RNA virus

Taxonomy and replication strategies of different types of RNA viruses

An RNA virus is a virus which has (ribonucleic acid) RNA as its genetic material.[1] The nucleic acid is usually single-stranded RNA (ssRNA) but it may be double-stranded RNA (dsRNA).[2] Notable human diseases caused by RNA viruses include the common cold, influenza, SARS, MERS, COVID-19, Dengue Virus, hepatitis C, hepatitis E, West Nile fever, Ebola virus disease, rabies, polio, mumps, and measles.

The International Committee on Taxonomy of Viruses (ICTV) classifies RNA viruses as those that belong to Group III, Group IV or Group V of the Baltimore classification system of classifying viruses and does not consider viruses with DNA intermediates in their life cycle as RNA viruses.[3] Viruses with RNA as their genetic material which also include DNA intermediates in their replication cycle are called retroviruses, and comprise Group VI of the Baltimore classification. Notable human retroviruses include HIV-1 and HIV-2, the cause of the disease AIDS.

All RNA viruses encoding an RNA-directed RNA polymerase, known as of May 2020, form a monophyletic group now known as the realm Riboviria.[4] The majority of such RNA viruses fall into the kingdom Orthornavirae and the rest have a positioning not yet defined.[5] The realm does not contain all RNA viruses: Deltavirus, Asunviroidae, and Pospiviroidae are taxa of RNA viruses that have been mistakenly included in 2019,[a] but corrected in 2020.[6]

Characteristics

Single-stranded RNA viruses and RNA Sense

RNA viruses can be further classified according to the sense or polarity of their RNA into negative-sense and positive-sense, or ambisense RNA viruses. Positive-sense viral RNA is similar to mRNA and thus can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA-dependent RNA polymerase before translation. Purified RNA of a positive-sense virus can directly cause infection though it may be less infectious than the whole virus particle. In contrast, purified RNA of a negative-sense virus is not infectious by itself as it needs to be transcribed into positive-sense RNA; each virion can be transcribed to several positive-sense RNAs. Ambisense RNA viruses resemble negative-sense RNA viruses, except they translate genes from their negative and positive strands.[7]

Double-stranded RNA viruses

Structure of the reovirus virion

The double-stranded (ds)RNA viruses represent a diverse group of viruses that vary widely in host range (humans, animals, plants, fungi,[b] and bacteria), genome segment number (one to twelve), and virion organization (Triangulation number, capsid layers, spikes, turrets, etc.). Members of this group include the rotaviruses, which are the most common cause of gastroenteritis in young children, and picobirnaviruses, which are the most common virus in fecal samples of both humans and animals with or without signs of diarrhea. Bluetongue virus is an economically important pathogen that infects cattle and sheep. In recent years, progress has been made in determining atomic and subnanometer resolution structures of a number of key viral proteins and virion capsids of several dsRNA viruses, highlighting the significant parallels in the structure and replicative processes of many of these viruses.[2]

Mutation rates

RNA viruses generally have very high mutation rates compared to DNA viruses,[9] because viral RNA polymerases lack the proofreading ability of DNA polymerases.[10] The genetic diversity of RNA viruses is one reason why it is difficult to make effective vaccines against them.[11] Retroviruses also have a high mutation rate even though their DNA intermediate integrates into the host genome (and is thus subject to host DNA proofreading once integrated), because errors during reverse transcription are embedded into both strands of DNA before integration.[12] Some genes of RNA virus are important to the viral replication cycles and mutations are not tolerated. For example, the region of the hepatitis C virus genome that encodes the core protein is highly conserved,[13] because it contains an RNA structure involved in an internal ribosome entry site.[14]

Replication

Animal RNA viruses are classified by the ICTV. There are three distinct groups of RNA viruses depending on their genome and mode of replication:

  • Double-stranded RNA viruses (Group III) contain from one to a dozen different RNA molecules, each coding for one or more viral proteins.
  • Positive-sense ssRNA viruses (Group IV) have their genome directly utilized as mRNA, with host ribosomes translating it into a single protein that is modified by host and viral proteins to form the various proteins needed for replication. One of these includes RNA-dependent RNA polymerase (RNA replicase), which copies the viral RNA to form a double-stranded replicative form. In turn, this dsRNA directs the formation of new viral RNA.
  • Negative-sense ssRNA viruses (Group V) must have their genome copied by an RNA replicase to form positive-sense RNA. This means that the virus must bring along with it the enzyme RNA replicase. The positive-sense RNA molecule then acts as viral mRNA, which is translated into proteins by the host ribosomes.

