Unit-I CLASSIFICATION OF VIRUSES

Prepared by Dr. R. Dhanapal Guest Lecturer of Zoologgy M.R.G.C

Content

1. Naming ofviruses 2. Classification of virus – ICTV,Baltimore Classification 3. Baltimore Classification – 7classes

Lesson Outcomes

• Understands the reasons on namingviruses • Briefly discuss the classification ofviruses • Explain the Baltimoreclassification

• Explain the characteristics of RNA andDNAviruses that cause humandisease. Nomenclature of Viruses Various approaches, (do not obey thebinomial nomenclature) derivedfrom:

1. Named after thediseases eg. Measles virus, smallpox virus

2. Name after the places where the diseasefirst reported eg. Newcastle disease virus, Ebola virus, Norwalk virus, Bunyaviridae

3. Host and signs ofdisease eg. Tobacco mosaic virus, cauliflowermosaic virus brome mosaicvirus 4. Latin and Greekwords eg.Coronaviridae – “crown” Parvoviridae – “small”

5. Virusdiscovers eg. Epstein-Barrvirus

6. How they were originally thought to becontractedeg. dengue virus (“evil spirit”), influenzavirus (the “influence”ofbad air)

7. Combinations of the aboveeg. Rous Sarcomavirus

7

Examples

Animal virus Plant virus

Reasons beyond classification  Classification of virus been determined bythe structural and chemical composition ofvirus  Are apply to all plant viruses, animal virusesand bacterialviruses  Virus is acellular cell – cannot becategorised using taxonomicclassification  It used International Committee onTaxonomy of Viruses (ICTV) to classify theviruses

Before discovery

• Dermotropic– infected skincell • Neurotrophic – infected nervecell • Viscerotropic – infect organ of digestivetract • Pneumotropic – infected respiratorysystem

After discovery

 Type and structure of their nucleicacids  Methods ofreplication  Hostrange  Chemical and physicalcharacteristics

Classification  Viruses are not classified as members ofthe kingdoms  Do not obey the biologicaltaxonomy  Generally basedon: 1. Classical - eg. animal, plant, bacterialvirus system - eg. naked or envelopedvirus

2. Genomic - Baltimoreclassification

3. Serology - classification based on Diagnosticvirology - eg. Infectious bronchitis virus (IBV) of chickens (a coronavirus) – 3 different types present, these types have significant antigenic differences, but perhaps very littlegenetic or biological difference between these viruses.

The following criteria are used to classify viruses: 1. Morphology – structure ofcapsid – presence or absence of envelope 2. Size of thevirion 3. Type of host/host structures the virusinfected - Bacteriophages: infect bacterialcells - Plant viruses infect plantcells - Animal viruses are subgrouped by thetissues theyattack: 1. Dermotrophic: if they infect theskin 2. Neurotrophic: if they infect nervetissue 4. Genome composition – DNA /RNA – ds/ss DNA and ds/ss RNA

1. Taxonomic groups – family, subfamily, genusand species 2. The names of virus families (family) areitalicized - End in Latin suffix –viridae 3. The genera (genus) end in the suffix –virus 4. The species – English common name

Virus taxonomy Order virales e.gMononegavirales

Family viridae e.g. Orthomyxoviridae Herpesviridae Subfamily virinae e.g. Alphaherpesvirinae Genus e.g. influenzavirusA Simplexvirus Species e.g. influenza Avirus human herpesvirus1 Informally: Type e.g. herpes simplex virus 1 Strain

In biology, binomial names are used. e.gRattus rattus, Saccharomycescerevisiae In virology, this does not happen: Bacteriophage have Tobacco etch potyvirus sounds OK their own rules Influenza A influenzavirus A does not!

• Viruses are divided into three groups, based on the morphology of the nucleocapsid and the arrangementof capsomeres.

Symmetry of viruses • Arrangement of capsomers in thevirus. • Two primary shapes of virus is rod andspherical. • Rod shaped virus-helicalsymmetry • Sphericalvirus-icosahedron

Cubic symmetry

Helical symmetry

Symmetry of viruses • 2- helical symmetry: The virus particle is elongatedorpleomorphic (not spherical), and the nucleic acid is spiral.Caposomeres are arranged round the nucleicacid. • 3- complexsymmetry:

The virus particle does not confirm either cubic or helical symmetry.

Helical symmetry (influenza & rabies viruses).

Complex symmetry (Poxviruses).

Classification of viruses

• The division of the viruses into classes based ongenome type and mode of replication andtranscription

• Suggested by David Baltimore – Seven Baltimoreclasses.

