Structure and Classification of Viruses (2023)

General Concepts

Structure and Function

Viruses are small obligate intracellular parasites, which by definition containeither a RNA or DNA genome surrounded by a protective, virus-coded protein coat.Viruses may be viewed as mobile genetic elements, most probably of cellularorigin and characterized by a long co-evolution of virus and host. Forpropagation viruses depend on specialized host cells supplying the complexmetabolic and biosynthetic machinery of eukaryotic or prokaryotic cells. Acomplete virus particle is called a virion. The main function of the virion isto deliver its DNA or RNA genome into the host cell so that the genome can beexpressed (transcribed and translated) by the host cell. The viral genome, oftenwith associated basic proteins, is packaged inside a symmetric protein capsid.The nucleic acid-associated protein, called nucleoprotein, together with thegenome, forms the nucleocapsid. In enveloped viruses, the nucleocapsid issurrounded by a lipid bilayer derived from the modified host cell membrane andstudded with an outer layer of virus envelope glycoproteins.

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.Helical morphology is seen in nucleocapsids of many filamentous and pleomorphicviruses. Helical nucleocapsids consist of a helical array of capsid proteins(protomers) wrapped around a helical filament of nucleic acid. Icosahedralmorphology is characteristic of the nucleocapsids of many“spherical” viruses. The number and arrangement of thecapsomeres (morphologic subunits of the icosahedron) are useful inidentification and classification. Many viruses also have an outer envelope.

Chemical Composition and Mode of Replication: The genome of a virusmay consist of DNA or RNA, which may be single stranded (ss) or double stranded(ds), linear or circular. The entire genome may occupy either one nucleic acidmolecule (monopartite genome) or several nucleic acid segments (multipartitegenome). The different types of genome necessitate different replicationstrategies.


Aside from physical data, genome structure and 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 RNAviruses is called sense (positive sense, plus sense) in orientation if it canserve as mRNA, and antisense (negative sense, minus sense) if a complementarystrand 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 Function

Viruses are inert outside the host cell. Small viruses, e.g., polio and tobaccomosaic virus, can even be crystallized. Viruses are unable to generate energy. Asobligate intracellular parasites, during replication, they fully depend on thecomplicated biochemical machinery of eukaryotic or prokaryotic cells. The mainpurpose of a virus is to deliver its genome into the host cell to allow itsexpression (transcription and translation) by the host cell.

A fully assembled infectious virus is called a virion. The simplest virions consistof two basic components: nucleic acid (single- or double-stranded RNA or DNA) and aprotein coat, the capsid, which functions as a shell to protect the viral genomefrom nucleases and which during infection attaches the virion to specific receptorsexposed on the prospective host cell. Capsid proteins are coded for by the virusgenome. Because of its limited size (Table41-1) the genome codes for only a few structural proteins (besidesnon-structural regulatory proteins involved in virus replication). Capsids areformed as single or double protein shells and consist of only one or a fewstructural protein species. Therefore, multiple protein copies must self assemble toform the continuous three-dimensional capsid structure. Self assembly of viruscapsids follows two basic patterns: helical symmetry, in which the protein subunitsand the nucleic acid are arranged in a helix, and icosahedral symmetry, in which theprotein subunits assemble into a symmetric shell that covers the nucleicacid-containing core.

Table 41-1

Chemical and Morphologic Properties of Animal Virus Families Relevant toHuman Disease.

Some virus families have an additional covering, called the envelope, which isusually derived in part from modified host cell membranes. Viral envelopes consistof a lipid bilayer that closely surrounds a shell of virus-encodedmembrane-associated proteins. The exterior of the bilayer is studded withvirus-coded, glycosylated (trans-) membrane proteins. Therefore, enveloped virusesoften exhibit a fringe of glycoprotein spikes or knobs, also called peplomers. Inviruses that acquire their envelope by budding through the plasma or anotherintracellular cell membrane, the lipid composition of the viral envelope closelyreflects that of the particular host membrane. The outer capsid and the envelopeproteins of viruses are glycosylated and important in determining the host range andantigenic composition of the virion. In addition to virus-specified envelopeproteins, budding viruses carry also certain host cell proteins as integralconstituents of the viral envelope. Virus envelopes can be considered an additionalprotective coat. Larger viruses often have a complex architecture consisting of bothhelical and isometric symmetries confined to different structural components. Smallviruses, e.g., hepatitis B virus or the members of the picornavirus or parvovirusfamily, are orders of magnitude more resistant than are the larger complex viruses,e.g. members of the herpes or retrovirus families.

