Shape of virus

Several viruses of medical importance still remain unclassified. Some are difficult or impossible to propagate in standard laboratory host systems and thus cannot be obtained in sufficient quantity to permit more precise characterization. Hepatitis E virus, the Norwalk virus and similar agents see Ch.

The fatal transmissible dementias in humans and other animals scrapie in sheep and goat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy; Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome in humans see Ch. The agents causing transmissible subacute spongiform encephalopathies have been linked to viroids or virinos i. Some of the transmissible amyloidoses show a familial pattern and can be explained by defined mutations which render a primary soluble glycoprotein insoluble, which in turn leads to the pathognomonic accumulation of amyloid fibers and plaques.

The pathogenesis of the sporadic amyloidoses, however, is still a matter of highly ambitious research. Turn recording back on. National Center for Biotechnology Information , U. Show details Baron S, editor. Search term. General Concepts Structure and Function Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.

Classification of Viruses Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication. Nomenclature Aside from physical data, genome structure and mode of replication are criteria applied in the classification and nomenclature of viruses, including the chemical composition and configuration of the nucleic acid, whether the genome is monopartite or multipartite.

Structure and Function Viruses are inert outside the host cell. Classification of Viruses Viruses are classified on the basis of morphology, chemical composition, and mode of replication. Morphology Helical Symmetry In the replication of viruses with helical symmetry, identical protein subunits protomers self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path.

Figure The helical structure of the rigid tobacco mosaic virus rod. Figure Fragments of flexible helical nucleocapsids NC of Sendai virus, a paramyxovirus, are seen either within the protective envelope E or free, after rupture of the envelope. Icosahedral Symmetry An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig.

Figure Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. Figure Adenovirus after negative stain electron microscopy. Virus Core Structure Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core.

Figure Two-dimensional diagram of HIV-1 correlating immuno- electron microscopic findings with the recent nomenclature for the structural components in a 2-letter code and with the molecular weights of the virus structural glyco- proteins.

Figure Schemes of 21 virus families infecting humans showing a number of distinctive criteria: presence of an envelope or double- capsid and internal nucleic acid genome. Virus Classification On the basis of shared properties viruses are grouped at different hierarchical levels of order, family, subfamily, genus and species. JB Lippincott, Philadelphia, Gajdusek DC. Unconventional viruses and the origin and disappearance of kuru.

Gelderblom HR. Assembly and morphology of HIV: potential effect of structure on viral function. Mattern CFT: Symmetry in virus architecture. Marcel Dekker, New York, Raven Press, New York, Springer-Verlag, New York, Elsevier, Amsterdam, Structure and Classification of Viruses. In: Baron S, editor. Chapter In this Page. Related information. PubMed Links to PubMed.

Similar articles in PubMed. Review Retroviral Virions and Genomes [Retroviruses. Review Molecular mechanisms of virus spread and virion components as tools of virulence. The number of structural units that creates each side is called the triangulation number T , because the structural units form the triangle face of the icosahedron.

The geometry and math involved with icosahedral capsid structure can be complex, and only the very basics are described here. In any case, by increasing the number of identical structural units on each face, the icosahedron can become progressively larger without requiring additional novel proteins to be produced.

Some viruses have triangulation numbers over 25, even! The proteins that compose the structural unit may form three dimensional structures known as capsomeres that are visible in an electron micrograph.

In icosahedral viruses, capsomeres generally take the form of pentons containing five units or hexons containing six units that form a visible pattern on the surface of the icosahedron See Fig. Capsomeres are morphological units that arise from the interaction of the proteins within the repeated structural units.

Why does the icosahedral virus structure appear so often? Research has shown that proteins forming icosahedral symmetry require lesser amounts of energy, compared to other structures, and so this structure is evolutionarily favored. Many viruses that infect animals are icosahedral, including human papillomavirus, rhinovirus, hepatitis B virus, and herpesviruses Fig. Like their helical counterparts, icosahedral viruses can be naked or enveloped, as well.

Poliovirus A , rotavirus B , varicella—zoster virus C , the virus that causes chickenpox and shingles, and reovirus D.

Note that C is enveloped. The majority of viruses can be categorized as having helical or icosahedral structure. A few viruses, however, have a complex architecture that does not strictly conform to a simple helical or icosahedral shape.

Poxviruses, geminiviruses, and many bacteriophages are examples of viruses with complex structure Fig. Poxviruses, including the viruses that cause smallpox or cowpox, are large oval or brick-shaped particles — nm long. The geminiviruses also exhibit complex structure. As their name suggests, these plant-infecting viruses are composed of two icosahedral heads joined together.

Bacteriophages , also known as bacterial viruses or prokaryotic viruses , are viruses that infect and replicate within bacteria. Many bacteriophages also have complex structure, such as bacteriophage P2, which has an icosahedral head, containing the nucleic acid, attached to a cylindrical tail sheath that facilitates binding of the bacteriophage to the bacterial cell.

Vaccinia virus A , a virus belonging to the poxvirus family, has a complex capsid architecture with a dumbbell-shaped core. Geminiviruses B have a double-icosahedron capsid. Bacteriophages, such as P2 C , often have complex capsid structure. The classification of viruses is useful for many reasons. It allows scientists to contrast viruses and to reveal information on newly discovered viruses by comparing them to similar viruses.