• Major groups of viruses are distinguished first bytheir nucleic acid content as either DNA orRNA

• RNA and DNA viruses can be single-stranded(ssRNA, ssDNA) or double-stranded (dsRNA,ssDNA)

7 class of virus classification Class Description of genomeand Example of Example of animal replicationstrategy bacterial virus virus

I Double stranded DNAgenome Lamda,T4 Herpesvirus, poxvirus

II Single stranded DNAgenome ØX174 Chicken anemia virus III Double stranded RNAgenome Ø6 Reovirus

IV Single stranded RNA genome plus MS2 Poliovirus sense V Single stranded RNA genome Influenza virus,Rabies minussense virus VI Single stranded RNA genome that Retrovirus replicated with DNAintermediate VII Double stranded DNA genome Hepatitis B virus that replicates with RNA intermediate Class I • Double-stranded (ds) DNA viruses are in class1 • The production of mRNA and genome replication insuch viruses occurs as it would from the hostgenome. Class II • Single-stranded(ss) DNAviruses. • These viruses form a double stranded DNAintermediate during replication and this intermediate used for transcription. • RNA polymerase requires double-stranded DNAas template.

Positive and Negative strand RNA viruses

• The productionofmRNA and genome replicationis much different with RNA viruses (ClassIII-VI). • mRNA is the complementary base sequence to the template strand ofDNA. • In virology, mRNA is said to be plus(+)configuration. • While its complement is said to be the minus (-) configuration.

How does these viruses replicate? • Cellular RNA polymerases do not catalyze formationof RNA from an RNA template but from DNAtemplate. • RNA viruses whether plus, negative or doublestranded require a specific RNA-dependant RNApolymerase. Class IV • Positive-strand of RNAviruses • Viral genome is of the plus configuration andhence serve directly asmRNA. • The viruses required other protein, thereforemRNA encodes a virus specific and RNA dependent RNA polymerase. • Once synthesized, this polymerase makes complementary minus strandsof RNA and then use as template to make more plusstrand. Class III and Class V

• ClassIII(double-stranded RNAviruses) • Class V(negative strand RNAvirus) • mRNA must be first synthesized, however cells doesnot have RNApolymerase. • To circumvent ,these viruses contain enzyme inthe virion,enters cell along with the genomicRNA. • Therefore, in this case complementary plus strand is synthesized by RNA dependant RNA polymeraseand used as mRNA. • Plus strand usedastemplate to make more negative- strandgenome.

Class VI • Single-stranded RNA genome that replicates withDNA intermediate. • This RNA virus require reverse transcriptase to copythe information found in RNA toDNA. Class VII • Double-stranded DNA genome that replicates withRNA intermediate. • Required reversetranscriptase • Mechanism producing mRNA is similar in virus ClassI

Retroviruses: are enveloped vi ruses that have two complete copies of (+) sense RNA. They also contain the enzyme reverse transcriptase, which uses the viral RNA to form a complementary strand of DNA, which is then replicated to form a dsDNA

retro, latin for “backward” (Class IV) in Baltimore Classification

Ambisense genome

• A virus genome composed of ssDNA orssRNA that is partly (+) sense and partly (-)sense. • Example: - Bunyaviridae ((-) sense RNA) and Arenavirus ((-) sense RNA)

Advantages

1. Can classify between the (+) strand RNA viruses that do (Class VI) and do not(Class IV) undergo reverse transcription

2. Can classify between the dsDNA virusesthat do (Class VII) and do not (Class I) carry out reverse transcription

General Properties of RNA Viruses  Many ssRNA viruses contain positive (+) sense RNA, and during an infection acts like mRNA and can be translated by host’sribosomes.

 Other ssRNA viruses have negative (-) sense RNAand the RNA acts as a template during transcription to make a complementary (+) sensemRNA.

 Negative (-) sense RNA must carry a RNApolymerase RNA within the virion. dependen t RNA polymera se – Class III, IV and V

 RNA viruses must either carry enzymes or have genes for those enzymes in order to copy RNA genomesafter infecting a hostcell

RNA VIRUSES

From Principles of Virology Flint et al., ASM Press

RNA viruses (+ve sense)

• Picornaviridae • Togaviridae • Flaviviridae • Retroviridae

RNA viruses (-ve sense)

• Paramyxoviridae • Rhabdoviridae • Orthomyxoviridae • Filoviridae • Bunyaviridae • Reoviridae(double-stranded)

DNA VIRUSES

From Principles of Virology Flint et al., ASM Press

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DNA viruses

Double – stranded Adenoviridae Herpesviridae Poxviridae Papovaviridae HepadnaviridaeSi ngle – stranded oParvoridae