Introduction to viruses Flashcards
What is the basic virus structure?
1 type of nucleic acid (either DNA or RNA)
Capsid core
Polymerase protein
May/may not be enveloped
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What are the main sources of exposure?
source:
Social
Food/Water
Environmental
Occupational
Nosocomial
Sexual
IVDU
Travel
Animal
arthropod-borne
Congenital
perinatal
What are the main routed of entry?
Oral
Respiratory
Percutaneous - skin/mucous membrane
Host can develop Short-lived infection
or
persistent carriage
What are viruses?
Obligate intracellular parasites that are metobollically inert
What does the genetic material of a virus encode?
Viruses contain either DNA or RNA genomes comprised of novel genetic material encoding structural and functional proteins needed for self perpetuation.
New virus particles are formed by assembly of components synthesised using host cell proteins.
What characteristics are used to classify viruses into families?
-Type of nucleic acid (DNA or RNA)
-Symmetry of nucleocapsid
-Lipid envelope (presence or absence)
-No. of strands of nucleic acid (ss/ds) & their physical
-construction (e.g. segmented)
-Polarity of viral genome (e.g. +ve or -ve strand DNA)
Virus Families infecting humans
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What are capsids constructed from?
A small number of virally-encoded protein subunits called capsomeres. So capsids are made from capsomeres.
The viral genome enclosed by the capsid protein coat is called nucleocapsid (core)
What 3 different types of symmetry can virus particles show?
-Icosahedral (e.g. adeno, herpes)
-Helical (e.g. orthomyxo, paramyxo)
-Complex (e.g. poxviridae)
Describe the structure of an icosahedral capsid.
-Capsid composed of 20 solid equilateral triangles arranged around the face of a sphere
-Structural units may be composed of more than one viral protein
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what is the minimum number of subunits in a icosahedral capsid
The simplest icosahedral capsid is made by 3 identical subunits to form each face, so the minimum number of subunits in an icosahedral capsid is 60 (20x3)
Describe the structure of a helical capsid.
-Capsomeres are wrapped around the RNA in a periodic fashion, thus winding the genome into a helix.
-Only occurs in RNA viruses
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Describe the structure of a complex capsid.
- Neither purely helical or icosahedral
-Only poxviruses have this symmetry
-Nucleocapsid is a continuous cylinder surrounded by lipid layer and and complex proteinaceous core wall (pallisade layer).
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What is the importance of viral surface proteins?
-They attach to membrane proteins (receptors) in the host cell- a determinant for tropism
-They’re also targets for antibodies- neutralisation
-They’re also determinants of antibody specificity (serotype)
Describe the structure of a viral envelope.
-Not present in all viruses
-Lipid bilayer is derived from the host cell membrane
-Contains viral glycoproteins (spikes) that project from the membrane
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What does the presence or absence of the envelope determine?
Ability to survive outside of the host cell & also its mode of transmission:
Non-enveloped “naked” viruses are stable in the environment; may be transmitted by food or water.
Enveloped viruses often survive only transiently outside host, and do not persist in the environment- spread by close or intimate contact
give examples of non and enveloped viruses?
non-enveloped virus:
e.g. picornaviruses (polio; hepatitis A), noroviruses
enveloped virus:
e.g. Influenza, HIV, Hepatitis B virus
what are steps of viral replication
- attachment and entry
- uncoating
3-5. Production of viral proteins and replication of viral Genome - assembly
- release
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What is viral protein synthesis completely dependent on?
Cellular translation machinery (ribosomes)
thus all viral genomes must produce mRNA to express the viral proteins
mRNA is needed for proteins
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How is viral mRNA produced?
By transcribing the genome with host or viral RNA polymerase.
-The route to produce viral mRNA depends on genome structure of each virus
How does gene expression & replication work in viruses with different genomes?
DNA genomes:
Large viruses (pox-, herpes-) encode many of the enzymes they need (e.g. DNA-dependent RNA polymerase) and are more “autonomous”.
