VIRUS MORPHOLOGY & VIRUS MORPHOLOGY & CLASSIFICATIONCLASSIFICATION

ELIZABETH P. QUILES, M.D., FPASMAPELIZABETH P. QUILES, M.D., FPASMAPOUR LADY OF FATIMA UNIVERSITYOUR LADY OF FATIMA UNIVERSITY

COLLEGE OF MEDICINECOLLEGE OF MEDICINE

• Not cellsNot cells

• Smallest infectious agent (20nm to Smallest infectious agent (20nm to 300 nm)300 nm)– smallest RNA virus smallest RNA virus Picornavirus Picornavirus– smallest DNA virus smallest DNA virus Parvovirus Parvovirus– largest virus largest virus Poxvirus Poxvirus

• Most have spherical shapes EXCEPTMost have spherical shapes EXCEPT– Rhabdovirus Rhabdovirus bullet-shaped bullet-shaped– Poxvirus Poxvirus brick-shaped brick-shaped– Bacteriophage Bacteriophage tadpole-shaped tadpole-shaped– Tobacco Mosaic virus Tobacco Mosaic virus rod- rod-

shapedshaped

• Composed of only one type of nucleic Composed of only one type of nucleic acidacid

• Obligate intracellular parasites Obligate intracellular parasites inert in the extracellular environment inert in the extracellular environment multiply only in living cells multiply only in living cells parasites at the genetic levelsparasites at the genetic levels

• Do not undergo mitosis or binary Do not undergo mitosis or binary fissionfission

• Do not have cell wall & ribosomeDo not have cell wall & ribosome

• Sensitive to interferonSensitive to interferon

STRUCTURE STRUCTURE • VIRAL NUCLEIC ACID (GENOME)VIRAL NUCLEIC ACID (GENOME)

– contain the genetic material necessary contain the genetic material necessary for replicationfor replication

– either a DNA or RNAeither a DNA or RNA

– maybe single-stranded or double-maybe single-stranded or double-strandedstranded

– can be linear or circularcan be linear or circular

– Segmented or nonsegmentedSegmented or nonsegmented

– haploid haploid contain only one copy of their contain only one copy of their genes EXCEPT Retrovirusesgenes EXCEPT Retroviruses

– used as major criterion in the used as major criterion in the classification of virusesclassification of viruses

• CAPSIDCAPSID

– protein coat made up of repeating protein coat made up of repeating subunits subunits CAPSOMERES (gives the CAPSOMERES (gives the virus its geometric symmetry)virus its geometric symmetry)

– 2 forms of symmetry:2 forms of symmetry:1. icosahedral – capsomeres 1. icosahedral – capsomeres

arranged in 20 arranged in 20 triangles triangles 2. helical – arranged in a hollow coil 2. helical – arranged in a hollow coil

that that appears rod-shapedappears rod-shaped

HELICAL SYMMETRYHELICAL SYMMETRY

ICOSAHEDRAL SYMMETRYICOSAHEDRAL SYMMETRY

Functions of the viral capsid:Functions of the viral capsid:1. protect the genetic material1. protect the genetic material2. site of receptors 2. site of receptors mediate mediate

attachmentattachment3. serve as antigenic determinants3. serve as antigenic determinants4. induce antibody production4. induce antibody production5. determinants of type specificity5. determinants of type specificity6. Provide the structural symmetry 6. Provide the structural symmetry

of the of the virusvirus

• ENVELOPE ENVELOPE

– a lipoprotein derived from the host a lipoprotein derived from the host cell membrane & virus specific cell membrane & virus specific proteinprotein

– composed of glycoproteins composed of glycoproteins (peplomers) that appear as spikes(peplomers) that appear as spikes

– Acquired through budding from the Acquired through budding from the host’s cell membrane in the course host’s cell membrane in the course of maturationof maturation

– Confers instability to the virus Confers instability to the virus because of the loss of infectivity because of the loss of infectivity due to disruption or loss of lipid due to disruption or loss of lipid more sensitive to heat, detergents more sensitive to heat, detergents & lipid solvents& lipid solvents

– Functions:Functions:1. site of receptors 1. site of receptors attach to attach to host cell receptors & host cell receptors & membrane membrane fusionfusion2. antigenic determinants2. antigenic determinants3. stimulate antibody production3. stimulate antibody production4. determinants of type specificity4. determinants of type specificity

TERMINOLOGIESTERMINOLOGIES• VIRION – a complete viral particleVIRION – a complete viral particle

- in naked viruses- in naked viruses virion is identical virion is identical to to the nucleocapsidthe nucleocapsid- in enveloped viruses - in enveloped viruses must acquire must acquire envelop before it is considered a virionenvelop before it is considered a virion

• NUCLEOPCAPSID – a protein-nucleic NUCLEOPCAPSID – a protein-nucleic acid complexacid complex

ATYPICAL VIRUS-LIKE ATYPICAL VIRUS-LIKE AGENTSAGENTS

• DEFECTIVE VIRUSES – composed of DEFECTIVE VIRUSES – composed of nucleic acid & proteins but cannot nucleic acid & proteins but cannot replicate without a helper virusreplicate without a helper virus

