Icosahedral virus parts

The structural unit repeats to form the capsid of the virion. A Virus capsids are composed of viral protein subunits that form structural units. The triangulation number T indicates the number of structural units per face of the icosahedron. The red lines outline a triangular face of the icosahedron, while the purple pentagons indicate the vertices fivefold axes of the icosahedron. But how can some viruses form very large icosahedral capsids?

The answer is repetition. The structural unit can be repeated over and over again to form a larger icosahedron side.

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. Figure 2. Virus and cell size comparison. Refresher: Orders of Magnitude and Scientific Notation.

Study Break. Structure of Viruses The infectious virus particle must be released from the host cell to infect other cells and individuals. Basic virus architecture. Refresher: Chemical Bonds. Comparison between a Naked and Enveloped Virion. Helical Capsid Structure Each virus possesses a protein capsid to protect its nucleic acid genome from the harsh environment.

Helical capsid structure. Electron micrographs of helical viruses. Icosahedral Capsid Structure Of the two major capsid structures, the icosahedron is by far more prevalent than the helical architecture.

Icosahedron terminology and axes of symmetry. Illustrations of viruses, as viewed on the twofold axis of rotation.

Graph , 12, —44 using 2G33 J. Capsid architecture and triangulation number. Electron micrographs of icosahedral viruses. Images courtesy of the CDC: Dr. Erskine Palmer B and D , and Dr. Erskine Palmer and B. Partin C. Complex Viral Structures The majority of viruses can be categorized as having helical or icosahedral structure.

Electron micrograph of viruses with complex architecture. Images courtesy of Ana Caceres et al. A, PLoS Pathog. Virus Classification and Taxonomy The classification of viruses is useful for many reasons. Table 2. Taxon Notes Example Order Ends in -virales suffix; only about half of viruses are currently classified in orders. Picornavirales Family Ends in -viridae suffix; subfamilies are indicated with -virinae suffix. Picornaviridae Genus Ends in -virus suffix.

Classifying and cataloging anything below the species classification such as subtypes, serotypes, strains, isolates, or variants is the responsibility of the specific field. Rhinovirus A Serotypes include Human rhinovirus 1, which includes strains human rhinovirus 1A and human rhinovirus 1B.

Taxa of viruses that infect vertebrates. Newest order, created in Was the first order created, in Summary of Key Concepts Section 2. Most viruses are in the range of 20— nm, although some viruses can exceed nm in length. Unlike cells that undergo mitosis and split in two, viruses completely disassemble within the cell and new virions infectious particles are assembled de novo from newly made components.

Most virus genomes fall within the range of 7—20 kb, but they range from 3 kb to over 2 mb. In addition, some viruses also have a lipid membrane envelope, derived from the cell. All helical animal viruses are enveloped. A helix is mathematically defined by amplitude and pitch.

The sides are composed of viral protein subunits that create a structural unit, which is repeated to form a larger side and the other sides of the icosahedron. The triangulation number refers to the number of structural units per side.

There are seven classes. The taxa used for classifying viruses are order, family, genus, and species. Because they are not alive, viruses are not categorized within the same taxonomical tree as living organisms. Chapter Review Questions 1.

Further Reading Bourne C. Global structural changes in hepatitis B virus capsids induced by the assembly effector HAP1. Involvement of the cellular phosphatase DUSP1 in vaccinia virus infection. PLoS Pathog. Virus taxonomy; pp. The structure of human parvovirus B Structure of dengue virus: implications for flavivirus organization, maturation, and fusion.

X-ray crystallographic structure of the Norwalk virus capsid. Principles of virus structural organization. Ambient occlusion and edge cueing to enhance real time molecular visualization.

IEEE Trans. Virus species, a much overlooked but essential concept in virus classification. Support Center Support Center. External link. The CK Theory enumerates the possible designs for icosahedral surface lattices by mapping the unfolded 20 triangular faces of an icosahedron onto a two dimensional hexagonal lattice. It is assumed this lattice consists entirely of regular hexagons, with the exception of each vertex from the triangulation should be a pentagon.

These pentagons curve the hexagonal matrice into a sphere. The triangulation number T-number is mathematically defined as the squared length of each triangle edge. It counts the number of symmetrically distinct but quasi-equivalent triangular facets in the triangulation per face of icosahedron. Contact Us Home.