Biological Sciences
Classification of Viruses
The classification of viruses involves organizing them into groups based on their genetic material, structure, replication process, and other characteristics. Viruses are classified into families, genera, and species, with the International Committee on Taxonomy of Viruses (ICTV) overseeing the naming and classification process. This system helps scientists understand the diversity and relationships among different viruses.
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11 Key excerpts on "Classification of Viruses"
eBook - PDF- Edward K. Wagner, Martinez J. Hewlett, David C. Bloom, David Camerini(Authors)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
More than 5450 viruses arranged in greater than 2000 species, 287 genera, 73 families, and three orders are described (Table 5.1). While this is a notable achievement, it is not a complete one – the pace of discovery of new viruses and characterization of the genes they encode ensures that the number will change. Further, it is increasingly evident that the very nature of virus replication and association with their hosts leads to complications not found in classification schemes for cell-based life. The rate of genetic change in viruses can be great due to the rapidity and frequency of genome replication with the associated opportunity for error. Viruses can also, however, exchange genetic elements with their hosts and any other genomes present in the same milieu in which the virus is replicating. Such an occurrence can lead to the creation of a new virus species in which some of its genes are derived from one lineage and some from another – clearly its classification will be complicated. The best generalization that can be made concerning virus classification is that it depends on analysis of a number of features, and the importance of such features may vary depending upon the use being made of the classification. A classification scheme that combines the Balti- more basis along with the nature of the host and detailed genetic characterization of critical viral proteins can be combined to generate a global view of viruses as a virosphere such as shown in Figure 5.5. The features of viruses discussed in this chapter provide the basis for this comprehensive classification scheme, but they are not complete – for example, diseases caused by viruses cannot be readily listed. Further, relationships between virus families will often transcend the nature of the host – this would require a third dimension to the scheme (appropriate to the concept of a sphere).
eBook - ePubViruses
Biology, Applications, and Control
- David Harper(Author)
- 2011(Publication Date)
- Garland Science(Publisher)
CHAPTER 2 Virus Classification and EvoultionAbout the chapter opener imageThree Picornaviruses(Courtesy of the Research Collaboratory for Structural Bioinformatics Protein Data Bank and David S. Goodsell, The Scripps Research Institute, USA.)Introduction
The earliest classification systems used for viruses date from before their nature was understood. These systems were based entirely on the clinical signs and symptoms produced by infection. We can still see their effects today, for example in the hepatitis viruses, named for their effects on the liver. Hepatitis A, B, C, and E are all very different types of virus with RNA (hepatitis A, C, E) or DNA (hepatitis B) genomes, while hepatitis D is not properly a virus at all. The poxviruses are a recognized group, with smallpox, orf virus, molluscum contagiosum virus, tanapox, and sometimes cowpox and monkeypox infecting humans. But chickenpox (from tzuiken pox, or itchy pox) is caused by a herpesvirus, and the great pox is a name formerly used for the bacterial disease syphilis. Thus, a more defined system was needed.One classification was established based on the observed characteristics of viruses (size, morphology).2.1 Virus Classification
The formal taxonomical system for the Classification of Viruses is administered by the International Committee for the Taxonomy of Viruses (ICTV), and is based on a broad range of characteristics including morphology and genome type (Box 2.1 ).Viruses are classified into families (ending -viridae) and genera (ending -virus) on the basis of these criteria and, increasingly, on genetic relatedness. Below this is the virus species, the lowest formal level of virus classification (Box 2.2 ).The classical taxonomical definition of a species as a group of organisms capable of interbreeding and producing fertile offspring is essentially meaningless for viruses, which do not reproduce by this means. Although the term is used by the ICTV, it is used in a much more flexible sense than is the case for more complex organisms (Box 2.2
eBook - PDF- Marc H.V. van Regenmortel, Brian W.J. Mahy(Authors)
- 2010(Publication Date)
- Academic Press(Publisher)
Genus A category in the virus taxonomic classification of related organisms, comprising one or more species. Nomenclature The assigning of names to organisms in a scientific classification system. Order A category in the virus taxonomic classification of related organisms, comprising one or more families. Species A virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche. Taxonomy The science of classifying plants, animals, and microorganisms into increasingly broader categories based on shared features. The practice or principles of classification. Introduction Virus taxonomy is a very important but controversial field of science. It was ranked as the first constraint for the modern development of virus databases, and the expo-nential increase in virus sequencing is worsening the situation. However, substantial progress has been made particularly in the last 20 years, both on the conceptual framework and practical implication of virus taxonomy. The International Committee on Taxonomy of Viruses (ICTV) is the only committee of the Virology Division, of the International Union of Microbiological Societies (IUMS), in charge of that task since 1966, for the interna-tional virology community. Virus Taxonomy Reports have been published regularly by ICTV and they became the reference in virus taxonomy and nomenclature. This article aims at providing some historical information about the establishment and changes in virus taxonomy and describes the current status of virus classification, nomenclature, and orthography. There is no such thing as a ‘natural’ or a ‘biological’ classi-fication; by essence any classification is an arbitrary human invention and viruses are no exception.