Retroviruses (Group VI) have a single-stranded RNA genome but, in general, are not considered RNA viruses because they use DNA intermediates to replicate. Reverse transcriptase, a viral enzyme that comes from the virus itself after it is uncoated, converts the viral RNA into a complementary strand of DNA, which is copied to produce a double-stranded molecule of viral DNA. After this DNA is integrated into the host genome using the viral enzyme integrase, expression of the encoded genes may lead to the formation of new virions.

Recombination

Numerous RNA viruses are capable of genetic recombination when at least two viral genomes are present in the same host cell.[15] RNA recombination appears to be a major driving force in determining genome architecture and the course of viral evolution among Picornaviridae ((+)ssRNA) (e.g. poliovirus).[16] In the Retroviridae ((+)ssRNA)(e.g. HIV), damage in the RNA genome appears to be avoided during reverse transcription by strand switching, a form of recombination.[17][18][19] Recombination also occurs in the Reoviridae (dsRNA)(e.g. reovirus), Orthomyxoviridae ((-)ssRNA)(e.g. influenza virus)[19] and Coronaviridae ((+)ssRNA) (e.g. SARS).[20] Recombination in RNA viruses appears to be an adaptation for coping with genome damage.[15] Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans.[20]

Classification

Classification of the RNA viruses is difficult. This is in part due to the high mutation rates these genomes undergo. Classification is based principally on the type of genome (double-stranded, negative- or positive-single-strand) and gene number and organization. Currently, there are 5 orders and 47 families of RNA viruses recognized. There are also many unassigned species and genera.

Related to but distinct from the RNA viruses are the viroids and the RNA satellite viruses. These are not currently classified as RNA viruses and are described on their own pages.

A study of several thousand RNA viruses has shown the presence of at least five main taxa: a levivirus and relatives group; a picornavirus supergroup; an alphavirus supergroup plus a flavivirus supergroup; the dsRNA viruses; and the -ve strand viruses.[21] The lentivirus group appears to be basal to all the remaining RNA viruses. The next major division lies between the picornasupragroup and the remaining viruses. The dsRNA viruses appear to have evolved from a +ve RNA ancestor and the -ve RNA viruses from within the dsRNA viruses. The closest relation to the -ve stranded RNA viruses is the Reoviridae.

Positive strand RNA viruses

This is the single largest group of RNA viruses[22] with 30 families. Attempts have been made to group these families in higher orders. These proposals were based on an analysis of the RNA polymerases and are still under consideration. To date, the suggestions proposed have not been broadly accepted because of doubts over the suitability of a single gene to determine the taxonomy of the clade.

The proposed classification of positive-strand RNA viruses is based on the RNA-dependent RNA polymerase. Three groups have been recognised:[23]

  1. Bymoviruses, comoviruses, nepoviruses, nodaviruses, picornaviruses, potyviruses, sobemoviruses and a subset of luteoviruses (beet western yellows virus and potato leafroll virus)—the picorna like group (Picornavirata).
  2. Carmoviruses, dianthoviruses, flaviviruses, pestiviruses, statoviruses, tombusviruses, single-stranded RNA bacteriophages, hepatitis C virus and a subset of luteoviruses (barley yellow dwarf virus)—the flavi like group (Flavivirata).
  3. Alphaviruses, carlaviruses, furoviruses, hordeiviruses, potexviruses, rubiviruses, tobraviruses, tricornaviruses, tymoviruses, apple chlorotic leaf spot virus, beet yellows virus and hepatitis E virus—the alpha like group (Rubivirata).

