTV began to consider establishing ranks higher than order as well as how

2925-417: The 3′-end of the mRNA. Additionally, some -ssRNA viruses are ambisense, as both the positive and negative strands separately encode viral proteins, and these viruses produce two separate mRNA strands: one directly from the genome and one from a complementary strand. -ssRNA viruses can be subdivided informally between those that have nonsegmented and segmented genomes. Nonsegmented -ssRNA viruses replicate in

3000-478: The 3′-end to the 5′-end of the antigenome and ignores all transcription signals when synthesizing genomic -ssRNA. Various -ssRNA viruses use special mechanisms for transcription. The manner of producing the polyA tail may be via polymerase stuttering , during which RdRp transcribes an adenine from uracil and then moves back in the RNA sequence with the mRNA to transcribe it again, continuing this process numerous times until hundreds of adenines have been added to

3075-450: The Baltimore groups would be treated among higher taxa. In two votes in 2018 and 2019, a 15-rank system ranging from realm to species was established by the ICTV. As part of this, the Baltimore groups for RNA viruses and RT viruses were incorporated into formal taxa. In 2018, the realm Riboviria was established and initially included the three RNA virus groups. A year later, Riboviria

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3150-521: The capsid protein organized on a pseudo T=2 lattice. The phylum is divided into three classes: Chrymotiviricetes , which primarily contains fungal and protozoan viruses, Resentoviricetes , which contains reoviruses, and Vidaverviricetes , which contains cystoviruses. The class Duplopiviricetes is the second clade of dsRNA viruses and is in the phylum Pisuviricota , which also contains positive-sense single-stranded RNA viruses. Duplopiviricetes mostly contains plant and fungal viruses and includes

3225-413: The capsid proteins and a plausible model for capsid assembly have been derived. While the structural proteins of RDV share no sequence similarity to other proteins, their folds and the overall capsid structure are similar to those of other Reoviridae . The L-A dsRNA virus of the yeast Saccharomyces cerevisiae has a single 4.6 kb genomic segment that encodes its major coat protein, Gag (76 kDa) and

3300-571: The capsid proteins of some positive-sense single-stranded RNA viruses, such as the nodaviruses and tetraviruses , as well as the T  = 13 capsid shell protein of the Reoviridae . The T  = 13 shell of the IBDV capsid is formed by trimers of VP2, a protein generated by removal of the C-terminal domain from its precursor, pVP2. The trimming of pVP2 is performed on immature particles as part of

3375-474: The class Revtraviricetes , phylum Arterviricota , kingdom Pararnavirae of the realm Riboviria . Excluding Caulimoviridae , which belongs to Group VII, all members of the Revtraviricetes order Ortervirales are ssRNA-RT viruses. The seventh Baltimore group contains viruses that have a double-stranded DNA genome that has an RNA intermediate (dsDNA-RT) in its replication cycle. dsDNA-RT viruses have

3450-422: The cytoplasm, and segmented -ssRNA viruses replicate in the nucleus. During transcription, the RdRp produces one monocistronic mRNA strand from each segment of the genome. All -ssRNA viruses are classified in the phylum Negarnaviricota in the kingdom Orthornavirae in the realm Riboviria . Negarnaviricota only contains -ssRNA viruses, so "-ssRNA virus" is synonymous with Negarnaviricota . Negarnaviricota

3525-402: The direction of DNA synthesis to move back and forth along the genome, producing numerous copies of the genome in a continuous process. Individual genomes are then excised from this molecule by the viral endonuclease. For parvoviruses, either the positive or negative sense strand may be packaged into capsids, varying from virus to virus. Nearly all ssDNA viruses have positive sense genomes, but

3600-455: The dsDNA family Sphaerolipoviridae , and in the family Pleolipoviridae , viruses contain both linear and circular genomes, varying from genus to genus. RNA editing is used by various ssRNA viruses to produce different proteins from a single gene. This can be done via polymerase slippage during transcription or by post-transcriptional editing. In polymerase slippage, the RNA polymerase slips one nucleotide back during transcription, inserting

