virology week6
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Virology - Introduction
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• Viruses cause disease in animals of economic and/or welfare importance
• Diagnose viral disease (clinical/lab tests)• Advise clients control (risk to other animals)
• Animal viruses may pose risk to human health (zoonosis)• Can act as important models for human disease•VIRUSES CAN BE USEFUL
VACCINE DEVELOPMENTGENE THERAPYTOOLS TO INVESTIGATE HOST CELLS
Why are viruses important ?
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What are What are Viruses?Viruses?
•A virus is a non-cellular particle made up of genetic material and protein that can invade living cells.
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Size of VirusesSize of Viruses
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Virus Classification
• International Committee on Taxonomy of Viruses
chemical characteristics, genome type, replication strategy, diseases, vectors, geographical distribution, host speciesnucleotide sequence
order family subfamily genus species/strain/type
-virales -viridae -virinae -virus -virus
mononegavirales
paramyxoviridae
paramyxovirinae morbillivirus canine distemper virus
herpesviridae alphaherpesvirinae varicellovirus
equid herpesvirus 1
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Basic virus structure
Capsid protein
NucleocapsidNaked capsid virus
DNA
RNA
or =+
NucleocapsidLipid
membrane, glycoproteins
Enveloped virus
+
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1. genome
± 3. envelopeenvelope glycoproteins
2. capsid
Virion (virus particle) structure
nucleocapsid
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DNA Viruses
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RNA Viruses
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DNA or RNA [cell genetic material is DNA] ss or ds ss -/+/mixed sense [mRNA= +sense] linear or circular segmented/non-segmented size 2-300 kb(p) [cell genome 3x106kbp]
1- Viral Nucleic Acids
Genetic ‘heritage’ Codes for virus proteins Controls virus protein production - promoters, transcriptional enhancers, splice signals Contains elements necessary for replication and genome packaging
Viral Proteins
Structural Components of capsid (protective coat) and other components of the virion
Non-structural Required for viral replication and interaction with host
Capsid is protein coat that protects the nucleic acid:
physical, chemical, enzymatic attack
Nucleocapsid comprises the capsid and enclosed nucleic acid
facilitates entry into cell and delivery of nucleic acid
exposed to immune system
2- Nucleocapsid
genome
capsid
nucleocapsid
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Viruses come in a variety of
shapes and sizes dictated
by their protein and nucleic acid
composition
- but there are common elements in their architecture due to SYMMETRY
ICOSAHEDRALHELICAL
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Capsid symmetry
Icosahedral Helical
Naked capsid
Enveloped
Lipid
Glycoprotein
Matrix
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Icosahedral (or cubic)
20 faces each face an equilateral triangleaxes of 2-, 3- and 5-fold rotational symmetryCapsomer structure enclosing maximum volume
Foot and mouth disease virus (picornavirus)
Some icosahedral animal viruses are enveloped
adenovirus
herpesvirus
Icosahedral naked capsid viruses
AdenovirusElectron
micrograph
Foot and mouth disease virus
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Icosahedral enveloped viruses
Herpes simplex virusElectron micrograph
Herpes simplex virusNucleocapsid cryoEM model
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Helical
Rabies virus (rhabdovirus)
Simple viruses with small genomes use this architecture to provide protection for the genome without the need to encode multiple capsid proteins.