(Video) Virus structure and classification | Cells | MCAT | Khan Academy

Classification of Viruses

Viruses are classified on the basis of morphology, chemical composition, and mode ofreplication. The viruses that infect humans are currently grouped into 21 families,reflecting only a small part of the spectrum of the multitude of different viruseswhose host ranges extend from vertebrates to protozoa and from plants and fungi tobacteria.


Helical Symmetry

In the replication of viruses with helical symmetry, identical proteinsubunits (protomers) self-assemble into a helical array surrounding thenucleic acid, which follows a similar spiral path. Such nucleocapsids formrigid, highly elongated rods or flexible filaments; in either case, detailsof the capsid structure are often discernible by electron microscopy. Inaddition to classification as flexible or rigid and as naked or enveloped,helical nucleocapsids are characterized by length, width, pitch of thehelix, and number of protomers per helical turn. The most extensivelystudied helical virus is tobacco mosaic virus (Fig. 41-1). Many important structural features ofthis plant virus have been detected by x-ray diffraction studies. Figure 41-2 shows Sendai virus, anenveloped virus with helical nucleocapsid symmetry, a member of theparamyxovirus family (see Ch.30).

Figure 41-1

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

Figure 41-2

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

Icosahedral Symmetry

An icosahedron is a polyhedron having 20 equilateral triangular faces and 12vertices (Fig. 41-3). Lines throughopposite vertices define axes of fivefold rotational symmetry: allstructural features of the polyhedron repeat five times within each360° of rotation about any of the fivefold axes. Lines through thecenters of opposite triangular faces form axes of threefold rotationalsymmetry; twofold rotational symmetry axes are formed by lines throughmidpoints of opposite edges. An icosaheron (polyhedral or spherical) withfivefold, threefold, and twofold axes of rotational symmetry (Fig. 41-3) is defined as having 532symmetry (read as 5,3,2).

Figure 41-3

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 passthrough the centers of each figure. Both polyhedral (upper) andspherical (lower) forms (more...)

(Video) Virus structure and Classification : DNA and RNA virus

Viruses were first found to have 532 symmetry by x-ray diffraction studiesand subsequently by electron microscopy with negative-staining techniques.In most icosahedral viruses, the protomers, i.e. the structural polypeptidechains, are arranged in oligomeric clusters called capsomeres, which arereadily delineated by negative staining electron microscopy and form theclosed capsid shell (Fig. 41-4 a/b). The arrangement of capsomeres into an icosahedral shell (compare Fig. 41-4 with the upper right modelin Fig. 41-3) permits theclassification of such viruses by capsomere number and pattern. Thisrequires the identification of the nearest pair of vertex capsomeres (calledpenton: those through which the fivefold symmetry axes pass) and thedistribution of capsomeres between them.

Figure 41-4

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

In the adenovirus model in Figure41-4, one of the penton capsomeres is arbitrarily assigned theindices h = 0, k = 0 (origin), where h and k are theindicated axes of the inclined (60°) net of capsomeres. The net axesare formed by lines of the closest-packed neighboring capsomeres. Inadenoviruses, the h and k axes also coincide with the edges of thetriangular faces. Any second neighboring vertex capsomere has indices h= 5, k = 0 (or h = 0, k = 5).The capsomere number (C) can be determined to be 252 from the h and kindices and the equation: C = 10(h2 +hk+ k2) + 2. This symmetry and number ofcapsomeres is typical of all members of the adenovirus family.

Virus Core Structure

Except in helical nucleocapsids, little is known about the packaging ororganization of the viral genome within the core. Small virions are simplenucleocapsids containing 1 to 2 protein species. The larger viruses containin a core the nucleic acid genome complexed with basic protein(s) andprotected 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 with the recent nomenclature forthe structural components in a 2-letter code and with themolecular weights of the virus structural (glyco-)proteins. SU stands for (more...)