3675-456: The dsRNA segments within the capsid , and the required enzymes are part of the virion structure. Double-stranded RNA viruses are classified into two phyla, Duplornaviricota and Pisuviricota (specifically class Duplopiviricetes ), in the kingdom Orthornavirae and realm Riboviria . The two phyla do not share a common dsRNA virus ancestor, but evolved their double strands two separate times from positive-strand RNA viruses . In

3750-476: The family Birnaviridae . These viruses have bipartite dsRNA genomes enclosed in single layered icosahedral capsids with T  = 13l geometry. IBDV shares functional strategies and structural features with many other icosahedral dsRNA viruses, except that it lacks the T  = 1 (or pseudo T  = 2) core common to the Reoviridae , Cystoviridae , and Totiviridae . The IBDV capsid protein exhibits structural domains that show homology to those of

3825-486: The following four families: Amalgaviridae , Hypoviridae , Partitiviridae , and Picobirnaviridae . Reoviridae are currently classified into nine genera . The genomes of these viruses consist of 10 to 12 segments of dsRNA , each generally encoding one protein . The mature virions are non-enveloped. Their capsids, formed by multiple proteins, have icosahedral symmetry and are arranged generally in concentric layers. The orthoreoviruses ( reoviruses ) are

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3900-414: The genome is linear or circular, and different methods of translating viral mRNA. Alternative splicing is a mechanism by which different proteins can be produced from a single gene by means of using alternative splicing sites to produce different mRNAs. It is found in various DNA, -ssRNA, and reverse transcribing viruses. Viruses may make use of alternative splicing solely to produce multiple proteins from

3975-554: The genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), the strandedness of the genome, which can be either single- or double-stranded, and the sense of a single-stranded genome, which is either positive or negative. The primary advantage of Baltimore classification is that by classifying viruses according to the aforementioned characteristics, viruses that behave in the same manner can be studied as distinct groups. There are seven Baltimore groups numbered with Roman numerals, listed hereafter. Baltimore classification

4050-517: The genome is single-stranded, however, it is first made into a double-stranded form by a DNA polymerase upon entering a host cell. mRNA is then synthesized from the double-stranded form. The double-stranded form of ssDNA viruses may be produced either directly after entry into a cell or as a consequence of replication of the viral genome. Eukaryotic ssDNA viruses are replicated in the nucleus. Most ssDNA viruses contain circular genomes that are replicated via rolling circle replication (RCR). ssDNA RCR

4125-418: The host cell's cytoplasm. mRNA is forced out from the capsid in order to be translated or to be translocated from a mature capsid to a progeny capsid. While dsRNA viruses typically have capsids, viruses in the families Amalgaviridae and Endornaviridae have not been observed to form virions and as such apparently lack capsids. Endornaviruses are also unusual in that unlike other RNA viruses, they possess

4200-399: The kingdom Orthornavirae in the realm Riboviria . The exceptions are generally viroids and other subviral agents . Some of the latter category, such as the hepatitis D virus , are classified in the realm Ribozyviria . The third Baltimore group contains viruses that have a double-stranded RNA (dsRNA) genome. After entering a host cell, the dsRNA genome is transcribed to mRNA from

4275-454: The kingdom Pararnavirae in the realm Riboviria . The sixth Baltimore group contains viruses that have a (positive-sense) single-stranded RNA genome that has a DNA intermediate ((+)ssRNA-RT) in its replication cycle. ssRNA-RT viruses are transcribed in the same manner as DNA viruses, but their linear genomes are first converted to a dsDNA form through a process called reverse transcription . The viral reverse transcriptase enzyme synthesizes

