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Baron S, editor. Medical Microbiology. fourth edition. Galveston (TX): College of Texas Medical Branch at Galveston; 1996.


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Structure and also Function

Viroffers are little obligate intracellular parasites, which by interpretation containeither a RNA or DNA genome surrounded by a protective, virus-coded protein coat.Viruses might be regarded as mobile hereditary elements, many most likely of cellularorigin and identified by a long co-evolution of virus and organize. Forpropagation viruses depfinish on specialized host cells offering the complexmetabolic and biosynthetic machinery of eukaryotic or prokaryotic cells. Afinish virus particle is referred to as a virion. The primary attribute of the virion isto supply its DNA or RNA genome into the hold cell so that the genome can beexpressed (transcribed and translated) by the host cell. The viral genome, oftenvia connected basic proteins, is packaged inside a symmetric protein capsid.The nucleic acid-linked protein, referred to as nucleoprotein, in addition to thegenome, forms the nucleocapsid. In enveloped viruses, the nucleocapsid issurrounded by a lipid bilayer acquired from the modified organize cell membrane andstudded through an external layer of virus envelope glycoproteins.


Group of Viruses

Morphology: Viroffers are grouped on the basis of dimension and also form,chemical complace and also structure of the genome, and mode of replication.Helical morphology is viewed in nucleocapsids of many filamentous and also pleomorphicvirsupplies. Helical nucleocapsids consist of a helical array of capsid proteins(protomers) wrapped roughly a helical filament of nucleic acid. Icosahedralmorphology is characteristic of the nucleocapsids of many“spherical” viroffers. The number and setup of thecapsomeres (morphologic subunits of the icosahedron) are valuable inidentification and classification. Many virprovides also have actually an external envelope.

Chemical Composition and Setting of Replication: The genome of a virusmay consist of DNA or RNA, which might be single stranded (ss) or double stranded(ds), straight or circular. The whole genome might occupy either one nucleic acidmolecule (monopartite genome) or several nucleic acid segments (multipartitegenome). The various forms of genome necessitate different replicationtactics.


Nomenclature

Aside from physical data, genome structure and also mode of replication are criteriaapplied in the classification and nomenclature of viruses, including thechemical composition and configuration of the nucleic acid, whether the genomeis monopartite or multipartite. The genomic RNA strand of single-stranded RNAviroffers is dubbed feeling (positive sense, plus sense) in orientation if it canserve as mRNA, and antifeeling (negative feeling, minus sense) if a complementarystrand also synthesized by a viral RNA transcriptase serves as mRNA. Also consideredin viral classification is the site of capsid assembly and, in envelopedviruses, the site of envelopment.


Structure and also Function

Viruses are inert exterior the host cell. Small viroffers, e.g., polio and tobaccomosaic virus, can also be crystallized. Virprovides are unable to generate power. Asobligate intracellular parasites, throughout replication, they totally depfinish on thecomplicated biochemical machinery of eukaryotic or prokaryotic cells. The mainpurpose of a virus is to supply its genome into the hold cell to enable itsexpression (transcription and also translation) by the hold cell.

A completely assembled contagious virus is called a virion. The most basic virions consistof 2 standard components: nucleic acid (single- or double-stranded RNA or DNA) and aprotein coat, the capsid, which functions as a shell to defend the viral genomefrom nucleases and which in the time of infection attaches the virion to specific receptorsexposed on the prospective hold cell. Capsid proteins are coded for by the virusgenome. Since of its restricted dimension (Table41-1) the genome codes for just a few structural proteins (besidesnon-structural regulatory proteins affiliated in virus replication). Capsids arecreated as single or double protein shells and also consist of just one or a fewstructural protein species. As such, multiple protein duplicates have to self assemble tocreate the continuous three-dimensional capsid structure. Self assembly of viruscapsids follows two standard patterns: helical symmetry, in which the protein subunitsand the nucleic acid are arranged in a helix, and icosahedral symmeattempt, in which theprotein subunits assemble right into a symmetric shell that covers the nucleicacid-containing core.