Chemical Composition and Mode of Replication

RNA Virus Genomes

RNA viruses, comprising 70% of all viruses, vary remarkably in genomestructure (Fig. 41-6). Because ofthe error rate of the enzymes involved in RNA replication, these virusesusually show much higher mutation rates than do the DNA viruses. Mutationrates of 10-4 lead to the continuous generation of virus variantswhich show great adaptability to new hosts. The viral RNA may besingle-stranded (ss) or double-stranded (ds), and the genome may occupy asingle RNA segment or be distributed on two or more separate segments(segmented genomes). In addition, the RNA strand of a single-stranded genomemay be either a sense strand (plus strand), which can function as messengerRNA (mRNA), or an antisense strand (minus strand), which is complementary tothe sense strand and cannot function as mRNA protein translation (see Ch. 42). Sense viral RNA alonecan replicate if injected into cells, since it can function as mRNA andinitiate translation of virus-encoded proteins. Antisense RNA, on the otherhand, has no translational function and cannot per se produce viralcomponents.

Figure 41-6

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

DsRNA viruses, e.g., members of the reovirus family, contain 10, 11 or 12separate genome segments coding for 3 enzymes involved in RNA replication, 3major capsid proteins and a number of smaller structural proteins. Eachsegment consists of a complementary sense and antisense strand that ishydrogen bonded into a linear 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 ashort terminal region. Retroviruses contain 2 envelope proteins encoded bythe env-gene, 4–6 nonglycosylated core proteins and 3non-structural functional proteins (reverse transcriptase, integrase,protease: RT, IN, PR) specified 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 into the DNA of the host cell to makepossible the subsequent transcription of the sense strands that eventuallygive rise to retrovirus progeny. After assembly and budding, retrovirusesshow structural and functional maturation. In immature virions thestructural proteins of the core are present as a large precursor proteinshell. After proteolytic processing by the viral protease the proteins ofthe mature virion are rearranged and form the dense isometric or cone-shapedcore typical of the mature virion, and the particle becomes infectious.

DNA Virus Genomes

Most DNA viruses (Fig. 41-6) containa single genome of linear dsDNA. The papovaviruses, comprising the polyoma-and papillomaviruses, however, have circular DNA genomes, about 5.1 and 7.8kb pairs in size. DsDNA serves as a template both for mRNA and forself-transcription. Three or 2 structural proteins make up the papovaviruscapsid: in addition, 5-6 nonstructural proteins are encoded that arefunctional in virus transcription, DNA replication and celltransformation.

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

Circular single-stranded DNA of only 1.7 to 2.3 kb is found in members of theCircovirus family which comprise the smallest autonomously propagatedviruses. The isometric capsid measures 17 nm and is composed of 2 proteinspecies only.

(Video) Virology lecture 1 | Virus structure and classification

Virus Classification

On the basis of shared properties viruses are grouped at different hierarchicallevels of order, family, subfamily, genus and species. More than 30,000 differentvirus isolates are known today and grouped in more than 3,600 species, in 164 generaand 71 families. Viral morphology provides the basis for grouping viruses intofamilies. A virus family may consist of members that replicate only in vertebrates,only in invertebrates, only in plants, or only in bacteria. Certain families containviruses that replicate in more than one of these hosts. This section concerns onlythe 21 families and genera of medical importance.

Besides physical properties, several factors pertaining to the mode of replicationplay a role 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. Alsoconsidered in classification is the site of viral capsid assembly and, in envelopedviruses, the site of nucleocapsid envelopment.Table 41-1 lists the major chemical and morphologic properties of thefamilies of viruses that cause disease in humans.

The use of Latinized names ending in -viridae for virus families and ending in -virusfor viral genera has gained wide acceptance. The names of subfamilies end in-virinae. Vernacular names continue to be used to describe the viruses within agenus. In this text, Latinized endings for families and subfamilies usually are notused. Table 41-2 shows the currentclassification of medically significant viruses.

(Video) Viral Structure and Functions


Current Classification of Major Groups Of viruses of MedicalSignificance.