4350-452: The main cytoplasmic area of the cell. +ssRNA viruses can be subdivided between those that have polycistronic mRNA, which encodes a polyprotein that is cleaved to form multiple mature proteins, and those that produce subgenomic mRNAs and therefore undergo two or more rounds of translation. +ssRNA viruses are included in three phyla in the kingdom Orthornavirae in the realm Riboviria : The fifth Baltimore group contains viruses that have

4425-424: The manner of replicating the genome, so Baltimore classification is useful for grouping viruses together for both transcription and replication. Certain subjects pertaining to viruses are associated with multiple, specific Baltimore groups, such as specific forms of translation of mRNA and the host range of different types of viruses. Structural characteristics such as the shape of the viral capsid , which stores

4500-402: The maturation process. The other major structural protein, VP3, is a multifunctional component lying under the T  = 13 shell that influences the inherent structural polymorphism of pVP2. The virus-encoded RNA-dependent RNA polymerase , VP1, is incorporated into the capsid through its association with VP3. VP3 also interacts extensively with the viral dsRNA genome. Bacteriophage Φ6 ,

4575-545: The molecular and structural levels. BTV, like other members of the family, is a complex non-enveloped virus with seven structural proteins and a RNA genome consisting of 10 variously sized dsRNA segments. Phytoreoviruses are non-turreted reoviruses that are major agricultural pathogens, particularly in Asia. One member of this family, Rice Dwarf Virus (RDV), has been extensively studied by electron cryomicroscopy and x-ray crystallography . From these analyses, atomic models of

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4650-508: The natural host. The family Totiviridae has five genera: An example of fungal totivirus is the L-A helper virus , a cytoplasmic virus found primarily in Saccharomyces cerevisiae . Double-stranded RNA viruses Double-stranded RNA viruses ( dsRNA viruses ) are a polyphyletic group of viruses that have double-stranded genomes made of ribonucleic acid . The double-stranded genome

4725-509: The negative strand by the viral RNA-dependent RNA polymerase (RdRp). The mRNA may be used for translation or replication. Single-stranded mRNA is replicated to form the dsRNA genome. The 5′-end of the genome may be naked, capped, or covalently bound to a viral protein. dsRNA is not a molecule made by cells, so cellular life has evolved antiviral systems to detect and inactivate viral dsRNA. To counteract this, many dsRNA genomes are constructed inside of capsids, thereby avoiding detection inside of

4800-408: The nucleus, hepadnaviruses and retroviruses contain their own proteins for exporting their unspliced genomic RNA out of the nucleus. Viral genomes can exist in a single, or monopartite, segment, or they may be split into more than one molecule, called multipartite . For monopartite viruses, all genes are on the single segment of the genome. Multipartite viruses typically package their genomes into

4875-411: The order Ortervirales , and Hepadnaviridae , which is the sole family in the order Blubervirales . A number of characteristics of viruses are not directly associated with Baltimore classification but nonetheless closely correspond to multiple, specific Baltimore groups. This includes alternative splicing during transcription, whether the viral genome is segmented, the host range of viruses, whether

4950-537: The other dsDNA. Viruses in the ssDNA plant virus family Geminiviridae likewise vary between being monopartite and bipartite. Different Baltimore groups tend to be found within different branches of cellular life. In prokaryotes, the large majority of viruses are dsDNA viruses, and a significant minority are ssDNA viruses. Prokaryotic RNA viruses, in contrast, are relatively rare. Most eukaryotic viruses, including most animal and plant viruses, are RNA viruses, although eukaryotic DNA viruses are also common. By group,

5025-416: The proteins of mammalian reovirus (MRV), which is the best-studied genotype. Electron cryo- microscopy (cryoEM) and X-ray crystallography have provided a wealth of structural information about two specific MRV strains, type 1 Lang (T1L) and type 3 Dearing (T3D). The cytoplasmic polyhedrosis viruses (CPVs) form the genus Cypovirus of the family Reoviridae . CPVs are classified into 14 species based on