A division of the alpha-like (Sindbis-like) supergroup on the basis of a novel domain located near the N termini of the proteins involved in viral replication has been proposed.[24] The two groups proposed are: the 'altovirus' group (alphaviruses, furoviruses, hepatitis E virus, hordeiviruses, tobamoviruses, tobraviruses, tricornaviruses and probably rubiviruses); and the 'typovirus' group (apple chlorotic leaf spot virus, carlaviruses, potexviruses and tymoviruses).

The alpha like supergroup can be further divided into three clades: the rubi-like, tobamo-like, and tymo-like viruses.[25]

Additional work has identified five groups of positive-stranded RNA viruses containing four, three, three, three, and one order(s), respectively.[26] These fourteen orders contain 31 virus families (including 17 families of plant viruses) and 48 genera (including 30 genera of plant viruses). This analysis suggests that alphaviruses and flaviviruses can be separated into two families—the Togaviridae and Flaviridae, respectively—but suggests that other taxonomic assignments, such as the pestiviruses, hepatitis C virus, rubiviruses, hepatitis E virus, and arteriviruses, may be incorrect. The coronaviruses and toroviruses appear to be distinct families in distinct orders and not distinct genera of the same family as currently classified. The luteoviruses appear to be two families rather than one, and apple chlorotic leaf spot virus appears not to be a closterovirus but a new genus of the Potexviridae.

Evolution

The evolution of the picornaviruses based on an analysis of their RNA polymerases and helicases appears to date to the divergence of eukaryotes.[27] Their putative ancestors include the bacterial group II retroelements, the family of HtrA proteases and DNA bacteriophages.

Partitiviruses are related to and may have evolved from a totivirus ancestor.[28]

Hypoviruses and barnaviruses appear to share an ancestry with the potyvirus and sobemovirus lineages respectively.[28]

Double-stranded RNA viruses

This analysis also suggests that the dsRNA viruses are not closely related to each other but instead belong to four additional classes—Birnaviridae, Cystoviridae, Partitiviridae, and Reoviridae—and one additional order (Totiviridae) of one of the classes of positive ssRNA viruses in the same subphylum as the positive-strand RNA viruses.

One study has suggested that there are two large clades: One includes the families Caliciviridae, Flaviviridae, and Picornaviridae and a second that includes the families Alphatetraviridae, Birnaviridae, Cystoviridae, Nodaviridae, and Permutotretraviridae.[29]

Negative strand RNA viruses

These viruses have multiple types of genome ranging from a single RNA molecule up to eight segments. Despite their diversity it appears that they may have originated in arthropods and to have diversified from there.[30]

Satellite viruses

A number of satellite viruses—viruses that require the assistance of another virus to complete their life cycle—are also known. Their taxonomy has yet to be settled. The following four genera have been proposed for positive sense single stranded RNA satellite viruses that infect plants—Albetovirus, Aumaivirus, Papanivirus and Virtovirus.[31] A family—Sarthroviridae which includes the genus Macronovirus—has been proposed for the positive sense single stranded RNA satellite viruses that infect arthropods.

Group III – dsRNA viruses

There are twelve families and a number of unassigned genera and species recognised in this group.[10]

  • Family Amalgaviridae
  • Family Birnaviridae
  • Family Chrysoviridae
  • Family Cystoviridae
  • Family Endornaviridae
  • Family Hypoviridae
  • Family Megabirnaviridae
  • Family Partitiviridae
  • Family Picobirnaviridae
  • Family Reoviridae – includes Rotavirus
  • Family Totiviridae
  • Family Quadriviridae
  • Genus Botybirnavirus
  • Unassigned species
    • Botrytis porri RNA virus 1
    • Circulifer tenellus virus 1
    • Colletotrichum camelliae filamentous virus 1
    • Cucurbit yellows associated virus
    • Sclerotinia sclerotiorum debilitation-associated virus
    • Spissistilus festinus virus 1

Group IV – positive-sense ssRNA viruses

There are three orders and 34 families recognised in this group. In addition, there are a number of unclassified species and genera.

