RNA viruses are of two types based on the RNA strand: single-stranded virus and double-stranded virus.
The single-stranded virus can be of three types:
- Positive-strand ssRNA: It can also be called as ‘same sense to viral mRNA.’ It can be immediately translated into proteins; transcription is not required. In other words, the viral strand is “same-sense“. Its genetic code is equivalent the coding strand (which is generally the 5′ to 3′ strand in double-stranded genomes). Examples are rotavirus (reovirus), poliovirus (picornavirus).
- Negative-strand ssRNA: They are antisense to viral mRNA. These are first copied to positive strand ssRNA; which is further translated. Examples are, rabies virus (rhabdovirus), influenza virus (orthomyxovirus), measles virus, mumps virus (paramyxovirus).
The important point is that there is no DNA in the life cycle of the above two types of viruses at all. RNA-dependent RNA polymerase is used for the replication of the above two.
Another type of ssRNA virus is there, known as the ssRNA-RT virus. The virus has a positive strand. This virus has a DNA intermediate in the life cycle. Reverse transcriptase enzyme first converts RNA into DNA; then this DNA is used for replication of the genome.