RNA viruses are viruses whose genetic makeup is solely made of the nucleic acid molecule known as ribonucleic acid (RNA). Viruses as aforementioned are basically classified based on their genome content either as DNA-containing viruses or RNA-containing viruses depending on the genetic makeup of the particular virus. RNA viruses are also different from the DNA viruses (whose genetic makeup is solely made up of deoxyribonucleic acid molecules) 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. Both Retroviruses and Orthomyxoviruses 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(s).
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 (Table 1). 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 (i.e., cDNA) 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 to RNA to 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, coupled and assembled for the formation of new virions to continue the viral life cycle.
The viruses in the Retroviridae family such as the human immunodeficiency virus (HIV) that causes AIDS in humans is one of the unique and diversified viral family because of their mode of replication. In addition to this, the Hepadnaviridae family such as the human hepatitis B virus (that causes liver inflammation in humans) is also among this category. Viruses in both Retroviridae and Hepadnaviridae families carryout their genome replication through a process called “reverse transcription”; and this is because they possess reverse transcriptase (RT) enzymes that allows them to copy genetic information backward as against the normal flow of genetic information from DNA to RNA and then to protein. Reverse transcription is the genetic process of copying the genetic information found in the RNA genome of an organism into DNA. And this process is opposed to the central molecular dogma of molecular biology which postulates that genetic information in living systems flows in this pattern: DNA to RNA to PROTEIN. In summary, both viruses in the Retroviridae family and Hepadnaviridae family replicates through the process of reverse transcription using RT enzyme.
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