  • Order Nidovirales
  • Order Picornavirales
    • Family Dicistroviridae
    • Family Iflaviridae
    • Family Marnaviridae
    • Family Picornaviridae – includes Poliovirus, Rhinovirus (a common cold virus), Hepatitis A virus
    • Family Secoviridae includes subfamily Comovirinae
    • Genus Bacillariornavirus
    • Species Kelp fly virus
  • Order Tymovirales
    • Family Alphaflexiviridae
    • Family Betaflexiviridae
    • Family Gammaflexiviridae
    • Family Tymoviridae
  • Unassigned
    • Family Alphatetraviridae
    • Family Alvernaviridae
    • Family Astroviridae
    • Family Barnaviridae
    • Family Benyviridae
    • Family Botourmiaviridae
    • Family Bromoviridae
    • Family Caliciviridae – includes Norwalk virus
    • Family Carmotetraviridae
    • Family Closteroviridae
    • Family Flaviviridae – includes Yellow fever virus, West Nile virus, Hepatitis C virus, Dengue fever virus, Zika virus
    • Family Fusariviridae
    • Family Hepeviridae
    • Family Hypoviridae
    • Family Leviviridae
    • Family Luteoviridae – includes Barley yellow dwarf virus
    • Family Polycipiviridae
    • Family Narnaviridae
    • Family Nodaviridae
    • Family Permutotetraviridae
    • Family Potyviridae
    • Family Sarthroviridae
    • Family Statovirus
    • Family Togaviridae – includes Rubella virus, Ross River virus, Sindbis virus, Chikungunya virus
    • Family Tombusviridae
    • Family Virgaviridae[32]
    • Unassigned genera
      • Genus Blunervirus
      • Genus Cilevirus
      • Genus Higrevirus
      • Genus Idaeovirus
      • Genus Negevirus
      • Genus Ourmiavirus
      • Genus Polemovirus
      • Genus Sinaivirus
      • Genus Sobemovirus
    • Unassigned species
      • Acyrthosiphon pisum virus
      • Bastrovirus
      • Blackford virus
      • Blueberry necrotic ring blotch virus
      • Cadicistrovirus
      • Chara australis virus
      • Extra small virus
      • Goji berry chlorosis virus
      • Harmonia axyridis virus 1
      • Hepelivirus
      • Jingmen tick virus
      • Le Blanc virus
      • Nedicistrovirus
      • Nesidiocoris tenuis virus 1
      • Niflavirus
      • Nylanderia fulva virus 1
      • Orsay virus
      • Osedax japonicus RNA virus 1
      • Picalivirus
      • Planarian secretory cell nidovirus
      • Plasmopara halstedii virus
      • Rosellinia necatrix fusarivirus 1
      • Santeuil virus
      • Secalivirus
      • Solenopsis invicta virus 3
      • Wuhan large pig roundworm virus

Satellite viruses

  • Family Sarthroviridae
  • Genus Albetovirus
  • Genus Aumaivirus
  • Genus Papanivirus
  • Genus Virtovirus
  • Chronic bee paralysis virus

An unclassified astrovirus/hepevirus-like virus has also been described.[33]

Group V – negative-sense ssRNA viruses

With the exception of the Hepatitis D virus, this group of viruses has been placed into a single phylum—Negarnaviricota. This phylum has been divided into two subphyla—Haploviricotina and Polyploviricotina. Within the subphylum Haploviricotina four classes are currently recognised: Chunqiuviricetes, Milneviricetes, Monjiviricetes and Yunchangviricetes. In the subphylum Polyploviricotina two classes are recognised: Ellioviricetes and Insthoviricetes.