In the early days of virology, viruses were named according to common pathogenicproperties, e.g. organ tropism and/or modes of transmission, and often also aftertheir discoverers. From the early 1950s until the mid-1960s, when many new viruseswere being discovered, it was popular to compose virus names by using sigla(abbreviations derived from a few or initial letters). Thus the name Picornaviridaeis derived from pico (small) and RNA; the name Reoviridae is derived fromrespiratory, enteric, and orphan viruses because the agents were found in bothrespiratory and enteric specimens and were not related to other classified viruses;Papovaviridae is from papilloma, polyoma, and vacuolating agent (simian virus 40[SV40]); 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 now in the familyPicornaviridae, genus Hepatovirus. Although the current rules for nomenclature donot prohibit the introduction of new sigla, they require that the siglum bemeaningful to workers in the field and be recognized by international studygroups.

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

Several viruses of medical importance still remain unclassified. Some are difficultor impossible to propagate in standard laboratory host systems and thus cannot beobtained in sufficient quantity to permit more precise characterization. Hepatitis Evirus, the Norwalk virus and similar agents (see Ch. 65) that cause nonbacterial gastroenteritis in humansare now assigned to the calicivirus family.

The fatal transmissible dementias in humans and other animals (scrapie in sheep andgoat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy;Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome inhumans) (see Ch. 71) are caused by theaccumulation of non-soluble amyloid fibrils in the central nervous systems. Theagents causing transmissible subacute spongiform encephalopathies have been linkedto viroids or virinos (i.e. plant pathogens consisting of naked, but very stablecircular RNA molecules of about 3-400 bases in size, or infectious genomes enwrappedinto a host cell coat) because of their resistance to chemical and physical agents.According to an alternative theory, the term “prion” has beencoined to point to an essential nonviral infectious cause for these fatalencephalopathies—prion standing for self-replicating proteinaceous agentdevoid of demonstrable nucleic acid. Some of the transmissible amyloidoses show afamilial pattern and can be explained by defined mutations which render a primarysoluble glycoprotein insoluble, which in turn leads to the pathognomonicaccumulation of amyloid fibers and plaques. The pathogenesis of the sporadicamyloidoses, however, is still a matter of highly ambitious research.


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  5. Mattern CFT: Symmetry in virus architecture. InNayak DP (ed): Molecular Biology of Animal Viruses. Marcel Dekker, New York,1977 .

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    (Video) Virus | Structure and Classification | Biology | Extraclass #bacteriophage

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What is the structure of a virus and its classification? ›

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.

What is the structure of virus answer? ›

Viral Structure. In the simpler viruses the virion consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid; the capsid and its enclosed nucleic acid together constitute the nucleocapsid. In some of the more complex viruses the capsid surrounds a protein core (Fig.

What are the 7 classifications of viruses? ›

The seven classes of viruses in the Baltimore Classification System are as follows:
  • Class I: Double stranded DNA (dsDNA) viruses. ...
  • Class II: Single stranded DNA (ssDNA) viruses. ...
  • Class III: Double stranded RNA (dsRNA) viruses. ...
  • Class IV: Single stranded RNA (ssRNA) viruses. ...
  • Class V: Single stranded RNA (ssRNA) viruses.

What are the main structures of viruses? ›

All viruses contain nucleic acid, either DNA or RNA (but not both), and a protein coat, which encases the nucleic acid. Some viruses are also enclosed by an envelope of fat and protein molecules. In its infective form, outside the cell, a virus particle is called a virion.

What three main structures do all viruses have name and describe? ›

These include: A protective protein shell, or capsid. A nucleic acid genome made of DNA or RNA, tucked inside of the capsid. A layer of membrane called the envelope (some but not all viruses)

Why is the structure of a virus? ›

Viruses vary in their structure. A virus particle consists of DNA or RNA within a protective protein coat called a ca psid. The shape of the capsid may vary from one type of virus to another. The capsid is made from the proteins that are encoded by viral genes within their genome.

What is a virus short answer? ›

A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat.

What is the structure of a virus biology discussion? ›

Viruses have a very simple structure. They consist of a nucleic acid core surrounded by a protein coat. In this respect they differ from typical cells which are made up of proteins, carbohydrates, lipids and nucleic acids.

What is the simple classification of viruses? ›

Viruses can be classified primarily on their phenotypic characteristics, core content, chemical composition, capsid structure, size, shape, modes of replication and other viral genome structures. The Baltimore classification is the most commonly used for studying the system of virus classification.

