RNA Viruses

RNA viruses remain in the cytoplasm except for retroviruses. RNA viruses can be classified based on their genome as follows:                   

Positive Strand RNA viruses

The RNA of positive strand RNA viruses is a mRNA (except retroviruses) and their genome is replicated by viral RNA dependent RNA polymerase (transcriptase) in the cytoplasm. The naked positive strand (coding, "sense") RNA viral genome is sufficient to initiate infection by itself. The positive strand RNA virus genomes act as mRNA, bind to ribosomes and direct protein synthesis. After an RNA-dependent RNA polymerase is produced, a negative strand RNA template is synthesized. This template can then be used to generate more mRNA and to replicate the genome. Examples of + strand RNA viruses include the following:

Alpha Viruses     Flavaviruses       Caronavirus      Picornaviruses   

                                                    Negative Strand RNA viruses

All of the negative strand RNA viruses are enveloped.

Negative strand RNA viruses carry in a virion transcriptase (RNA-dependent RNA polymerase) to produce a positive complementary mRNA as well as to replicate their genome in the cytoplasm. The negative strand (non-coding, "antisense") RNA virus genomes are templates for production of mRNA. The negative-strand RNA genome is not infectious by itself. This mRNA can then be translated into proteines. The mRNA or another positive-strand RNA species can also act as a template to generate more copies of the genome.

Examples of - strand RNA viruses include Orthomyxoviruses,    Paramyxoviruses,     Hantaviruses   

                            Ambisense Strand RNA viruses

Viruses like the following can have either a + or - RNA strand for their genome.

Bunyaviruses   

                            Double Stranded RNA viruses

Examples of double stranded RNA viruses are: Reoviruses   

                                         Retroviruses

Retroviruses convert their RNA to a hydrid RNA/DNA by virion reverse transcriptase (RT). RT then removes the RNA strand while synthesizing a new DNA strand to produce dsDNA which then integrates into the host nuclear DNA followed by production of mRNA by the host RNA polymerase.