eBook - PDF- Frank J. Fenner, B. R. McAuslan, C. A. Mims(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
12 1. Nature and Classification of Animal Viruses of diseases caused by them, which tended to classify the host responses rather than the viruses. Bawden (1941) made the pioneering suggestion that viral no-menclature and classification should be based upon properties of the virus particle. In the early 1950's Bawden's approach was exploited by animal virologists (Andrewes, 1952), and viruses were allocated to groups which were usually given latinized names constructed from a chosen prefix plus the word virus. Thus, myxovirus (Andrewes et ah, 1955), poxvirus (Fenner and Burnet, 1957), herpesvirus (Andrewes, 1954), reovirus (Sabin, 1959), papovavirus (Melnick, 1962), picornavirus (International Enterovirus Study Group, 1963), and adeno-virus (Pereira et ah, 1963) groups were described. In the meantime, a classifica-tion using quite different criteria had been established by epidemiologists. Since they were so concerned with the transmission of infection, epidemiologists have used a classification based on the mode of transmission of disease; they have grouped viruses together as respiratory viruses, enteric viruses, or ar-thropod-borne (arbo-) viruses. The last term, in particular, has been widely used, but it is generally agreed that this epidemiological classification, although useful is in no sense taxonomic. Concurrently with these suggestions relating to the viruses of vertebrates, Lwoff (1957) insisted upon the similarities between viruses, whatever their natural host, and the differences between viruses and all other biological entities. He was instrumental in arranging for the establishment of an international com-mittee (Anon., 1965; Lwoff and Tournier, 1966) to discuss nomenclature. Its major proposal was to select type species upon which names for groups would be based.
eBook - PDFViruses
Biology, Applications, and Control
- David Harper(Author)
- 2011(Publication Date)
- Garland Science(Publisher)
Below this is the virus species , the lowest formal level of virus clas-sification ( Box 2.2 ). The classical taxonomical definition of a species as a group of organisms capable of interbreeding and producing fertile offspring is essentially meaningless for viruses, which do not reproduce by this means. Although the term is used by the ICTV, it is used in a much more flexible sense than is the case for more complex organisms (Box 2.2). Generally, viruses are identified by clinical, immunological, structural, and molecular means and are then grouped into the categories defined above. Such groupings may change as more information about the viruses Box 2.1 Criteria for classifying viruses (from Fauquet 2006) I. Virion properties A. Morphology properties of virions 1. Virion size 2. Virion shape 3. Presence or absence of an envelope and peplomers 4. Capsomeric symmetry and structure B. Physical properties of virions 1. Molecular mass of virions 2. Buoyant density of virions 3. Sedimentation coeffi cient 4. pH stability 5. Thermal stability 6. Cation (Mg 2+ , Mn 2+ ) stability 7. Solvent stability 8. Detergent stability 9. Radiation stability C. Properties of genome 1. Type of nucleic acid—DNA or RNA 2. Strandedness—single stranded or double stranded 3. Linear or circular 4. Sense—positive, negative, or ambisense 5. Number of segments 6. Size of genome or genome segments 7. Presence or absence and type of 5 ¢ -terminal cap 8. Presence or absence of 5 ¢ -terminal covalently linked polypeptide 9. Presence or absence of 3 ¢ -terminal poly(A) tract (or other specific tract) 10. Nucleotide sequence comparisons D. Properties of proteins 1. Number of proteins 2. Size of proteins 3. Functional activities of proteins (especially virion transcriptase, virion reverse transcriptase, virion haemagglutinin, virion neuraminidase, virion fusion protein) 4. Amino acid sequence comparisons E. Lipids 1. Presence or absence of lipids 2.
