infectious agents of small size and simple composition that can
multiply only in living cells. Viruses are obligate intracellular
parasites that are metabolically inert when they are outside their
hosts. The virus genome is composed either of DNA or RNA.
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Question? No, The Viroids and Prions are smaller than
viruses.
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Relative Size of Viruses Eye
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Capsid: The protein shell, that encloses the nucleic acid
genome. Capsomer: Morphological Morphologic units (seen in the
electron microscope) making up the capsid. Structural units:
Protomer: The basic protein building blocks of the coat. They are
usually a collection of more than one nonidentical protein
subunit.
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Nucleocapsid: The protein-nucleic acid complex representing the
packaged form of the viral genome. Envelope: A lipid-containing
membrane that surrounds some virus particles. It is acquired during
viral maturation by a budding process through a cellular membrane.
Spikes: Peplomer: Rod-like proteins projecting from the envelope or
surface of a naked virion. Matrix:
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Virion : The complete virus particle. In non-enveloped viruses,
the virion is identical with the nucleocapsid. In more complex
virions (Evneloped viruses), this includes the nucleocapsid plus a
surrounding envelope. Defective viruses: virus particle that is
functionally deficient in some aspect of replication.
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to protect the fragile nucleic acid genome from physical,
chemical, or enzymatic damage. The outer surface of the virus is
also responsible for recognition of and the first interaction with
the host cell. The capsid also has a role to play in initiating
infection by delivering the genome in a form in which it can
interact with the host cell. Many viral proteins are enzymes or
factors that used in viral replication and transcription.
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Viruses may be derived from DNA or RNA nucleic acid components
of host cells that became able to replicate autonomously and evolve
independently. Viruses may be degenerate forms of intracellular
parasites Viruses may be the first form of the living material May
be they come from another planet !?
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Classification of Viruses The following properties have been
used as a basis for the classification of viruses: 1.Virion
morphology, including size, shape, type of symmetry, presence or
absence of peplomers, and presence or absence of membranes. 2.Virus
genome properties, including type of nucleic acid (DNA or RNA),
size of genome, strandedness (single or double), whether linear or
circular, sense (positive, negative, ambisense), segments (number,
size), nucleotide sequence, G + C content, and 3.Physicochemical
properties of the virion, including molecular mass, buoyant
density, pH stability, thermal stability, and susceptibility to
physical and chemical agents, especially ether and detergents.
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4.Genome organization and replication, including gene order,
strategy of replication, and cellular sites (accumulation of
proteins, virion assembly, virion release). 5.Biologic properties,
including natural host range, mode of transmission, vector
relationships, pathogenicity, tissue tropisms, and pathology.
6.Antigenic properties. 7.Virus protein properties, including
number, size, and functional activities of structural and
nonstructural proteins, amino acid sequence, and special functional
activities (transcriptase, reverse transcriptase, neuraminidase,
fusion activities).
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Virus family names have the suffix -viridae Genus names carry
the suffix -virus In four families (Poxviridae, Herpesviridae,
Parvoviridae, Paramyxoviridae), a larger grouping called
subfamilies has been defined ( -Virinae) Virus orders may be used
to group virus families that share common characteristics (
-Virales) mononegavirales, Nidovirales, Caudovirales.
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ICTV: International Committee on Taxonomy of Viruses By 2005,
the International Committee on Taxonomy of Viruses had organized
more than 5500 known viruses into 73 families, 9 subfamilies, and
287 genera and 3 order, with hundreds of viruses still unassigned.
Currently, 24 families contain viruses that infect humans and
animals.
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Viroids: Prions: NORMAL PrP C PRION PrP SC
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Electron microscopy, cryoelectron microscopy, and x-ray
diffraction techniques have made it possible to resolve fine
differences in the basic morphology of viruses.