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Some virus families have a second covering, referred to as the envelope, which isusually derived in part from modified hold cell membranes. Viral envelopes consistof a lipid bilayer that carefully surrounds a shell of virus-encodedmembrane-connected proteins. The exterior of the bilayer is studded withvirus-coded, glycosylated (trans-) membrane proteins. Therefore, enveloped virusesfrequently exhibit a fringe of glycoprotein spikes or knobs, likewise called peplomers. Inviruses that acquire their envelope by budding via the plasma or anotherintracellular cell membrane, the lipid complace of the viral envelope closelyshows that of the particular organize membrane. The external capsid and also the envelopeproteins of virsupplies are glycosylated and also important in determining the organize range andantigenic complace of the virion. In enhancement to virus-mentioned envelopeproteins, budding virprovides carry also particular host cell proteins as integralconstituents of the viral envelope. Virus envelopes deserve to be considered an additionalprotective coat. Larger viroffers frequently have actually a facility architecture consisting of bothhelical and isometric symmetries confined to various structural components. Smallvirsupplies, e.g., hepatitis B virus or the members of the picornavirus or parvovirusfamily members, are orders of magnitude more resistant than are the bigger facility virsupplies,e.g. members of the herpes or retrovirus families.


Group of Viruses

Virsupplies are classified on the basis of morphology, chemical composition, and also mode ofreplication. The viruses that infect humans are presently grouped right into 21 households,reflecting just a small component of the spectrum of the multitude of different viruseswhose organize arrays extend from vertebprices to protozoa and also from plants and also fungi tobacteria.


Helical Symmetry

In the replication of viruses through helical symmetry, similar proteinsubunits (protomers) self-assemble right into a helical array bordering thenucleic acid, which follows a similar spiral course. Such nucleocapsids formrigid, very elongated rods or flexible filaments; in either instance, detailsof the capsid framework are frequently discernible by electron microscopy. Inenhancement to classification as flexible or rigid and as naked or enveloped,helical nucleocapsids are characterized by length, width, pitch of thehelix, and also variety of protomers per helical revolve. The many extensivelystudied helical virus is tobacco mosaic virus (Fig. 41-1). Many kind of crucial structural features ofthis plant virus have actually been detected by x-ray diffraction research studies. Figure 41-2 mirrors Sendai virus, anenveloped virus via helical nucleocapsid symmeattempt, a member of theparamyxovirus household (see Ch.30).


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Figure 41-1

The helical structure of the rigid tobacco mosaic virusrod. About 5 percent of the size of the virion is shown.Individual 17,400-Da protein subdevices (protomers) assemble in ahelix with an axial repeat of 6.9 nm (49 subunits per threeturns). Each (more...)


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Figure 41-2

Fragments of flexible helical nucleocapsids (NC) of Sendaivirus, a paramyxovirus, are watched either within the protectiveenvelope (E) or free, after rupture of the envelope. The intact nucleocapsid is about 1,000 nm long and also 17 nm indiameter; its pitch (even more...)


Icosahedral Symmetry

An icosahedron is a polyhedron having 20 equilateral triangular deals with and 12vertices (Fig. 41-3). Lines throughoppowebsite vertices define axes of fivefold rotational symmetry: allstructural features of the polyhedron repeat five times within each360° of rotation around any of the fivefold axes. Lines with thecenters of opposite triangular encounters form axes of threefold rotationalsymmetry; twofold rotational symmetry axes are formed by lines throughmidpoints of oppowebsite edges. An icosaheron (polyhedral or spherical) withfivefold, threefold, and also twofold axes of rotational symmetry (Fig. 41-3) is characterized as having actually 532symmetry (review as 5,3,2).


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Figure 41-3

Icosahedral models watched, left to right, on fivefold,threefold, and also twofold axes of rotational symmetry. These axes are perpendicular to the aircraft of the page and also passvia the centers of each figure. Both polyhedral (upper) andspherical (lower) develops (more...)