What are the 5 classification of viruses? ›

The first broad division used in virus classification separates viruses into vertebrate viruses, invertebrate viruses, plant viruses, bacterial viruses, and algae, fungi, yeast, and protozoan viruses.

How are viruses classified and named? ›

The highest taxonomic group among viruses is the family; families are named with a suffix -viridae. Subfamilies have the suffix -virinae; genera the suffix -virus.

What are the 7 characteristics of viruses? ›

  • Non living structures.
  • Non-cellular.
  • Contain a protein coat called the capsid.
  • Have a nucleic acid core containing DNA or RNA (one or the other - not both)
  • Capable of reproducing only when inside a HOST cell.

What are the 3 basic shapes of viruses? ›

What are the 3 shapes of viruses? The 3 Shapes of viruses are helical, polyhedral (which includes icosahedral and prolate shapes), and complex shapes.

What structure is found in viruses but not in cells? ›

Living things have cells.

Viruses do not have cells. They have a protein coat that protects their genetic material (either DNA or RNA). But they do not have a cell membrane or other organelles (for example, ribosomes or mitochondria) that cells have.

What is a virus made up of? ›

Viruses are simple, and are made up of up to three constitutive elements: A genome, made up of nucleic acids that can be DNA (like humans) or RNA. RNA is very similar to DNA; both are made up of of chains of nucleotides (ACGT/U) that make up genes that are translated in proteins.

Does virus have DNA or RNA? ›

Viruses do not have DNA or RNA.

What is the importance of viruses? ›

Viruses also keep us alive. They form part of the body's microbiome and safeguard our health. They can be harnessed to treat illness, deliver vaccines, and diagnose infections. They're wielded as research tools to illuminate biology and disease and develop new drugs.

What is the life cycle of a virus? ›

The life cycle of virus. The virus life cycle could be divided into six steps: attachment, penetration, uncoating, gene expression and replication, assembly, and release.

What is the smallest virus? ›

Adenovirus is the smallest virus and Mycoplasma is the smallest bacteria. Both bacteria and viruses are contagious organisms that lead to many diseases in both plants and animals.

What is the largest virus? ›

Mimivirus is the largest and most complex virus known. Is it an evolutionary bridge between nonliving viruses and living organisms, or is it just an anomaly? Viruses are small and fairly simple.

Are viruses living or non living? ›

Viruses are not living things. Viruses are complicated assemblies of molecules, including proteins, nucleic acids, lipids, and carbohydrates, but on their own they can do nothing until they enter a living cell. Without cells, viruses would not be able to multiply.

Do viruses have cell structure? ›

Viruses are acellular, meaning they are biological entities that do not have a cellular structure. They therefore lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane.

What is the best virus definition? ›

Listen to pronunciation. (VY-rus) In medicine, a very simple microorganism that infects cells and may cause disease. Because viruses can multiply only inside infected cells, they are not considered to be alive.

What is virus in one sentence? ›

A virus is an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria.

How to identify a virus? ›

Direct methods assay for the presence of the virus itself, while indirect methods observe the effects of the virus, such as cell death or the production of antibodies by the infected individual. Tissue culture is a way to identify a virus based upon the effects of the virus upon the cells.

What are the structure and phases of virus? ›

A virus particle consists of a genome, which can comprise either single- or double-stranded RNA or DNA molecule(s) protected by a protein or a proteolipid outer capsid. Viruses insert their genome into a host organism where they can replicate and assemble.

Why is classification of viruses difficult? ›

Viruses are notoriously difficult to classify due to their enormous diversity, high rates of change and tendency to exchange genetic material.

What are the types of viruses explain? ›

File Infector Virus – As the name suggests, it first infects a single file and then later spreads itself to other executable files and programs. The main source of this virus are games and word processors. Spacefiller Virus – It is a rare type of virus which fills in the empty spaces of a file with viruses.

What is the most important factor for virus classification *? ›

The classification of viruses is based on chemical and morphologic criteria. The two major components of the virus used in classification are (1) the nucleic acid (its molecular weight and structure) and (2) the capsid (its size and symmetry and whether it is enveloped).

Which was the first virus in the world? ›

Abstract. Two scientists contributed to the discovery of the first virus, Tobacco mosaic virus. Ivanoski reported in 1892 that extracts from infected leaves were still infectious after filtration through a Chamberland filter-candle.

