Structure and classification of virus


Virology is often considered a part of microbiology or pathology. Virology is the study of viruses, virus-like agents, including, but not limited to, their taxonomy, disease-producing properties, cultivation, and genetics.

Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication. It focuses on the following aspects of viruses, their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is a subfield of microbiology.

Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.

The study of the manner in which viruses cause disease is viral pathogenesis. The degree to which a virus causes disease is its virulence. Virologists also study sub-viral particles, infectious entities notably smaller and simpler than viruses. A major branch of virology is virus classification.

Structure and classification:

The shape and structure of viruses has been studied by electron microscopy, NMR spectroscopy, and X-ray crystallography.

Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms.

Viruses may be viewed as mobile genetic elements, most probably of cellular origin and characterized by a long co-evolution of virus and host. For propagation viruses depend on specialized host cells supplying the complex metabolic and biosynthetic machinery of eukaryotic or prokaryotic cells. A complete virus particle is called a virion. The main function of the virion is to deliver its DNA or RNA genome into the host cell so that the genome can be expressed that means transcribed and translated by the host cell. The viral genome, often with associated basic proteins, is packaged inside a symmetric protein capsid. The nucleic acid-associated protein, called nucleoprotein, together with the genome, forms the nucleocapsid. In enveloped viruses, the nucleocapsid is surrounded by a lipid bilayer derived from the modified host cell membrane and studded with an outer layer of virus envelope glycoproteins.

Classification of Virus involves,

Morphology: Helical morphology is seen in nucleocapsids of many filamentous and pleomorphic viruses. Helical nucleocapsids consist of a helical array of capsid proteins i.e., promoters wrapped around a helical filament of nucleic acid. Icosahedral morphology is characteristic of the nucleocapsids of many spherical viruses. The number and arrangement of the morphologic subunits of the icosahedron are useful in identification and classification. Many viruses also have an outer envelope.

Chemical Composition and Mode of Replication: The genome of a virus may consist of DNA or RNA, which may be single stranded or double stranded, linear or circular. The entire genome may occupy either one nucleic acid molecule (monopartite genome) or several nucleic acid segments (multipartite genome). The different types of genome necessitate different replication strategies.

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Journal of Clinical and Experimental Pathology


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