Viruses were initially found to have 532 symmeattempt by x-ray diffractivity studiesand consequently by electron microscopy with negative-staining methods.In many icosahedral viruses, the protomers, i.e. the structural polypeptidechains, are arranged in oligomeric clusters dubbed capsomeres, which areconveniently delineated by negative staining electron microscopy and also form theclosed capsid shell (Fig. 41-4 a/b). The setup of capsomeres into an icosahedral shell (compare Fig. 41-4 via the upper ideal modelin Fig. 41-3) permits theclassification of such viroffers by capsomere number and also pattern. Thisneeds the identification of the nearemainder pair of vertex capsomeres (calledpenton: those with which the fivefold symmeattempt axes pass) and also thedistribution of capsomeres in between them.


Figure 41-4

Adenovirus after negative stain electron microscopy. (A) The capsid reveals the typical isometric shell consisted of from20 equilateral triangular encounters. The 252 capsomeres, 12 pentonsand the 240 hollow hexon capsomeres are arranged in a T= 25 symmeattempt (more...)


In the adenovirus design in Figure41-4, among the penton capsomeres is arbitrarily assigned theindices h = 0, k = 0 (origin), wright here h and also k are thesuggested axes of the inclined (60°) net of capsomeres. The net axesare developed by lines of the closest-packed surrounding capsomeres. Inadenoviroffers, the h and also k axes additionally coincide through the edges of thetriangular faces. Any second bordering vertex capsomere has indices h= 5, k = 0 (or h = 0, k = 5).The capsomere number (C) deserve to be figured out to be 252 from the h and kindices and also the equation: C = 10(h2 +hk+ k2) + 2. This symmetry and also number ofcapsomeres is typical of all members of the adenovirus family members.


Virus Core Structure

Except in helical nucleocapsids, little is known about the packaging orcompany of the viral genome within the core. Small virions are simplenucleocapsids containing 1 to 2 protein species. The larger virsupplies containin a core the nucleic acid genome complexed via fundamental protein(s) anddefended by a single- or double layered capsid (consisting of more than onespecies of protein) or by an envelope (Fig.41-5).


Figure 41-5

Two-dimensional diagram of HIV-1 correlating (immuno-)electron microscopic findings via the current nomenclature forthe structural components in a 2-letter code and with themolecular weights of the virus structural (glyco-)proteins. SU means (even more...)


RNA Virus Genomes

RNA viroffers, comprising 70% of all virsupplies, differ remarkably in genomestructure (Fig. 41-6). Due to the fact that ofthe error price of the enzymes associated in RNA replication, these virusesgenerally show much greater mutation rates than perform the DNA viroffers. Mutationrates of 10-4 cause the constant generation of virus variantswhich display great adaptcapacity to new hosts. The viral RNA may besingle-stranded (ss) or double-stranded (ds), and the genome may occupy asingle RNA segment or be dispersed on 2 or even more separate segments(segmented genomes). In enhancement, the RNA strand also of a single-stranded genomemight be either a sense strand also (plus strand), which have the right to function as messengerRNA (mRNA), or an antifeeling strand also (minus strand), which is complementary tothe sense strand and cannot function as mRNA protein translation (check out Ch. 42). Sense viral RNA alonecan replicate if injected into cells, since it have the right to attribute as mRNA andinitiate translation of virus-encoded proteins. Antifeeling RNA, on the otherhand, has actually no translational function and also cannot per se produce viralcomponents.


Figure 41-6

Schemes of 21 virus families infecting people showing avariety of distinctive criteria: presence of an envelope or(double-) capsid and also internal nucleic acid genome. +, Sense strand; -, antifeeling strand; ±,dsRNA or DNA; 0, circular DNA; C, number (more...)