Are viruses single celled? ›

Viruses are not classified as cells and therefore are neither unicellular nor multicellular organisms. Most people do not even classify viruses as "living" as they lack a metabolic system and are dependent on the host cells that they infect to reproduce.

Who discovered the virus? ›

Dmitri Ivanowsky in 1892. He recognized an infectious agent, which caused tobacco mosaic disease and were smaller than bacteria. M.W. Beijerinek in 1898 called the filter 'Contagium vivum fluidum' and named it the 'virus'.

What is the smallest virus name and size? ›

AAV is the smallest DNA virus with an average size of 20 nm.

What do viruses need to reproduce? ›

Viruses cannot replicate on their own, but rather depend on their host cell's protein synthesis pathways to reproduce. This typically occurs by the virus inserting its genetic material in host cells, co-opting the proteins to create viral replicates, until the cell bursts from the high volume of new viral particles.

Where do viruses reproduce? ›

They also do not grow or reproduce on their own. A virus needs a living cell in order to reproduce. The living cell in which the virus reproduces is called a host cell.

What are the 5 stages of viral replication? ›

The viral life cycle can be divided into several major stages: attachment, entry, uncoating, replication, maturation, and release.

What are 5 characteristics of a virus? ›

  • Non living structures.
  • Non-cellular.
  • Contain a protein coat called the capsid.
  • Have a nucleic acid core containing DNA or RNA (one or the other - not both)
  • Capable of reproducing only when inside a HOST cell.

What is the structure of a virus a level biology? ›

→What is the structure of viruses? Viruses are biological structures that have a nucleic acid genome surrounded by protein and lipids. They have no nucleus, no ribosomes, and none of the membrane bound organelles. Viruses carry genetic material.

Are viruses structurally organized? ›

Many of these viruses exhibit exquisitely symmetric organization. Irrespective of their shape and size, the underlying theme in all these viruses is that the virus structure is designed to contain and protect the viral genome and deliver it to a specific host cell for subsequent replication of the virus.

How are virus classified by genome structure and core? ›

1: Virus classification by genome structure and core: The type of genetic material (DNA or RNA) and its structure (single- or double-stranded, linear or circular, and segmented or non-segmented) are used to classify the virus core structures. Viruses can also be classified by the design of their capsids.

What are the 7 properties of viruses? ›

General Properties of Viruses
  • Single and/or double stranded.
  • Glycosylated and/or.
  • Gaps present in double stranded molecule.
  • Circular or linear.
  • Bound protein molecules.
  • Unique purine and/or pyrimidine bases present.
  • Ribonucleotides present.

What are the main general characteristics of viruses? ›

They can mutate.
  • They are acellular, that is, they contain no cytoplasm or cellular organelles.
  • They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. ...
  • The vast majority of viruses possess either DNA or RNA but not both.
Apr 9, 2022

Do viruses have DNA? ›

A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat.

How do viruses change structure? ›

Viruses are continuously changing as a result of genetic selection. They undergo subtle genetic changes through mutation and major genetic changes through recombination. Mutation occurs when an error is incorporated in the viral genome.

Do viruses have a basic cell structure? ›

Viruses are acellular, meaning they are biological entities that do not have a cellular structure. They therefore lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane.

What is virus classification based mainly on? ›

Virus classification is based mainly on characteristics of the viral particles, including the capsid shape, the type of nucleic acid (DNA or RNA, double stranded (ds) or single stranded (ss)) within the capsid, the proces s of replication, their host organisms, or the type of disease they cause.

Why is classifying viruses important? ›

Virus taxonomy is important because it allows the clinical, biological and evolutionary features of a virus to be placed into a framework that accommodates and connects all viruses. The understanding that this brings has immense practical value.

How many groups are viruses classified into? ›

The Baltimore system of virus classification

There are seven distinct groups in the Baltimore system, based on the type of nucleic acid making up the genome, whether the genome is single- or double-stranded, and in the case of single-stranded RNA viruses, whether the RNA is positive or negative sense.


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6. Virus Structure and Classification // shape of viruses // DNA VIRUS & RNA virus
(Dr.Anurag Saini {MBBS})


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