Six classes, seven orders and twenty four families are currently recognized in this group. A number of unassigned species and genera are yet to be classified.[10]

  • Phylum Negarnaviricota[34]
    • Subphylum Haploviricotina
      • Class Chunqiuviricetes
        • Order Muvirales
          • Family Qinviridae
      • Class Milneviricetes
        • Order Serpentovirales
          • Family Aspiviridae
      • Class Monjiviricetes
        • Order Jingchuvirales
          • Family Chuviridae
        • Order Mononegavirales
          • Family BornaviridaeBorna disease virus
          • Family Filoviridae – includes Ebola virus, Marburg virus
          • Family Mymonaviridae
          • Family Nyamiviridae[35]
          • Family Paramyxoviridae – includes Measles virus, Mumps virus, Nipah virus, Hendra virus, and NDV
          • Family Pneumoviridae – includes RSV and Metapneumovirus
          • Family Rhabdoviridae – includes Rabies virus
          • Family Sunviridae
          • Genus Anphevirus
          • Genus Arlivirus
          • Genus Chengtivirus
          • Genus Crustavirus
          • Genus Wastrivirus
      • Class Yunchangviricetes
        • Order Goujianvirales
          • Family Yueviridae
    • Subphylum Polyploviricotina
      • Class Ellioviricetes
        • Order Bunyavirales
          • Family Arenaviridae – includes Lassa virus
          • Family Cruliviridae
          • Family Feraviridae
          • Family Fimoviridae
          • Family Hantaviridae
          • Family Jonviridae
          • Family Nairoviridae
          • Family Peribunyaviridae
          • Family Phasmaviridae
          • Family Phenuiviridae
          • Family Tospoviridae
          • Genus Tilapineviridae
      • Class Insthoviricetes
        • Order Articulavirales
  • Unassigned genera:
    • Genus Deltavirus – includes Hepatitis D virus (not a true virus, but a subviral agent)

Gallery

See also

Notes

  1. ^ This inclusion was due to TaxoProp 2017.006G, which proposed Riboviria. The confusion might be due to the TaxoProp's reference to a "monophyly of all RNA viruses", improperly termed as it was only demonstrated with RdRP. On the other hand, the proposed definition of Riboviria did correctly mention RdRP.
  2. ^ The majority of fungal viruses are double-stranded RNA viruses. A small number of positive-strand RNA viruses have been described. One report has suggested the possibility of a negative stranded virus.[8]

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VIH - HIV / SIDA - AIDS viruses.
Mumps virus, negative stained TEM 8758 lores.jpg

ID#: 8758 Description: This 1973 negative stained transmission electron micrograph (TEM) depicted the ultrastructural features displayed by the mumps virus. The mumps virus replicates in the upper respiratory tract and is spread through direct contact with respiratory secretions or saliva or through fomites, i.e., inanimate objects that are contaminated by the virus, and are subsequently handled. The infectious period or time that an infected person can transmit mumps to a non-infected person is from 3 days before symptoms appear to about 9 days after the symptoms appear. The incubation time, which is the period from when a person is exposed to virus to the onset of any symptoms, can vary from 16 to 18 days (range 12-25 days).

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Computer assisted reconstruction of a rotavirus
Lymphocytic choriomeningitis virus.jpg
Methylamine tungstate negative-stain electron micrograph of arenavirus isolated from mouse spleen homogenate cultures that tested positive by immunofluorescence assay for lymphocytic choriomeningitis virus infection. Viral envelope spikes and projections are visible, and virion inclusions show a sandy appearance, indicating Arenaviridae.
Parainfluenza virus TEM PHIL 271 lores.jpg

ID#: 271 Description: Transmission electron micrograph of parainfluenza virus. Transmission electron micrograph of parainfluenza virus. Two particles and free filamentous nucleocapsid.

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ID#: 2175 Description: Thic electron micrograph reveals the morphologic traits of the Respiratory Syncytial Virus (RSV). The virion is variable in shape, and size (average diameter of between 120-300nm). RSV is the most common cause of bronchiolitis and pneumonia among infants and children under 1 year of age.

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Copyright Restrictions: None - This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image.
Vesicular stomatitis virus (VSV) EM 18 lores.jpg

ID#: 5611 Description: This electron micrograph depicts the vesiculovirus responsible for vesicular stomatitis (VS) in horses, cattle and pigs. As a member of the Rhabdoviridae family of viruses, you’ll note the morphologic similarity, i.e., bullet-shaped virion, between this vesicular stomatitis virus (VSV), and the rabies virus.

