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17.1 Virion properties

Virions are spherical in shape and are having uniform appearance. The diameter of virus varies between 70-100 nm. The enveloped virion particle encircles the icosahedral capsid. Envelope contains spikes of viral glycoprotein which are major antigenic determinants of the virus. The spike glycoproteins are highly variable among strains and also between different serotypes.

 

17.2 Structure of positive strand RNA genome

 

They are positive sense, single-stranded RNA, vary between 9-12 kb in size, with the exception of coronaviruses. Terminal 5' end of the genome is capped while 3' end is polyadenylated. Viruses with single stranded RNA genome do not require secondary or tertiary fold in their capsid to accommodate its genome. That means they are highly organized and tightly packed. Generally a dimer of coat protein interacts with the 3' end of the RNA, which is essential for the virus replication. This interaction is also needed for the packaging of the genomic RNA inside the virion. The viral genomic RNA is arranged in the icosahedral capsid in various ways in order to neutralize the negative charge of the nucleic acids.

 

17.3 Replication of positive strand RNA

 

The positive strand RNA virus transfers its genome directly to the ribosome and starts translation for the synthesis of viral proteins. Infectious cycle begins with the entry of virus into the cell through endocytosis. The genomic RNA uncoats after getting into the cytoplasm of the infected cells. The RNA is then translated into the viral polyprotein precursors which are later cleaved by proteolysis to form the structural and non structural viral proteins. The structural proteins are involved in the maturation and assembly of the virion while nonstructural proteins act as RNA replicating enzyme for genomic RNA synthesis. Some of the viruses in this class form the subgenomic RNA during replication process (Coronavirus, Caliciviruses, and Togaviruses).