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Introduction to the Virology
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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
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Viral StructureEach virus particle or virion is composed of:
A protein coat capsidA nucleic acid core
Many viruses are naked but some are enveloped
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Viral capsidIt is the protein coat surrounding the nucleic
acidIt is composed of small protein subunits
arranged symmetrically around the nucleic acid called capsomeres
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General structure: A. nonenveloped; B. enveloped viruses.
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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)
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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
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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.
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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)
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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
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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
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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
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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
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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
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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)