• PSEUDOVIRION – contain host cell PSEUDOVIRION – contain host cell DNA instead of viral DNA within the DNA instead of viral DNA within the capsidcapsid

• VIROIDS – consist solely of a single VIROIDS – consist solely of a single molecule of circular RNA without a molecule of circular RNA without a protein coat or envelopeprotein coat or envelope

• PRIONS – infectious protein particles PRIONS – infectious protein particles composed solely of proteinscomposed solely of proteins

FEATUREFEATURE PRIONSPRIONS CONVENTIONAL CONVENTIONAL VIRUSESVIRUSES

Nucleic acidNucleic acid NoNo YesYesProteinProtein Yes, encoded by Yes, encoded by

cellular genes cellular genes Yes, encoded by Yes, encoded by

viral genes viral genes Heat Heat InactivationInactivation

NoNo YesYes

AppearanceAppearance Amyloid-like Amyloid-like Icosahedral or Icosahedral or helical helical

Antibody Antibody ResponseResponse

NoNo YesYes

InflammatorInflammatory Responsey Response

NoNo YesYes

VIRUS CLASSIFICATIONVIRUS CLASSIFICATION1.1. Virion morphology - size, shape, Virion morphology - size, shape,

symmetry, +/- of peplomers, +/- of symmetry, +/- of peplomers, +/- of membranemembrane

2.2. Virus genome – DNA or RNA, size, Virus genome – DNA or RNA, size, strandedness, segmentsstrandedness, segments

3.3. Physicochemical properties of the Physicochemical properties of the virion virion – molecular mass, bouyant – molecular mass, bouyant density, density, stability & susceptabilitystability & susceptability

4.4. Antigenic propertiesAntigenic properties5.5. Virus protein properties – number, Virus protein properties – number,

size, size, activitiesactivities6.6. Biological properties – host range, Biological properties – host range,

MOT, vector relationships, MOT, vector relationships, pathogenicity, tissue tropism & pathogenicity, tissue tropism & pathologypathology

FAMILYFAMILY ENVELOPEENVELOPE SYMMETRYSYMMETRY DNA DNA STRUCTURESTRUCTURE

IMPORTANT IMPORTANT VIRUSESVIRUSES

ParvovirusParvovirus -- IcosahedralIcosahedral SS, linearSS, linear B19 virusB19 virusPapovavirusPapovavirus -- IcosahedralIcosahedral DS, circularDS, circular HPV, HPV,

PolyomaPolyomaAdenovirusAdenovirus -- IcosahedralIcosahedral DS, linearDS, linear AdenoviruseAdenoviruse

ssHepadnaviruHepadnaviruss

++ IcosahedralIcosahedral DS, DS, incomplete incomplete circularcircular

Hepatitis B Hepatitis B virusvirus

HerpesvirusHerpesviruseses

++ IcosahedralIcosahedral DS, linearDS, linear HSV, VZV, HSV, VZV, CMV, EBV, CMV, EBV, HHV6HHV6

PoxvirusesPoxviruses ++ ComplexComplex DS, linearDS, linear Vaccinia, Vaccinia, Variola, Variola, Molluscum Molluscum contangiosucontangiosumm

IcosahedralIcosahedral NakedNaked SSSSSSSSSSSSDS segmentedDS segmented

PicornaviridaePicornaviridaeAstroviridaeAstroviridaeCaliciviridaeCaliciviridaeReoviridaeReoviridae

EnvelopedEnveloped SSSS TogaviridaeTogaviridaeUnknown Unknown or complexor complex

EnvelopedEnveloped SSSSSS segmentedSS segmentedSSSSSS diploidSS diploid

FlaviviridaeFlaviviridaeArenaviridaeArenaviridaeCoronaviridaeCoronaviridaeRetroviridaeRetroviridae

HelicalHelical EnvelopedEnveloped SS segmentedSS segmented

SS SS

Orthoviridae, Orthoviridae, BunyaviridaeBunyaviridaeRhabdoviridae, Rhabdoviridae, ParamyxoviridaeParamyxoviridae,,FiloviridaeFiloviridae

GENERAL PRINCIPLESGENERAL PRINCIPLESDNA VIRUSES:DNA VIRUSES:• Double stranded DNA except ParvovirusDouble stranded DNA except Parvovirus• Naked except Herpes, Hepadna & Naked except Herpes, Hepadna &

PoxvirusesPoxviruses• Icosahedral symmetry except PoxvirusesIcosahedral symmetry except Poxviruses• Replicate in the nucleus except Replicate in the nucleus except

Poxviruses Poxviruses

RNA VIRUSESRNA VIRUSES::• Single stranded RNA except ReovirusSingle stranded RNA except Reovirus• Enveloped except Reovirus & Enveloped except Reovirus &

PicornavirusPicornavirus• Helical symmetry except Reovirus, Helical symmetry except Reovirus,

Picornavirus & TogavirusPicornavirus & Togavirus• Replicate in the cytoplasm except Replicate in the cytoplasm except

Orthomyxovirus & retrovirusesOrthomyxovirus & retroviruses