Viruses as aforementioned are basically classified based on their genome content as DNA-containing viruses and RNA-containing viruses. RNA viruses are also different from the DNA viruses in terms of their site of replication aside their genomic difference from DNA viruses. All RNA viruses replicate in the cytoplasm of their host cell. The only exception in this case is Retroviruses and Orthomyxoviruses found in the Retroviridae family and Orthomyxoviridae family respectively which can replicate in the nucleus of their host cell. In addition, Retroviruses and some Hepadnaviruses (that have RNA genomes) can replicate in both compartments of the nucleus and the cytoplasm of their host cell. All RNA viruses have a single-stranded RNA (ssRNA) genome. The only exceptions in this case are Rotavirus, Reoviruses or Orthoreoviruses (found in the Reoviridae family) that have a double-stranded RNA (dsRNA) genome. RNA-containing viruses must provide a replicase enzyme (i.e. an RNA-dependent RNA polymerase) for the replication of its genome since the host cell (especially those of eukaryotic origin) do not innately possess enzymes for the replication of RNA genomes.
Structurally, RNA viruses have icosahedral shape or helical shape; and they can either be enveloped or naked viruses. The major families of RNA viruses include Retroviridae, Orthomyxoviridae, Paramyxoviridae, Filoviridae, Togaviridae, Bunyaviridae, Picornaviridae, Caliciviridae, Reoviridae, and Arenaviridae. These varieties of RNA viruses have unique replication strategies; and typical amongst these are the retroviruses in the Retroviridae family that have a reverse transcriptase (RT) enzyme. Retroviruses (such as the human immunodeficiency virus, HIV) use RT, an RNA-dependent DNA polymerase to produce a DNA copy of its own RNA genome. And in this case the RNA genome of the virus serves as a replication template for the biosynthesis of a DNA copy of the RNA genome of the virus.
The normal operation of the flow of genetic information in living systems according to the central dogma of molecular biology is from DNA — RNA — protein. But in the case of replication in retroviruses, genetic information now flows from the RNA to the DNA because the virus has an enzyme (reverse transcriptase) that carries out this function in a reverse fashion; and this process occurs in the cytoplasm of the cell. The dsDNA synthesized is then transported to the nucleus of the cell where it is inserted into the genome of the host cell as a provirus (a viral DNA). The provirus is later transcribed by the host cell into an RNA molecule (i.e. mRNA) that enters the cytoplasm or ribosome where viral proteins are finally synthesized and assembled for the formation of new virions.
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