Viral Packaging of Nucleic Acids

Replication of Viral Nucleic Acid. In addition to producing molecules for the formation of new capsids, the virus must replicate its nucleic acid to provide genetic material for packaging into the capsids. The way in which this is done might vary. In positive-sense, single-strand RNA viruses, a polymerase translated from viral mRNA produces negative-sense RNA from the positive-sense template which is then repeatedly transcribed into more positive strands. 

Viruses may be constructed that express the HIV-receptor CD4 and coreceptor on the viral surface, either as part of the virus itself, or as the result of viral pseudotyping (55,56). These molecules then define the target-cell specificity of the virions, and they bind to and infect target cells expressing the HIV envelope on the cell surface (i.e., HIV-infected cells). If the virus itself is cytopathic, then it may kill the target cell (55). Alternatively, these viruses may be used to package nucleic acids for gene therapy, toxins, or other anti-HIV materials. 

Some viruses also have a lipid membrane, called a viral envelope, surrounding the nucleocapsid. The envelope is derived from the host-cell plasma membrane. Viruses have been classified based on both the type of viral-particle packaging (nucleocapsid, viral envelope, etc.), and on the nucleic-acid composition (RNA or DNA) of the viral genome. 

Many viruses have icosahedral symmetry. Their nucleic-acid component (DNA or RNA) is contained in the hole of a capsid formed by the package of coat proteins. A first question then arises, whether in these vimses the external envelope of the capsid is related according to a crystallographic scaling to the viral hole in a way similar to that in axially symmetric biomacromolecules. 

As viral genome replication slows down, late proteins are synthesized in increasing quantities these include certain soluble proteins (eventually packaged with the nucleic acid in the mature phage), maturation proteins and structural proteins (e.g. capsid and envelope proteins). The phase of late protein synthesis is followed by the maturation phs, in which new virus particles are assembled from the newly synthesized nucleic acid and proteins. 

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