eBook - PDF- A. O. Betts, C. J. York, A. O. Betts, C. J. York(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
CHAPTER 1 General Nature of Viruses SIR CHRISTOPHER ANDREWES I. Introduction 1 II. History of Virology 2 III. Definition of a Virus 4 IV. Origin of Viruses 5 V. Replication of Viruses 6 VI. Pathogenesis 7 VII. Basis of Virus Classification 9 VIII. Chemical Composition 10 IX. Morphology 11 X. RNA Viruses 13 A. Picornaviruses 13 B. Reoviruses 14 C. Arboviruses 14 D. Myxoviruses 15 XI. DNA Viruses 16 A. Adenoviruses 16 B. Papovaviruses 16 C. Herpesviruses 16 D. Poxviruses 17 E. Parvoviruses 17 XII. Other Agents 17 XIII. Immunity 18 XIV. Virus Ecology , 19 XV. Control of Virus Infections 23 References 24 I. I N T R O D U C T I O N Within recent years virology, the study of viruses, has attained the dignity of a branch of biology in its own right. No longer are viruses considered to be merely small parasitic agents, discussion of which can be tacked on as an addendum to an extended account of bacteria. Viruses, it is now realized, constitute a separate category of living things—for they have almost all the attributes of life—having a mode of replication that sets them apart from larger beings. The growth of virology has been greatly helped by the techniques of experimental pathology, histopathology, electron microscopy, genetics, and biochemistry. Lately virology has begun to repay this debt: study of the details of virus replication has been throwing light 1 2 Sir Christopher Andrewes on intimate biological processes of enormous interest to other branches of biology. Virology, in its more academic aspects, is thus in the forefront of exciting new advances in knowledge. In practical ways, too, it is contributing greatly. The discovery of new methods of cultivating viruses and of modifying their properties is opening up new possibilities of specific protection against virus infections of man and domestic animals. Perhaps even more important for this purpose is the increased knowledge of the virus ecology.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- College Publishing House(Publisher)
A unified taxonomy (a universal system for classifying viruses) has been established. The 7th lCTV Report formalised for the first time the concept of the virus species as the lowest taxon (group) in a branching hierarchy of viral taxa. However, at present only a small part of the total diversity of viruses has been studied, with analyses of samples from humans finding that about 20% of the virus sequences recovered have not been seen before, and samples from the environment, such as from seawater and ocean sediments, finding that the large majority of sequences are completely novel. The general taxonomic structure is as follows: Order (-virales) Family (-viridae) Subfamily (-virinae) Genus ( -virus ) Species ( -virus ) In the current (2008) ICTV taxonomy, five orders have been established, the Caudo-virales, Herpesvirales, Mononegavirales, Nidovirales, and Picornavirales. The committee does not formally distinguish between subspecies, strains, and isolates. In total there are 5 orders, 82 families, 11 subfamilies, 307 genera, 2,083 species and about 3,000 types yet unclassified. ________________________ WORLD TECHNOLOGIES ________________________ Baltimore classification The Baltimore Classification of Viruses is based on the method of viral mRNA synthesis The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification. The Baltimore Classification of Viruses is based on the mechanism of mRNA production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). Additionally, ssRNA viruses may be either sense (+) or antis ense (−).
eBook - PDF- Nigel J. Dimmock, Andrew J. Easton, Keith N. Leppard(Authors)
- 2015(Publication Date)
- Wiley-Blackwell(Publisher)
Similarly, the identification of a positive sense RNA genome is not sufficient to classify the virus unambiguously since viruses of classes 4 and 6 have similar genome nucleic acids. 3.5 Classification on the basis of taxonomy The International Committee on Taxonomy of Viruses (ICTV), first founded in the late 1960s, Chapter 3 Classification of Viruses 35 has established a taxonomic classification scheme for viruses. This uses the familiar systematic taxonomy scheme of Order, Family, Subfamily and Genus (no Kingdoms, Phyla or Classes of virus have been described within this scheme). However, the concept of a species in the Classification of Viruses is complex and in many cases remains a point of ongoing debate. For viruses with RNA genomes, the concept of a species is made difficult by the absence of a proofreading function during genome replication. The result of this is that a virus exists as a member of a population where each member has a genome sequence which may be different to the others but which belongs to a collection of sequences which will combine to form a consensus for that virus. The virus is said to be a quasispecies, and there is no defined ‘correct’ genome sequence (this is discussed in Chapter 4). In assigning a virus to a taxonomic group, the ICTV considers a range of characteristics. These include host range (eukaryote or prokaryote, animal, plant, etc.), morphological features of the virion (enveloped, shape of capsid or nucleocapsid, etc.) and nature of the genome nucleic acid (DNA or RNA, single stranded or double stranded, positive or negative sense, etc.). Within these parameters additional features are considered. These include such things as the length of the tail of a phage or the presence or absence of specific genes in the genomes of similar viruses, and these aspects allow allocation of subdivisions in the taxonomic designation.