DsRNA virprovides, e.g., members of the reovirus family, contain 10, 11 or 12separate genome segments coding for 3 enzymes involved in RNA replication, 3significant capsid proteins and also a number of smaller structural proteins. Eachsegment consists of a complementary sense and antifeeling strand also that ishydrogen bonded right into a direct ds molecule. The replication of these virusesis complex; only the sense RNA strands are released from the infectingvirion 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 abrief terminal area. Retrovirprovides contain 2 envelope proteins encoded bythe env-gene, 4–6 nonglycosylated core proteins and 3non-structural practical proteins (reverse transcriptase, integrase,protease: RT, IN, PR) mentioned by the gag-gene (Fig. 41-5). The RT transcribes the viral ssRNA intodouble-stranded, circular proviral DNA. This DNA, mediated by the viralintegrase, becomes covalently bonded right into the DNA of the hold cell to makefeasible the succeeding transcription of the sense strands that eventuallyprovide increase to retrovirus progeny. After assembly and also budding, retrovirusespresent structural and functional maturation. In immature virions thestructural proteins of the core are existing as a big precursor proteinshell. After proteolytic processing by the viral protease the proteins ofthe mature virion are rearranged and develop the dense isometric or cone-shapedcore typical of the mature virion, and the pwrite-up becomes infectious.


DNA Virus Genomes

Most DNA virsupplies (Fig. 41-6) containa solitary genome of direct dsDNA. The papovaviruses, making up the polyoma-and also papillomavirsupplies, but, have circular DNA genomes, about 5.1 and 7.8kb pairs in dimension. DsDNA serves as a design template both for mRNA and also forself-transcription. Three or 2 structural proteins consist of the papovaviruscapsid: in addition, 5-6 nonstructural proteins are encoded that arepractical in virus transcription, DNA replication and also celltransdevelopment.

Single-stranded direct DNA, 4–6 kb in size, is discovered with themembers of the Parvovirus household that comprises the parvo-, the erythro- andthe dependovirsupplies. The virion contains 2–4 structural proteinspecies which are differently derived from the very same gene product (view Ch. 64). The adeno-associatedvirus (AAV, a dependovirus) is incapable of producing progeny virions exceptin the presence of helper viroffers (adenovirus or herpesvirus). It istherefore shelp to be replication defective.

Circular single-stranded DNA of only 1.7 to 2.3 kb is discovered in members of theCircovirus family which make up the smallest autonomously propagatedviroffers. The isometric capsid steps 17 nm and is created of 2 proteinspecies just.


Virus Classification

On the basis of shared properties viruses are grouped at various hierarchicallevels of order, family members, subfamily members, genus and also species. More than 30,000 differentvirus isolates are well-known today and grouped in even more than 3,600 species, in 164 generaand 71 families. Viral morphology provides the basis for grouping viroffers intohouseholds. A virus household might consist of members that replicate only in vertebrates,just in invertebprices, just in plants, or only in bacteria. Certain family members containviruses that replicate in more than one of these hosts. This area concerns onlythe 21 households and genera of clinical prominence.

Besides physical properties, a number of factors pertaining to the mode of replicationplay a function in classification: the configuration of the nucleic acid (ss or ds,linear or circular), whether the genome consists of one molecule of nucleic acid oris segmented, and whether the strand of ss RNA is sense or antisense. Alsotaken into consideration in classification is the site of viral capsid assembly and, in envelopedvirsupplies, the site of nucleocapsid envelopment.Table 41-1 lists the significant chemical and also morphologic properties of thehouseholds of viruses that cause disease in human beings.

The usage of Latinized names finishing in -viridae for virus families and also ending in -virusfor viral genera has obtained wide acceptance. The names of subfamily members finish in-virinae. Vernacular names continue to be used to describe the viroffers within agenus. In this text, Latinized endings for families and also subfamilies usually are notprovided. Table 41-2 shows the currentclassification of medically substantial virsupplies.


