Viruses are infectious agents that have a simple acellular structure that is mainly made up of a protein coat or capsid and a nucleic acid genome which can either be DNA or RNA. Some viruses also have envelopes (which are lipid-containing outer membranous layer that surround the nucleocapsid in some viruses) while others lack them, and are thus generally known as naked viruses. Viruses with envelopes are known as enveloped viruses. Viruses are unique group of microorganisms that are composed of several chemical molecules or structures that are vital to their development and replication within a particular host cell that they infect or invade.
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These chemical compositions of viruses include the viral proteins, viral envelopes, viral glycoproteins and viral genomes; and they form the integral parts of mature viruses.
- Viral proteins: Viral proteins are important components of a virus because they serve as the main antigenic determinants or epitopes of an infecting pathogenic virus. The viral proteins bind specifically to the receptor molecules on host cells, and this facilitates their penetration or entry into the cell. Viral proteins form parts of the viral capsid or protein coat which serves to protect the nucleic acid genome (DNA or RNA) of the infecting virus. This protective function provided by the viral capsid prevents any external damage to the viral nucleic acid either by enzymes that readily hydrolyze or destroy the viral genome. Generally, the viral proteins provide and maintain the structural integrity or symmetry of viruses.
- Viral enzymes: Viruses like other microorganisms contain enzymes (which are proteinous in nature) that facilitate their development and replicative processes within their host cell. These enzymes which include reverse transcriptase (RT), proteases, integrase enzyme and RNA polymerase enzymes play important role in the replication process of the viruses in vivo or within a living host cell. Though some of the enzymes may have little or no function in the protein coat of the viruses, they play important role in viral genome integration and coupling in infected host cell.
- Viral genomes: The nucleic acid genome of a virus is either DNA or RNA. No virus contains both DNA and RNA as its nucleic acid genome. Viral nucleic acid genome is important because it is what contains the genetic information necessary for the replication of an infecting pathogenic virus within a living host cell. The genome of a virus can assume several orientations. The genome of a virus can either be circular or linear in structure. Some are double-stranded (ds) while others are single-stranded (ss). The viral genome can also be segmented or non-segmented; and while some RNA-containing viruses have genomes with negative-sense strands (-) others have genomes with positive-sense (+) strands. The ‘+’ and ‘–’ signs are used to indicate positive-sense and negative-sense viruses respectively. Some RNA-containing viruses have genomes that are ambisense– becausethey have a partial genetic coding sequence in the positive-sense as well as in the negative-sense. Arenaviruses are examples of viruses with ambisense genomes. There is a great variation in the nucleic acid genome of a virus depending on whether the virus is a DNA-containing virus or an RNA-containing virus. Also, the genome size of DNA viruses (which is usually in the range of 3.0-370 kbp) is different from the genome size of RNA viruses (which is usually in the range of 7-30 kb). All animal viruses including those that infect humans have a dsDNA genome. The only exception is parvoviruses (in Parvoviridae family)which have ssDNA genome. All RNA viruses that infect animals and humans have ssRNA genome with the exception of reoviruses (in Reoviridae family) which have dsRNA genome.
- Viral glycoproteins: Viral glycoproteins are viral molecules that comprise proteins and carbohydrates which are attached together. They are mainly found on enveloped viruses. Viral glycoproteins are virus-encoded, and this means that they are generated by the viruses themselves. Viral glycoproteins form the surface glycoproteins of enveloped viruses and they aid in the adsorption or attachment of enveloped viruses to specific molecules on the surface of the host cell. Naked viruses lack viral glycoproteins.
- Viral envelopes: Some viruses are known as enveloped viruses because they contain envelope while others are referred to as naked viruses because they lack envelopes. Enveloped viruses acquire their envelopes (which are usually lipid-like in nature) through the cytoplasmic membrane of their host cell by a budding process. And this occurs during the maturation and release of the virus from the host cell. Though they also contain some viral glycoproteins, the viral envelope is also made up of some portions of the host cell membrane through which they bud or exit the cell. Generally, viral envelopes are a host-cell-derived lipid bilayer membrane, which is acquired in part by viruses as they exit their infected host cell. Since they contain lipids, enveloped viruses are sensitive to lipid-solvents, ether and other organic solvents unlike naked viruses that lack lipid-envelopes and are resistant to treatment with organic solvents.
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