Content Providers(s): CDC Creation Date: 1981

Copyright Restrictions: None - This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image.
Measles virus.JPG
This thin-section transmission electron micrograph (TEM) revealed the ultrastructural appearance of a single virus particle, or “virion”, of measles virus. The measles virus is a paramyxovirus, of the genus Morbillivirus. It is 100-200 nm in diameter, with a core of single-stranded RNA, and is closely related to the rinderpest and canine distemper viruses. Two membrane envelope proteins are important in pathogenesis. They are the F (fusion) protein, which is responsible for fusion of virus and host cell membranes, viral penetration, and hemolysis, and the H (hemagglutinin) protein, which is responsible for adsorption of virus to cells.

There is only one antigenic type of measles virus. Although studies have documented changes in the H glycoprotein, these changes do not appear to be epidemiologically important (i.e., no change in vaccine efficacy has been observed).

Prior to 1963, almost everyone got measles; it was an expected life event. Each year in the U.S. there were approximately 3 to 4 million cases and an average of 450 deaths, with epidemic cycles every 2 to 3 years. More than half the population had measles by the time they were 6 years old, and 90 % had the disease by the time they were 15. This indicates that many more cases were occurring than were being reported. However, after the vaccine became available, the number of measles cases dropped by 98 % and the epidemic cycles drastically diminished.

Measles virus is rapidly inactivated by heat, light, acidic pH, ether, and trypsin. It has a short survival time (<2 hours) in the air, or on objects and surfaces.
Marburg virus.jpg
This negative stained transmission electron micrograph (TEM) depicts a number of filamentous Marburg virions, which had been cultured on Vero cell cultures, and purified on sucrose, rate-zonal gradients. Note the virus’s morphologic appearance with its characteristic “Shepherd’s Crook” shape; Magnified approximately 100,000x. Marburg hemorrhagic fever is a rare, severe type of hemorrhagic fever which affects both humans and non-human primates. Caused by a genetically unique zoonotic (that is, animal-borne) RNA virus of the filovirus family, its recognition led to the creation of this virus family. The four species of Ebola virus are the only other known members of the filovirus family. Marburg virus was first recognized in 1967, when outbreaks of hemorrhagic fever occurred simultaneously in laboratories in Marburg and Frankfurt, Germany and in Belgrade, Yugoslavia (now Serbia).
Structure of the reovirus virion.png
Author/Creator: Nicholas H. Acheson, Licence: CC BY-SA 4.0
The T=1 inner capsid encloses the 10 double-stranded RNA gene segments and is covered by a T=13 outer capsid. Twelve spikes project from the core through the outer capsid. Gene segments are classified as large (L), medium (M), or small (S) based on relative electrophoretic mobilities. Viral capsid proteins are labeled. Nine of the 10 reovirus gene segments code for a single protein; the exception is the S1 gene, which codes for both the (sigma)1 attachment protein and the (sigma)1s nonstructural protein in overlapping reading frames. Viral proteins are named with the Greek letters (lambda), (mu), and (sigma), corresponding to the size classes (L, M, and S) of their respective gene segments.
Influenza virus particle 8430 lores.jpg
This negative-stained transmission electron micrograph (TEM) depicts the ultrastructural details of an influenza virus particle, or “virion”. A member of the taxonomic family Orthomyxoviridae, the influenza virus is a single-stranded RNA organism

The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. The best way to prevent this illness is by getting a flu vaccination each fall.

Every year in the United States, on average:

- 5% to 20% of the population gets the flu

- more than 200,000 people are hospitalized from flu complications, and

- about 36,000 people die from flu. Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications. See PHIL 10073 for a colorized version of this image.