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The Shape of Viruses Cubic symmetry (Icosahedral ) DNA viruses
RNA viruses
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Helical Symmetry In animals, Only RNA viruses TMV
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Complex Structure
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Direct Observation in the Electron Microscope Sedimentation in
the Ultracentrifuge Comparative Measurements
Nucleic Acid DNA RNA Double Stranded (DS) Positive Sense
Negative Sense RNADNA Single Stranded (SS) Double Stranded (DS)
Single Stranded (SS) Virus Genomes
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Positive Sense = (+) Sense: Viral RNA acts as an mRNA Within
the infected cells Picornaviridae, Flaviviridae, Togaviridae,
Caliciviridae, Coronaviridae, Arteriviridae, Astroviridae, and
Retroviridae. Negative Sense = (-) Sense: Viral RNA is
complementary to mRNA. Orthomyxoviridae, Paramyxoviridae,
Rhabdoviridae, Filoviridae, Bornaviridae.
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Group I Like : Adenoviridae, Herpesviridae, Poxviridae,
Hepadnaviridae,Papilloma viridae, Polyomavridae Group II :
Parvoviridae, Circoviridae Group III: Reoviridae, Birnaviridae
Group IV: Picornaviridae, Flaviviridae, Togaviridae, Coronaviridae,
Caliciviridae, Astroviridae Group V: Orthomyxoviridae,
Paramyxoviridae, Rhabdoviridae, Filoviridae, Arenaviridae,
Bonyaviridae, Bornaviridae Group VI: Retroviridae
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Heat & Cold Stabilization of Viruses by Salts pH Radiation
Photodynamic Inactivation Ether Susceptibility Detergents
Formaldehyde Antibiotics & Other Antibacterial Agents
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Sterilization: steam under pressure, dry heat, ethylene oxide,
and gamma irradiation Vaccine production: use of formaldehyde, -
propiolactone, psoralen + ultraviolet irradiation, or detergents
(subunit vaccines) to inactivate the vaccine virus. Surface
disinfectants: sodium hypochlorite, glutaraldehyde, formaldehyde,
and peracetic acid. Skin disinfectants: include chlorhexidine, 70%
ethanol, and iodophores
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Attachment
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Rhinovirus: ICAM-1 EBV: CD21=CR 2 Polio: PVR= CD 155 HIV: CD 4
.. Definition: Susceptibility & Permissivity
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Penetration or Engulfment Penetration of the target cell
normally occurs a very short time after attachment of the virus to
its receptor in the cell membrane. Three main mechanisms are
involved in penetration: Three main mechanisms are involved in
penetration: Receptor-mediated endocytosis (or Viropexis) Fusion
Direct penetration of virus particles across the plasma membrane
(or Translocation)
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clathrin-independent endocytosis
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Endocytosis ( Viropexis)
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Fusion
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Direct penetration
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Uncoating Uncoating is the physical separation of the viral
nucleic acid from the outer structural components of the virion.
often, the genome is released as free nucleic acid. Or rarely, as a
nucleocapsid (reoviruses). Eclipse period: Indeed, Viruses are the
only infectious agents for which dissolution of the infecting agent
is an obligatory step in the replicative pathway.
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Expression of Viral Genomes and Synthesis of Viral
Components
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maturation phase may occur before or after release
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Assembly Newly synthesized viral genomes and capsid
polypeptides assemble together to form progeny viruses. Icosahedral
capsids can condense in the absence of nucleic acid, whereas
nucleocapsids of viruses with helical symmetry cannot form without
viral RNA. Release Nonenveloped viruses accumulate in infected
cells, and the cells eventually lyse and release the virus
particles. Enveloped viruses mature by a budding process.
Detection of Virus-Infected Cells Development of cytopathic
effects (CPE) : cell lysis or necrosis giant cell formation (
Syncytia ) Viral Ag detection (serologically) Viral NA detection
(Molecular) Hemadsorption Inclusion Body Formation
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Negri bodies in a rabies infected cell Inclusion bodies
Guarnieri bodies