5100-530: The prototypic members of the virus Reoviridae family and representative of the turreted members, which comprise about half the genera. Like other members of the family, the reoviruses are non-enveloped and characterized by concentric capsid shells that encapsidate a segmented dsRNA genome . In particular, reovirus has eight structural proteins and ten segments of dsRNA. A series of uncoating steps and conformational changes accompany cell entry and replication. High-resolution structures are known for almost all of

5175-448: The realm Duplodnaviria , usually the tailed bacteriophages of the order Caudovirales , and tailless or non-tailed dsDNA viruses of the realm Varidnaviria . dsDNA viruses are classified into three of the four realms and include many taxa that are unassigned to a realm: The second Baltimore group contains viruses that have a single-stranded DNA (ssDNA) genome. ssDNA viruses have the same manner of transcription as dsDNA viruses. Because

5250-709: The transcription and replication parts of the viral life cycle . Structural characteristics of a virus particle, called a virion, such as the shape of the viral capsid and the presence of a viral envelope , a lipid membrane that surrounds the capsid, have no direct relation to Baltimore groups, nor do the groups necessarily show genetic relation based on evolutionary history. DNA viruses have genomes made of deoxyribonucleic acid (DNA) and are organized into two groups: double-stranded DNA (dsDNA) viruses, and single-stranded DNA (ssDNA) viruses. They are assigned to four separate realms: Adnaviria , Duplodnaviria , Monodnaviria , and Varidnaviria . Many have yet to be assigned to

5325-607: The vast majority of dsDNA viruses infect prokaryotes, ssDNA viruses are found in all three domains of life, dsRNA and +ssRNA viruses are primarily found in eukaryotes but also in bacteria, and -ssRNA and reverse transcribing viruses are only found in eukaryotes. Viral genomes may be either linear with ends or circular in a loop. Whether a virus has a linear or circular genome varies from group to group. A significant percentage of dsDNA viruses are both, ssDNA viruses are primarily circular, RNA viruses and ssRNA-RT viruses are typically linear, and dsDNA-RT viruses are typically circular. In

5400-475: The viral RNA that are then attached to sgRNA strands. Because replication is required for sgRNA synthesis, RdRp is always translated first. Because the process of replicating the viral genome produces intermediate dsRNA molecules, +ssRNA viruses can be targeted by the host cell's immune system. To avoid detection, +ssRNA viruses replicate in membrane-associated vesicles that are used as replication factories. From there, only viral +ssRNA, which may be mRNA, enters

5475-738: The viral genome, and the evolutionary history of viruses are not necessarily related to Baltimore groups. Baltimore classification was created in 1971 by virologist David Baltimore . Since then, it has become common among virologists to use Baltimore classification alongside standard virus taxonomy, which is based on evolutionary history. In 2018 and 2019, Baltimore classification was partially integrated into virus taxonomy based on evidence that certain groups were descended from common ancestors. Various realms, kingdoms, and phyla now correspond to specific Baltimore groups. Baltimore classification groups viruses together based on their manner of mRNA synthesis. Characteristics directly related to this include whether

5550-524: Was expanded to also include both RT groups. Within the realm, RT viruses are included in the kingdom Pararnavirae and RNA viruses in the kingdom Orthornavirae . Furthermore, the three Baltimore groups for RNA viruses are used as defining characteristics of the phyla in Orthornavirae . Unlike RNA viruses and RT viruses, DNA viruses have not been united under a single realm but are instead dispersed across four realms and various taxa that are not assigned to

5625-505: Was proposed in 1971 by virologist David Baltimore in a paper titled Expression of Animal Virus Genomes . It initially contained the first six groups but was later expanded to include group VII. Because of the utility of Baltimore classification, it has come to be used alongside standard virus taxonomy, which is based on evolutionary relationships and governed by the International Committee on Taxonomy of Viruses (ICTV). From

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