Polyhedral virus structure

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 exterior of the bilayer is studded with virus-coded, glycosylated trans- membrane proteins. Therefore, enveloped viruses often exhibit a fringe of glycoprotein spikes or knobs, also called peplomers. In viruses that acquire their envelope by budding through the plasma or another intracellular cell membrane, the lipid composition of the viral envelope closely reflects that of the particular host membrane.

The outer capsid and the envelope proteins of viruses are glycosylated and important in determining the host range and antigenic composition of the virion. In addition to virus-specified envelope proteins, budding viruses carry also certain host cell proteins as integral constituents of the viral envelope.

Virus envelopes can be considered an additional protective coat. Larger viruses often have a complex architecture consisting of both helical and isometric symmetries confined to different structural components. Viruses are classified on the basis of morphology, chemical composition, and mode of replication.

The viruses that infect humans are currently grouped into 21 families, reflecting only a small part of the spectrum of the multitude of different viruses whose host ranges extend from vertebrates to protozoa and from plants and fungi to bacteria. 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.

Such nucleocapsids form rigid, highly elongated rods or flexible filaments; in either case, details of the capsid structure are often discernible by electron microscopy. In addition to classification as flexible or rigid and as naked or enveloped, helical nucleocapsids are characterized by length, width, pitch of the helix, and number of protomers per helical turn. The most extensively studied helical virus is tobacco mosaic virus Fig. Many important structural features of this plant virus have been detected by x-ray diffraction studies.

Figure shows Sendai virus, an enveloped virus with helical nucleocapsid symmetry, a member of the paramyxovirus family see Ch. The helical structure of the rigid tobacco mosaic virus rod. About 5 percent of the length of the virion is depicted. Individual 17,Da protein subunits protomers assemble in a helix with an axial repeat of 6.

Each more 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. The intact nucleocapsid is about 1, nm long and 17 nm in diameter; its pitch more An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig.

Lines through the centers of opposite triangular faces form axes of threefold rotational symmetry; twofold rotational symmetry axes are formed by lines through midpoints of opposite edges. An icosaheron polyhedral or spherical with fivefold, threefold, and twofold axes of rotational symmetry Fig.

Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. These axes are perpendicular to the plane of the page and pass through the centers of each figure.

Both polyhedral upper and spherical lower forms more Viruses were first found to have symmetry by x-ray diffraction studies and subsequently by electron microscopy with negative-staining techniques.

In most icosahedral viruses, the protomers, i. The arrangement of capsomeres into an icosahedral shell compare Fig. This requires the identification of the nearest pair of vertex capsomeres called penton: those through which the fivefold symmetry axes pass and the distribution of capsomeres between them.

Adenovirus after negative stain electron microscopy. A The capsid reveals the typical isometric shell made up from 20 equilateral triangular faces. The net axes are formed by lines of the closest-packed neighboring capsomeres. In adenoviruses, the h and k axes also coincide with the edges of the triangular faces.

This symmetry and number of capsomeres is typical of all members of the adenovirus family. Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core. Small virions are simple nucleocapsids containing 1 to 2 protein species. The larger viruses contain in a core the nucleic acid genome complexed with basic protein s and protected by a single- or double layered capsid consisting of more than one species of protein or by an envelope Fig.

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. SU stands for more Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much higher mutation rates than do the DNA viruses.

Mutation rates of 10 -4 lead to the continuous generation of virus variants which show great adaptability to new hosts. The viral RNA may be single-stranded ss or double-stranded ds , and the genome may occupy a single RNA segment or be distributed on two or more separate segments segmented genomes. In addition, the RNA strand of a single-stranded genome may be either a sense strand plus strand , which can function as messenger RNA mRNA , or an antisense strand minus strand , which is complementary to the sense strand and cannot function as mRNA protein translation see Ch.

Sense viral RNA alone can replicate if injected into cells, since it can function as mRNA and initiate translation of virus-encoded proteins.

Antisense RNA, on the other hand, has no translational function and cannot per se produce viral components. 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. DsRNA viruses, e.

Each segment consists of a complementary sense and antisense strand that is hydrogen bonded into a linear ds molecule. The replication of these viruses is complex; only the sense RNA strands are released from the infecting virion to initiate replication. The retrovirus genome comprises two identical, plus-sense ssRNA molecules, each monomer 7—11 kb in size, that are noncovalently linked over a short terminal region.

Retroviruses contain 2 envelope proteins encoded by the env-gene, 4—6 nonglycosylated core proteins and 3 non-structural functional proteins reverse transcriptase, integrase, protease: RT, IN, PR specified by the gag-gene Fig. This DNA, mediated by the viral integrase, becomes covalently bonded into the DNA of the host cell to make possible the subsequent transcription of the sense strands that eventually give rise to retrovirus progeny.

After assembly and budding, retroviruses show structural and functional maturation. In immature virions the structural proteins of the core are present as a large precursor protein shell. After proteolytic processing by the viral protease the proteins of the mature virion are rearranged and form the dense isometric or cone-shaped core typical of the mature virion, and the particle becomes infectious.

Most DNA viruses Fig. The papovaviruses, comprising the polyoma- and papillomaviruses, however, have circular DNA genomes, about 5. Three or 2 structural proteins make up the papovavirus capsid: in addition, nonstructural proteins are encoded that are functional in virus transcription, DNA replication and cell transformation. Find Yourself First. John Kim. Virus structure 1. History of virology 2. Definition 3.

Characteristics Of Virus 6. Viral Structure 7. General Morphology 8. Virus Replication 9. Virus Classification Viruses In Dental Diseases Oncogenic viruses Transmission Of Viruses Methods of Inactivating Viruses 3.

How Viruses Multiply4 - Viruses This step is referred to as penetration Classification of major virus groups 1. Herpesvirus 2. Poxvirus 3.

Adenovirus 4. Parvovirus 5. Papovavirus 1. Orthomyxovirus 2. Paramyxovirus 3. Rhabdovirus 4. Tagovirus 5. Retrovirus 6. Reovirus 7. Picornavirus 8. Gillespie, 4th edition