It also allows scientists to study the origin of viruses and how they have evolved over time. The classification of viruses is not simple, however—there are currently over different viral species with very different properties! One classification scheme was developed in the s by Nobel laureate David Baltimore. The Baltimore classification system categorizes viruses based on the type of nucleic acid genome and replication strategy of the virus.

As will be further discussed in the next chapter, positive-strand also positive-sense or plus-strand RNA is able to be immediately translated into proteins; as such, messenger RNA mRNA in the cell is positive strand. Negative-strand also negative-sense or minus-strand RNA is not translatable into proteins; it first has to be transcribed into positive-strand RNA. Baltimore also took into account viruses that are able to reverse transcribe , or create DNA from an RNA template, which is something that cells are not capable of doing.

Together, the seven classes are. There are a variety of ways by which viruses could be classified, however, including virion size, capsid structure, type of nucleic acid, physical properties, host species, or disease caused.

Because of this formidable challenge, the International Committee on Taxonomy of Viruses ICTV was formed and has been the sole body charged with classifying viruses since Taxonomy is the science of categorizing and assigning names nomenclature to organisms based on similar characteristics, and the ICTV utilizes the same taxonomical hierarchy that is used to classify living things. It is important to note that viruses, since they are not alive, belong to a completely separate system that does not fall under the tree of life.

Whereas a living organism is classified using domain, kingdom, phylum, class, order, family, genus, and species taxa singular: taxon , or categories, viruses are only classified using order, family, genus, and species Table 2. The ICTV classifies viruses based upon a variety of different characteristics with the intention of categorizing the most similar viruses with each other.

The chemical and physical properties of the virus are considered, such as the type of nucleic acid or number of different proteins encoded by the virus. DNA technologies now allow us to sequence viral genomes relatively quickly and easily, allowing scientists to compare the nucleic acid sequences of two viruses to determine how closely related they are.

Other virion properties are also taken into account, including virion size, capsid shape, and whether or not an envelope is present. The taxa of viruses that infect vertebrates are shown in Fig. Also note the size difference between viruses of different families. Viruses are categorized based upon their type of nucleic acid DNA viruses in yellow boxes and RNA viruses in blue boxes and further classified based upon distinguishing characteristics.

Note the nucleic acid, size, and architectural differences between viruses of different families. Viruses in color will be discussed in later chapters. Seventy-seven virus families, however, have yet to be assigned to an order, including notable viruses such as the retroviruses, papillomaviruses, and poxviruses. New orders have been proposed, and it is likely that more will be created as the taxonomical process continues.

The ICTV has established guidelines for naming newly discovered viruses. The Latin binomial names that are used for living organisms, where the genus and species are listed together such as Homo sapiens or Yersinia pestis , are not used for naming viruses. When directly referring to a viral order, family, genus, or species the virus name should be written in italics with the first letter capitalized. When not referring specifically to viral classification, however, capitalization and italics are not required unless a proper name is encountered.

Section 2. What is the function of the capsid? Why must viruses repeat the same capsid protein subunits over and over again, rather than having hundreds of different capsid proteins? What is a structural unit? What taxa are used to classify viruses? How does this differ from the classification of a living organism? National Center for Biotechnology Information , U. Essential Human Virology. Published online May 6.

Jennifer Louten. Author information Copyright and License information Disclaimer. Elsevier hereby grants permission to make all its COVIDrelated research that is available on the COVID resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source.

Abstract Viruses have several common characteristics: they are small, have DNA or RNA genomes, and are obligate intracellular parasites.

Taken together, we have learned that although they can be quite diverse, viruses share several common characteristics: 1. Open in a separate window.

The Cost of Deadly Virus Infections Viruses gain their infamy through a combination of large infection rates and death, even when their power has been relatively subdued. The Three Categories of Viruses There are three different virus types that are made distinct by their shape. How a Virus Spreads The first thing a virion does is enter a cell and becomes a virus. Treating Viruses Antiviral drugs can treat viruses by inhibiting viral development and slowing down disease progression.

The Role of Advanced Practice Nurses Worldwide Around the world, nurses contribute to the prevention, management and containment of viral outbreaks by caring for infected patients and educating the public on prevention strategies. The Invaluable Contribution of Nurses Across a variety of roles and specializations, nursing professionals fight viruses in numerous ways.

Slowing the Spread Advanced practice nurses are uniquely qualified to conduct research and assist government leaders and public health officials in creating an informed response to viral outbreaks. Prepare to become a leader in nursing with your D. Get Program Details. In North Dakota, we do work worth doing. Join us, and you can do work worth doing, too. This will take only a moment.

What program are you most interested in? Please select an option D. Transmission electron micrograph of poliomyelitis viruses ; courtesy of Dennis Kunkel's Microscopy.

Transmission electron micrograph showing envelope and glycoprotein spikes Coronaviruses ; courtesy of CDC.

Transmission electron micrograph of herpes simplex viruses ; courtesy of Dennis Kunkel's Microscopy. Transmission electron micrograph of the bacteriophage coliphage T4 ; courtesy of Dennis Kunkel's Microscopy. Exercise: Think-Pair-Share Questions We just learned that most viruses are much smaller than bacteria.