In the beforehand days of virology, virprovides were named according to prevalent pathogenicproperties, e.g. body organ tropism and/or settings of transmission, and also regularly also aftertheir discoverers. From the at an early stage 1950s till the mid-1960s, when many new viruseswere being uncovered, it was popular to write virus names by making use of sigla(abbreviations obtained from a couple of or initial letters). Thus the name Picornaviridaeis derived from pico (small) and also RNA; the name Reoviridae is acquired fromrespiratory, enteric, and orphan viruses bereason the agents were uncovered in bothrespiratory and also enteric specimens and also were not pertained to various other classified viruses;Papovaviridae is from papilloma, polyoma, and vacuolating agent (simian virus 40); retrovirus is from reverse transcriptase; Hepadnaviridaeis from the replication of the virus in hepatocytes and their DNA genomes, as seenin hepatitis B virus. Hepatitis A virus is classified currently in the familyPicornaviridae, genus Hepatovirus. Although the existing rules for nomenclature donot prohilittle bit the development of brand-new sigla, they require that the siglum besystematic to employees in the field and be known by international studygroups.

The names of the various other households that contain viroffers pathogenic for people arederived as follows: Adenoviridae (adeno, “gland”; describes theadenoid tissue from which the virsupplies were initially isolated); Astroviridae (astronsuggests star); Arenaviridae (arena “sand”) defines the sandyappearance of the virion. Bunyaviridae (from Bunyamwera, the place in Africa wherethe form strain was isolated); Calicivirus (calix, “cup” or“goblet” from the cup-shaped depressions on the viral surfaces);Coronaviridae (corona, “crown”) explains the appearance of thepeplomers protruding from the viral surface; Filoviridae (from the Latin filum,“thread” or “filament”) explains themorphology of these virprovides. Herpesviridae (herpes, “creeping”)defines the nature of the lesions; Orthomyxoviridae (ortho,“true,” plus myxo “mucus,” a substance forwhich the virprovides have actually an affinity; Paramyxoviridae obtained from para,“closely resembling” and myxo; Parvoviridae (parvus means,“small”); Poxviridae (pock means,“pustule”); Rhabdoviridae (rhabcarry out, “rod”defines the form of the viruses and also Togaviridae (toga,“cloak”) refers to the tight viral envelope.

Several viroffers of medical prominence still remain unclassified. Some are difficultor impossible to propagate in typical laboratory host devices and also for this reason cannot beobtained in adequate quantity to permit even more precise characterization. Hepatitis Evirus, the Norwalk virus and also equivalent agents (check out Ch. 65) that cause nonbacterial gastroenteritis in humansare currently assigned to the calicivirus household.

The fatal transmissible dementias in people and also other animals (scrapie in sheep andgoat; bovine spongidevelop encephalopathy in cattle, transmissible mink encephalopathy;Kuru, Creutzfeldt-Jakob condition, and also Gerstmann-Straussler-Scheinker syndrome inhumans) (watch Ch. 71) are brought about by theaccumulation of non-soluble amyloid fibrils in the main nervous systems. Theagents resulting in transmissible subacute spongidevelop encephalopathies have actually been linkedto viroids or virinos (i.e. plant pathogens consisting of naked, but exceptionally stablecircular RNA molecules of about 3-400 bases in dimension, or contagious genomes enwrappedright into a host cell coat) because of their resistance to chemical and also physical agents.According to an different concept, the term “prion” has beencoined to allude to an essential nonviral transmittable reason for these fatalencephalopathies—prion standing for self-replicating proteinaceous agentdevoid of demonstrable nucleic acid. Some of the transmissible amyloidoses display afamilial pattern and deserve to be described by characterized mutations which render a primarysoluble glycoprotein insoluble, which consequently leads to the pathognomonicbuildup of amyloid fibers and also plaques. The pathogenesis of the sporadicamyloidoses, but, is still a issue of very ambitious research.


Caspar DLD: Design ethics in virus particlebuilding. In Horsautumn FL, Tamm I (eds): Viral and Rickettsial Infections inMan. fourth Ed. JB Lippincott, Philadelphia, 1975 .
Mattern CFT: Symmetry in virus design. InNayak DP (ed): Molecular Biology of Animal Viruses. Marcel Dekker, New York,1977 .

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Murphy FA, Fauquet CM, Bishop DHL, et al. (eds):Virus Taxonomy: Sixth Report of the International Committee on Taxonomy ofVirsupplies. Springer-Verlag, New York, 1995 .