(+) Single stranded RNA viruses are members of a large family of viruses also called picornaviruses because they have small RNA genomes. The single stranded RNA genome of the picornaviruses is formally and functionally identical to an mRNA molecules and as such is termed "+". This viral RNA molecule can be directly translated by the host cell ribosomes to make viral proteins. Importantly, the host cell does not have a mechanism to replicate RNA
Thus this genome must encode a viral enzyme that can replicate the ssRNA genome as well as the proteins needed for the capsid. In principle the viral enzyme could convert ssRNA into dsRNA but that is not what is seen. As the viral polymerase moves along the (+) stranded RNA template it elongates a (-) stranded RNA molecule that remains single-stranded. No hydrogen bonds are formed between the complementary RNA strands
The resulting (-) ssRNA molecules are then used as templates by the viral enzyme for polymerizing new (+) ssRNA molecules destined to serve as the genome in a progeny virion, or as mRNA for viral proteins. Members of this class of virus include many of the common cold viruses as well as poliovirus. Cold viruses replicate in the epithelial lining of the respiratory tract. Poliovirus replicates in the intestinal lining but on rare occasions escapes from the gut and infects the nerve cells in the spinal column resulting in paralysis
After entering the cell the ssRNA genome of the poliovirus is released from the nucleocapsid and is immediately translated. The genome of poliovirus is carried as a single (+) ssRNA molecule. However you know of at least two viral proteins required for propagation of this virus
Indeed poliovirus requires about six proteins yet it has a single RNA molecule. Does this pose a problem Translation of the (+) strand of RNA from the poliovirus results in a large polyprotein that is then cut into the separate required viral proteins. Interestingly, the protease needed to cut the polyprotein is part of the polyprotein. This creates a chicken and egg problem for you to ponder
Thus this genome must encode a viral enzyme that can replicate the ssRNA genome as well as the proteins needed for the capsid. In principle the viral enzyme could convert ssRNA into dsRNA but that is not what is seen. As the viral polymerase moves along the (+) stranded RNA template it elongates a (-) stranded RNA molecule that remains single-stranded. No hydrogen bonds are formed between the complementary RNA strands
The resulting (-) ssRNA molecules are then used as templates by the viral enzyme for polymerizing new (+) ssRNA molecules destined to serve as the genome in a progeny virion, or as mRNA for viral proteins. Members of this class of virus include many of the common cold viruses as well as poliovirus. Cold viruses replicate in the epithelial lining of the respiratory tract. Poliovirus replicates in the intestinal lining but on rare occasions escapes from the gut and infects the nerve cells in the spinal column resulting in paralysis
After entering the cell the ssRNA genome of the poliovirus is released from the nucleocapsid and is immediately translated. The genome of poliovirus is carried as a single (+) ssRNA molecule. However you know of at least two viral proteins required for propagation of this virus
Indeed poliovirus requires about six proteins yet it has a single RNA molecule. Does this pose a problem Translation of the (+) strand of RNA from the poliovirus results in a large polyprotein that is then cut into the separate required viral proteins. Interestingly, the protease needed to cut the polyprotein is part of the polyprotein. This creates a chicken and egg problem for you to ponder