(B.) RNA viruses

Important Characteristics:

• •  RNA as their genetic material

•  Genome may be ssRNA or dsRNA

•  ssRNA viruses may be of positive (+) sense or negative (-) sense

•  Replicationoccurs in the cytoplasm using a RNA-dependent RNA polymerase

•  RNA dependent RNA polymerases are not having proofreading activity and hence replicate their templates with a higher error rate.

Positive (+) sense viral RNA  is similar to mRNA and thus can be immediately translated by the host cell. Flaviviruses,togaviruses, poliovirus are some examples of (+) sense RNA viruses. 

Negative (-) sense viral RNA is complementary to mRNA and thus must be converted to positive (+) sense RNA by an RNA dependent RNA polymerase before translation. Influenza virus, Measles virus, Rabies virus are some examples of negative (-) sense RNA viruses.

Double-stranded RNA viruses need to package an RNA dependent RNA polymerase to make their mRNA after infection of the host cell. Examples of dsRNA viruses are Rotaviruses which belong to family Reoviruses.

Important groups of RNA viruses are:

• •  Picornaviridae

•  Togaviridae

•  Flaviviridae

•  Retroviridae

•  Paramyxoviridae

•  Orthomyxoviridae

•  Bunyaviridae

•  Reoviridae

The different families of RNA viruses are distinguished from one another by their nucleic acid content, their capsid shape and presence or absence of an envelope (as shown in Table 2)