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Helical
All animal viruses with helical symmetry are ENVELOPED
paramyxovirus07/04/23 MASDIANA PADAGA
Helical enveloped viruses
Influneza A virusElectron
micrograph
ParamyxovirusElectron
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3 – Virus Envelope Envelopes are LIPID BILAYERS acquired from cellular membranes e.g. endoplasmic reticulum, nuclear membrane, plasma membrane
viral proteins are associated with/inserted into membrane – surface proteins often glycosylated
Adsorption and entry of virus into cells (and exit) -access to target cells-binding to receptors-fusion of envelope with cellular membranes to release genomeInteraction with immune system components- binding of antibody - Targets of immune system
Complex Virus StructuresMost animal viruses fall into three structural classes,
helical capsid (enveloped)icosahedral capsid (nonenveloped)or icosahedral capsid (enveloped)
However, more complex structures do exist e.g. pox viruses
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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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Stability of Viruses
•Non enveloped viruses more ‘hardy’ than enveloped viruses (e.g. foot and mouth disease hardier than influenza virus)
•Different viruses have differential ability to survive
• sensitive to temperature, pH, dessication, lipid solvents, detergents
Most inactivated at >55-60oCDetergents used to disrupt viral envelopes Rotavirus survives pH of stomach
•Clinical sample collection / Diagnostics
Properties of enveloped viruses
• Envelope is sensitive to– Drying– Heat– Detergents– Acid
• Consequences– Must stay wet during
transmission– Transmission in large
droplets and secretions– Cannot survive in the
gastrointestinal tract– Do not need to kill
cells in order to spread– May require both a
humoral and a cellular immune response
Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical Microbiology, 5th edition, Elsevier Mosby, Philadelphia,
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Properties of naked capsid viruses
• Capsid is resistant to– Drying– Heat– Detergents– Acids– Proteases
• Consequences– Can survive in the
gastrointestinal tract– Retain infectivity on drying– Survive well on
environmental surfaces– Spread easily via fomites– Must kill host cells for
release of mature virus particles
– Humoral antibody response may be sufficient to neutralize infection
Adapted from Murray, P.R. Rosenthal K.S., Pfaller, M.A. (2005) Medical Microbiology, 5th edition, Elsevier Mosby, Philadelphia, PA , Box 6-407/04/23 26MASDIANA PADAGA
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BASIC STEPS IN VIRAL LIFE CYCLE
• ADSORPTION• PENETRATION (injection) of viral DNA or
RNA• UNCOATING AND ECLIPSE• SYNTHESIS OF VIRAL NUCLEIC ACID AND
PROTEIN (REPLICATION)• ASSEMBLY (maturation) of the new viruses• RELEASE of the new viruses into the
environment (cell lyses)07/04/23 MASDIANA PADAGA
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ADSORPTION
• TEMPERATURE INDEPENDENT
• REQUIRES VIRAL ATTACHMENT PROTEIN
• CELLULAR RECEPTORS
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PENETRATION enveloped virus
•FUSION WITH PLASMA MEMBRANE •ENTRY VIA ENDOSOMES
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PENETRATION - ENVELOPED VIRUSES
from Schaechter et al, Mechanisms of Microbial Disease, 3rd ed, 199807/04/23 MASDIANA PADAGA
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PENETRATIONNON-ENVELOPED VIRUSES
entry directly across plasma membrane:
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UNCOATING
• NEED TO MAKE GENOME AVAILABLE
• ONCE UNCOATING OCCURS, ENTER ECLIPSE PHASE
• ECLIPSE PHASE LASTS UNTIL FIRST NEW VIRUS PARTICLE FORMED
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SYNTHESIS OF VIRAL NUCLEIC ACID AND
PROTEIN
• MANY STRATEGIES
• NUCLEIC ACID MAY BE MADE IN NUCLEUS OR CYTOPLASM
• PROTEIN SYNTHESIS IS ALWAYS IN THE CYTOPLASM
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ASSEMBLY AND MATURATION
• NUCLEUS• CYTOPLASM• AT MEMBRANE
RELEASE• LYSIS• BUDDING
THROUGH PLASMA MEMBRANE
• NOT EVERY RELEASED VIRION IS INFECTIOUS
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epithelial cells - adenovirus
uninfected early infection
late infectionslides from CDC
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Bacteriophages
• Bacteriophages ( phages ) are obligate intracellular parasites that multiply inside bacteria by making use of some or all of the host biosynthetic machinery . They are viruses that infect bacteria.
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Composition of Bacteriophage
• Nucleic acid: either DNA or RNA but not both– ds DNA, ss RNA, ss DNA – unusual or modified bases– encode 3-5 gene products to over 100 gene
products
• Protein: function in infection and protect the nucleic acid
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Tail
Tail Fibers
Base Plate
Head
Contractile Sheath
Capsid
DNA
Structure of Bacteriophage
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PrionsPrions•Prions are “infectious proteins”• They are normal body proteins that get converted into an alternate configuration by contact with other prion proteins• They have no DNA or RNA•The main protein involved in human and mammalian prion diseases is called “PrP”07/04/23 MASDIANA PADAGA
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Prion Prion DiseasesDiseases
•Prions form Prions form insoluble insoluble deposits in the braindeposits in the brain•Causes neurons to Causes neurons to rapidly degeneration.rapidly degeneration.•Mad cow disease Mad cow disease (bovine spongiform (bovine spongiform encephalitis: BSE) is an encephalitis: BSE) is an exampleexample•People in New Guinea People in New Guinea used to suffer from used to suffer from kurukuru, which they got , which they got from eating the brains from eating the brains of their enemiesof their enemies07/04/23 MASDIANA PADAGA