Small viruses (e.g., papilloma-) use host cell enzymes.
RNA genomes:
Most examples encode their own RNA-dependent RNA polymerase, which uses a complementary RNA as template (+ve –ve or –ve +ve )
Retroviruses (and hepadna-):
Use reverse transcriptase to copy a +ve ssRNA genome into dsDNA, which is the template for protein and new genome synthesis
What cell functions are required for viral propagation (replication)?
-Machinery for translation of viral mRNAs
-Enzymes for replication of genome & assembly of new virions
-Transport pathways to reach the sites of replication, viral assembly, etc.
-Energy source
What are some examples of DNA viruses?
- parvoviridae - parvo virus B19
- papovaviridae - papilloma viruses
- hepadnaviridae - hepatitis B virus
- adenoviridae - usually find resp tract, eye
- herpesviridae - herpes simplex virus varicella-zoster virus,
- cytomegalovirusm, epstein-barr virus
- poxviridae - smallpox virus
Classify the DNA viruses.
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What is Hepatitis B virus?
A blood borne virus (from Hepadnavirus family) that can cause acute hepatitis.
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what does Hepatitis B virus do
virus that causes inflammation of the liver; transmitted through any body fluid, including vaginal secretions, semen, and blood
What are some examples of RNA viruses?
- Picornaviridae - e.g. rhinoviruses, polio
- Orthomyxoviridae - e.g. Influenza A, B
- Paramyxoviridae - e.g. mumps and measles
- Retroviruses - e.g. HIV 1 & 2
Classify the RNA viruses.
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How do you diagnose viral infections?
- Detection of virus or viral components:
-via PCR which comes under Nucleic acid amplification technology (NAT)
-Antigen detection
-Culture
-Electron microscopy - Detection of virus-specific antibody responses by serological technique
What is the Serological technique?
Blood from the patient is mixed with serum containing known antibodies to determine which antigens were present. This was the traditional method of HLA typing.
What is viral tropism?
Efficient virus infection of host cells depends on specificity interaction between proteins on the surface of the virus and “cognate” molecules expressed on the cell surface.
-The ability of a virus to infect some tissues and not others
-This involves a specific cell surface receptor on the host cell
The need for a specific cell receptor narrows the species and the type of cells the virus can enter
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How do non-enveloped viruses mediate attachment?
Via external capsomere proteins
what receptor in the host cell does the influenzavirus bind to?
Hemagglutinin in the influenza virus binds to sialic acid in the host cell.
what receptor in the host cell does the ebola virus bind to?
glycoprotein in the ebola virus binds to TIM-1 and NPC1 receptors in the host cell.
what receptor in the host cell does the rhinovirus bind to?
the caspid protein binds to ICAM-1
what receptor in the host cell does the HIV bind to?
envelope Gp120 protein binds to CD4 and co recptioes CXCR4/CCR5 on T-cells
What happens in acute viral infections?
- Virus infects host
- Establishes a focus of infection (at site of infection or inoculation)
- Overcomes host defences
- Spreads from initial focus
- Further replication
- Exit from host
Site of acute viral infection can be superficial or systemic
What is the difference between superficial & systemic infection?
Superficial infection: replication in epithelium at portal of entry, short incubation period (2-4 days), acute infection or short duration
Systemic infection: complicated traffic of virus during long incubation period (~ 2 weeks), replication of virus at multiple sites, natural infection → lifelong immunity
What are the possible outcomes after viral infection of cell?
- Clearance following acute phase of cell death
- Persistent infection with continued production of infectious viruses & immune evasion
- Latent infection with viral persistence after initial clearance, & asymptomatic or symptomatic reactivation
What is a latent virus infection?
- Viral DNA persists but doesn’t replicate to produce new infectious virus
-May never cause signs of disease
-May activate on one or more occasions and be asymptomatic or cause disease
-Some latent infections may lead to malignant disease.
What is persistent infection and give an example?
Continued production of infectious virus and immune evasion e.g hepatitis B and HIV
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