eBook - PDF- Mary Ann Clark, Jung Choi, Matthew Douglas(Authors)
- 2018(Publication Date)
- Openstax(Publisher)
However, these earlier classification methods grouped viruses differently, because they were based on different sets of characters of the virus. The most commonly used classification method today is called the Baltimore classification scheme, and is based on how messenger RNA (mRNA) is generated in each particular type of virus. Past Systems of Classification Viruses contain only a few elements by which they can be classified: the viral genome, the type of capsid, and the envelope structure for the enveloped viruses. All of these elements have been used in the past for viral classification (Table 21.1 and Figure 21.6). Viral genomes may vary in the type of genetic material (DNA or RNA) and its organization (single- or double-stranded, linear or circular, and segmented or non-segmented). In some viruses, additional proteins needed for replication are associated directly with the genome or contained within the viral capsid. Virus Classification by Genome Structure Genome Structure Examples RNA DNA Rabies virus, retroviruses Herpesviruses, smallpox virus Single-stranded Double-stranded Rabies virus, retroviruses Herpesviruses, smallpox virus Linear Circular Rabies virus, retroviruses, herpesviruses, smallpox virus Papillomaviruses, many bacteriophages Non-segmented: genome consists of a single segment of genetic material Segmented: genome is divided into multiple segments Parainfluenza viruses Influenza viruses Table 21.1 564 Chapter 21 | Viruses This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Figure 21.6 Viruses can be classified according to their core genetic material and capsid design. (a) Rabies virus has a single-stranded RNA (ssRNA) core and an enveloped helical capsid, whereas (b) variola virus, the causative agent of smallpox, has a double-stranded DNA (dsDNA) core and a complex capsid. Rabies transmission occurs when saliva from an infected mammal enters a wound.
- Diwakar, R K(Authors)
- 2018(Publication Date)
- Daya Publishing House(Publisher)
Most of the studies were centred on their ability to cause infections and diseases. Such system have difficiencies e.g. The same virus produces different disease syndromes in different hosts, different strains of the same virus can produce different syndromes in the same host and different viruses can produces the same pictures. Epidemeological data based classification was also tried earlier. The most important criteria for classification are I) Virion structure II) Genome of the viruses III) Mode of replication Bawden in 1941-50, proposed first time that viruses grouped on the basis of shared virion properties. The International Committee on Taxonomy of Viruses (ICTV) Against this background, in 1966 the the International Committee on Nomenclature of Viruses (ICNV) was established at the International Congress of Microbiology in Moscow. At that time, Virologists already sensed a need for a single, universal taxonomic scheme. There was dispute This ebook is exclusively for this university only. Cannot be resold/distributed. that the viruses were isolated from humans, animals, plants and invertebrates should be classifed in a single system and that this system should separate the viruses from all other biological entities. The International Committee on Nomenclature of Viruses (ICNV) become the International Committee on taxonomy of Viruses (ICtV) in 1973. Today ICTV operates under the auspices of the Virology division of the international union of Microbiological Societies. In the ICTV have six sub-committees, 45 study groups and more than 400 virologists. ICTV develop universal scheme, Virion characteristics are considered and weight by these criteria to division into the families, in some cases sub-families and genera. Now three orders (Mononegavirales, Nidovirales, and Caudovirales) have been approved.
eBook - PDF- Julianne Zedalis, John Eggebrecht(Authors)
- 2018(Publication Date)
- Openstax(Publisher)
only a single, specific RNA contains the virus genome CHAPTER SUMMARY 21.1 Viral Evolution, Morphology, and Classification Viruses are tiny, acellular entities that can usually only be seen with an electron microscope. Their genomes contain either DNA or RNA—never both—and they replicate using the replication proteins of a host cell. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the Classification of Viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA. 876 Chapter 21 | Viruses This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 21.2 Virus Infection and Hosts Viral replication within a living cell always produces changes in the cell, sometimes resulting in cell death and sometimes slowly killing the infected cells. There are six basic stages in the virus replication cycle: attachment, penetration, uncoating, replication, assembly, and release. A viral infection may be productive, resulting in new virions, or nonproductive, which means that the virus remains inside the cell without producing new virions. Bacteriophages are viruses that infect bacteria. They have two different modes of replication: the lytic cycle, where the virus replicates and bursts out of the bacteria, and the lysogenic cycle, which involves the incorporation of the viral genome into the bacterial host genome. Animal viruses cause a variety of infections, with some causing chronic symptoms (hepatitis C), some intermittent symptoms (latent viruses such a herpes simplex virus 1), and others that cause very few symptoms, if any (human herpesviruses 6 and 7).
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