Everything about Positive-sense Ssrna Virus totally explained
An
RNA virus is a
virus that has
RNA (ribonucleic acid) as its
genetic material. This
nucleic acid is usually single-stranded RNA (ssRNA) but may be double-stranded RNA (dsRNA). The
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 doesn't consider viruses with DNA intermediates as RNA viruses. Notable human diseases caused by RNA viruses include
SARS,
influenza and
hepatitis C.
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 identical to viral 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 polymerase before translation. As such, purified RNA of a positive-sense virus can directly cause infection though it may be less infectious than the whole virus particle. Purified RNA of a negative-sense virus isn't infectious by itself as it needs to be
transcribed into positive-sense RNA.
Ambisense RNA viruses resemble negative-sense RNA viruses, except they also translate genes from the positive strand.
Double-stranded RNA viruses
The
double-stranded (ds)RNA viruses represent a diverse group of viruses that vary widely in host range (humans, animals, plants,
fungi, and
bacteria),
genome segment number (one to twelve), and
virion organization (T-number,
capsid layers, or turrets). Members of this group include the
rotaviruses, renowned globally as the commonest cause of
gastroenteritis in young children, and
bluetongue virus , an economically important pathogen of cattle and sheep. In recent years, remarkable progress has been made in determining, at atomic and subnanometeric levels, the structures of a number of key viral proteins and of the virion capsids of several dsRNA viruses, highlighting the significant parallels in the structure and replicative processes of many of these viruses.
Mutation rates
RNA viruses generally have very high
mutation rates as they lack
DNA polymerases which can find and fix mistakes, and are therefore unable to conduct
DNA repair of damaged genetic material.
DNA viruses have considerably lower mutation rates due to the proof-reading ability of DNA polymerases within the host cell.
Retroviruses 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 prior to integration.
Although RNA usually mutates rapidly, recent work found that the SARS virus and related RNA viruses contain a gene that mutates very slowly. The gene in question has a complex three-dimensional structure which is hypothesized to provide a chemical function necessary for viral propagation, perhaps as a
ribozyme. If so, most mutations would render it unfit for that purpose and wouldn't propagate.
Replication
Animal RNA viruses are
classified
into three distinct groups depending on their genome and mode of replication (and the numerical groups based on the older
Baltimore classification):
- Double-stranded RNA viruses (Group III) contain from one to a dozen different RNA molecules, each of which codes for one or more viral proteins.
- Positive-sense ssRNA viruses (Group IV) have their genome directly utilized as if it were mRNA, producing a single protein which is modified by host and viral proteins to form the various proteins needed for replication. One of these includes RNA-dependent RNA polymerase, which copies the viral RNA to form a double-stranded replicative form, in turn this directs the formation of new virions.
- Negative-sense ssRNA viruses (Group V) must have their genome copied by an RNA polymerase to form positive-sense RNA. This means that the virus must bring along with it the RNA-dependent RNA polymerase enzyme. The positive-sense RNA molecule then acts as viral mRNA, which is translated into proteins by the host ribosomes. The resultant protein goes on to direct the synthesis of new virions, such as capsid proteins and RNA replicase, which is used to produce new negative-sense RNA molecules.
Retroviruses (Group VI) have a single-stranded RNA genome but are usually not considered RNA viruses because they use DNA intermediates to replicate.
Reverse transcriptase, a viral enzyme that comes from the virus itself after it's 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, expression of the encoded genes may lead the formation of new virions.
Group III - dsRNA viruses
Family Birnaviridae
Family Chrysoviridae
Family Cystoviridae
Family Hypoviridae
Family Partitiviridae
Family Reoviridae - includes Rotavirus
Family Totiviridae
Unassigned genera
Group IV - positive-sense ssRNA viruses
Order Nidovirales
Unassigned
Group V - negative-sense ssRNA viruses
Order Mononegavirales
Unassigned
Further Information
Get more info on 'Positive-sense Ssrna Virus'.
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