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, with host ribosomes translating it into 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 (RNA replicase), 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 replicase 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 generally 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.
Read more about this topic: RNA Virus