Introduction to the Virology

General properties of viruses1-They are very small in size, from 20-300 nm

2-They contain one kind of nucleic acid (RNA or DNA) as their genome

3-They are metabolically inert because they do not possess enzyme systems necessary for the synthesis of new viral material (ribosomes)

4-They are obligate intracellular parasites as they replicate inside living cells

5-They are only seen by electron microscope

Viral StructureEach virus particle or virion is composed of:

A protein coat capsidA nucleic acid core

Many viruses are naked but some are enveloped

Viral capsidIt is the protein coat surrounding the nucleic

acidIt is composed of small protein subunits

arranged symmetrically around the nucleic acid called capsomeres

General structure: A. nonenveloped; B. enveloped viruses.

Viral nucleic acid and viral EnvelopeViruses contain either DNA or RNA but not both

Most DNA viruses are double strandedsome are single stranded

Most RNA viruses are single strandedsome are double stranded

Virus envelopeMany viruses are surrounded by a lipid or lipoprotein

envelopes which may be covered by spikes (glycoproteins)

Virus SymmetryViruses have three types of symmetry

Cubical symmetryThese viruses resemble a crystal and are called icosahedral

virusExample: Adenoviruses

Helical symmetryIn which the particle is elongatedMost helical viruses are envelopedExample: Influenza virus

Complex symmetryIn which the viruses are complicated in structureExample: Poxviruses and Bacteriophage

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5 BASIC TYPES OF VIRAL STRUCTURE

HELICAL ENVELOPED HELICAL

ENVELOPED ICOSAHEDRAL

COMPLEX

ICOSAHEDRAL

Adapted from Schaechter et al., Mechanisms of Microbial Disease

nucleocapsid icosahedral nucleocapsid

nucleocapsid

helical nucleocapsid

lipid bilayer

lipid bilayer

glycoprotein spikes= peplomers

Virus particle = virion

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Koneman et al. Color Atlas and Textbook of Microbiology 5th Ed. 1997

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  Growth on artificial media

Division by binary fission

Contain DNA and RNA

Contain protein synthesis machinery

Contain muramic acid

Sensitive to antibiotics

 Bacteria

 often

 yes

 yes

 yes

 often

 yes

 Viruses

 never

 no

 Either DNA or RNA

 no*

 no

 no

* The arenavirus family appears to ‘accidentally’ package ribosomes, but these appear to play no role in protein synthesis.

Classification of VirusesClassical virus classification schemes have been

based on the consideration of major properties of viruses

1- The type of nucleic acid which is found in the virion(RNA or DNA, single stranded or double stranded)

2-The symmetry and shape of the capsid(Cubic, helical, complex)

3- The presence or absence of an envelope(enveloped, naked)

4- The size of the virus particle

5- Antigenic properties

6- Biologic properties, including natural host range, mode of transmission, vector relationship, pathogenicity, and tissue tropism

DNA

Icosahedral Complex

Pox virusesNaked Enveloped

Papilloma virus Herpes viruses

Hepatitis B

Classification of Viruses

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HERPESVIRIDAEHEPADNAVIRIDAE

ENVELOPED

PAPILLOMAVIRIDAEPOLYOMAVIRIDAE(formerly grouped together as the PAPOVAVIRIDAE)

CIRCULAR

ADENOVIRIDAE

LINEAR

NON-ENVELOPED

DOUBLE STRANDED

PARVOVIRIDAE

SINGLE STRANDEDNON-ENVELOPED

POXVIRIDAE

COMPLEXENVELOPED

DNA VIRUSES

Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

All families shown are icosahedral except for poxviruses

a

Classification of Viruses

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FLAVIVIRIDAETOGAVIRIDAE

RETROVIRIDAE

ICOSAHEDRAL

CORONAVIRIDAE

HELICAL

ENVELOPED

ICOSAHEDRAL

PICORNAVIRIDAECALICIVIRIDAE

NONENVELOPED

SINGLE STRANDEDpositive sense

BUNYAVIRIDAEARENAVIRIDAE

ORTHOMYXOVIRIDAEPARAMYXOVIRIDAE

RHABDOVIRIDAEFILOVIRIDAE

SINGLE STRANDEDnegative sense

REOVIRIDAE

DOUBLE STRANDED

RNA VIRUSES

ENVELOPED

HELICAL ICOSAHEDRAL

NONENVELOPED

Modified from Volk et al., Essentials of Medical Microbiology, 4th Ed. 1991

Viral replicationViruses multiply only in living cells

Steps of viral replication

1- AdsorptionVirus attaches to the cell surface

2- Penetration (Entry)Enveloped viruses: receptor mediated endocytosis

3- UncoatingUncoating is the physical separation of viral nucleic acid from

the outer structural components

4- Transcription of mRNA

5- Synthesis of viral componentsThis involves the synthesis of viral proteins and

viral genomes

6- AssemblyNew virus particles are assembled by packaging

of the genome into capsid

7- ReleaseVirus may be released due to cell lysis, or, if

enveloped, may bud from the cell

Viral replication

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ADSORPTION

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ADSORPTION

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PENETRATION

herpesviruses, paramyxoviruses, HIV

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PENETRATION - ENVELOPED VIRUSES

from Schaechter et al, Mechanisms of Microbial Disease, 3rd ed, 1998

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PENETRATIONNON-ENVELOPED VIRUSES

entry directly across plasma membrane:

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Smallpox virus cytoplasmic assembly and maturation

F. A. Murphy, School of Veterinary Medicine, University of California, Davis. http://www.vetnet.ucdavis.edu/fam_graphics/download.html

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HIV budding and maturation

Hsiung, GD et al., Diagnostic Virology 1994 p204 (D. Medina)