Influenza A and B are the two types of influenza viruses that cause epidemic human disease. Influenza A viruses are further categorized into subtypes on the basis of two surface antigens: hemagglutinin and neuraminidase. Influenza B viruses are not categorized into subtypes. Since 1977, influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses have been in global circulation. In 2001, influenza A (H1N2) viruses that probably emerged after genetic reassortment between human A (H3N2) and A (H1N1) viruses began circulating widely. Both influenza A and B viruses are further separated into groups on the basis of antigenic characteristics. New influenza virus variants result from frequent antigenic change (i.e., antigenic drift) resulting from point mutations that occur during viral replication. Influenza B viruses undergo antigenic drift less rapidly than influenza A viruses.
Ebola virions.png
Author/Creator: See Source, Licence: CC BY 2.5
Scanning electron microscopic image of Ebola virions.
18 2014 1695 Fig1 HTML.webp
Author/Creator: Aartjan J. W. te Velthuis, Licence: CC BY 4.0
Taxonomy and replication strategies of RNA viruses. a Simplified taxonomy of the genome architecture of the RNA viruses described in this review. See main text for used abbreviations. b (+RNA virus) Infection with a +RNA virus—as exemplified here with a CoV-like virion—releases a single-stranded RNA genome into the cytoplasm (1) [81, 173, 174]. (2) Translation of the 5′-terminal open-reading frame of the genome produces the viral replicase. (3) This multi-enzyme complex includes RdRp activity (orange) and associates with intracellular membranes before −RNA synthesis commences. Newly synthesised −RNAs are subsequently used to produce new +RNAs (4), which are typically capped (yellow) and polyadenylated (polyA). (Retrovirus) HIV-1 genomes are packaged as ssRNA in virions. When the ssRNA is released (1) a cDNA copy is synthesised by the RT (2). The RNA is next degraded by the intrinsic RNase H activity in the RT (3) and the single stranded cDNA converted to dsDNA (4). The dsDNA is imported in the nucleus (5) for integration into the host’s genetic material. (−RNA virus) (1) As illustrated here with an IAV-like particle, infection with an −RNA virus releases a viral RNA genome that is associated with a viral polymerase (orange) and nucleoprotein (green). (2) In the case of non-segmented −RNA viruses, these complexes support transcription to produce viral mRNAs or cRNAs. (3) Viral mRNAs are next translated and new viral proteins complex with cRNAs to synthesise new vRNAs. (5) The vRNA-containing complexes of some segmented −RNA viruses are imported into the nucleus of the host cell, where (6) the RdRp produces mRNAs or cRNAs. (7) mRNAs are transported to the cytoplasm, while cRNAs are bound by new viral proteins to form cRNPs for −RNA synthesis. (dsRNA virus) Fully duplexed RNA genomes lack cap and polyA elements. (1) The RdRp (orange), therefore, transcribes the viral genome inside the capsid of the virion (blue and red), so viral mRNAs can be (2) released into the cytoplasm as illustrated here with a rotavirus-like virion. In the cytoplasm the mRNA is translated (3) or replicated by newly synthesised viral RdRps (4) [175, 176]
Sin Nombre virus Hanta TEM 1137 lores.jpg

ID#: 1137 Description: Transmission electron micrograph of Sin Nombre virus. Transmission electron micrograph of Sin Nombre virus. Hantavirus.

Content Providers(s): CDC/ Cynthia Goldsmith, Luanne Elliott

Copyright Restrictions: None - This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image.
Lassa virus.JPG

A transmission electron micrograph (TEM) of a number of Lassa virus virions adjacent to some cell debris. The virus, a member of the virus family en:Arenaviridae, causes en:Lassa fever.

Source:CDC's Public Health Image Library Image #8700

Photo Credit: C. S. Goldsmith
Rabies Virus EM PHIL 1876.JPG

ID#: 1876 Description: Electron micrograph of the Rabies Virus. This electron micrograph shows the rabies virus, as well as Negri bodies, or cellular inclusions.

Content Providers(s): CDC/Dr. Fred Murphy Creation Date: 1975

Copyright Restrictions: None - This image is in the public domain and thus free of any copyright restrictions. As a matter of courtesy we request that the content provider be credited and notified in any public or private usage of this image.