Introduction to Viruse Flashcards
What is the basic outline of the organism and host?
Exposure leads to infection which then leads to clinical symptoms.
Host response can make the symptoms worse.
Name some viral factors
Natural history/life cycle Mode and efficiency of transmission Tropism Invasivness Opportunism Virulence
Name some host factors
Age Genetic predisposition Immune status Specific immunity Trauma/surgery/foreign body
What are some sources of exposure to viruses?
Social Food/water Environmental Occupational Hospital Sexual Drugs Travel Animals Maternal
Ultimately the type of exposure is orally (mucosal membranes in mouth), respiratory or percutaneous (skin).
What is the basic viral structure?
DNA or RNA
Caspid core (protects genetic material)
Polymerase protein (enzyme packaged with the virus)
Envelope possibly
What are some distinguishing characteristics of viruses?
They are obligate intracellular parasites.
Viruses contain either DNA or RNA genomes compromised of genetic material encoding structural or functional proteins. Structural genes encode the capsids and the polymerase proteins or glycoproteins sticking out of enveloped proteins. Functional proteins (enzymes) - the smaller the genome, the fewer of these. Smaller genome viruses rely on host cell.
New versus are formed by assembly of components synthesised using host cell proteins. They enter a cell and release genetic material. This material undergoes transcription and then assembly of this protein to make capsins. Eventual release of virus from cell.
How do we characterise viruses into families?
- Type of nucleic acid (DNA or RNA)
- Symmetry of nucleocaspid
- Lipid envelope
- Number of strands of nucleic acid & their physical construction
- Polarity of viral genome (positive or single stranded RNA)
Give some details on the stature of the caspid
Caspids are constructed from a smaller number of virally-encoded protein subunits called capsomeres.
The viral genome enclosed by the caspid coat is called nucleocaspid.
Virus particles show 3 types of symmetry:
- Icosahedral
- Helical
- Complex
Explain the isosahderal caspid
A caspid composed of 20 solid equilateral triangles arranged around the face of the sphere.
The simplest icosahedral caspid is made by 3 identical subunits to form each face, so the minimum number of subunits is 60 (2x3).
The structural units may be composed of more than one viral protein.
What is the structure of the helical caspid?
Cashmeres are bound in a periodic fashion to the viral genome, thus winding the genome into a helix/
Only occurs in RNA viruses.
Explain the complex caspid structure
Neither purely helical or icosahedral.
Poxviruses are the only human viruses with this symmetry.
The nucelocaspid is a continuous cylinder surrounded by a lipid layer and complex proteinaceous core wall (pallisade layer).
What is the importance of viral surface proteins?
They attach to membrane proteins (Receptors) in host cell - determinants of tropism.
Target for antibodies - neutralisation.
Determinants of antibody specificity (Serotype)
What is the basic structure of a viral envelope?
Lipid bilayer derived from the host cell membrane.
Contains viral glycoproteins form the host cell membrane.
Not present in all viruses.
What does the presence or absence of the envelope determine?
ability to survive outside cell and the mode of transmission
non-enveloped viruses are able in the environment - may be transmitted by food or water
e.g. noroviruses
enveloped viruses do not perish long in the external environment - they are spread by close or intimate contact
e.g. influenza, HIV, Hep B
How do viruses replace and spread within host?
- Attachment of virus
- Entry of virus
- Uncoating of virus
- Production of viral proteins
- Replication of viral genome
- Assembly at cell surface membrane
- Release of virus
How does gene replication of a virus work?
Dependant on cellular translation machinery (ribosomes), thus all viral genomes must produce mRNA to express the viral proteins.
The route to produce viral mRNA depends on genome structure of each virus.
Ultimately the production of mRNA is made by transcribing the genome either using the host mechanisms or viral RNA polymerase.
Explain the DNA genomes and RNA genomes in gene expression
How are retroviruses different?
DNA genomes:
In large viruses encoding many enzymes needed.
(small viruses use host cell enzymes)
RNA genomes:
Encode their own RNA dependant RNA polymerase, which uses a complementary RNA as template.
Retroviruses are RNA viruses but instead…
They encode an enzyme called reverse transcriptase to make a positive RNA to make it into DNA.
Single sense RNA is used as a template to make double stranded DNA which then acts as the template for proteins. HIV is the best example of this.
What cell functions are required for viral propagation?
- Machinery for translation of viral mRNAs
- Enzymes for replication of genome and assembly of new virions
- Transport pathways to reach the sites of replication
- Energy source
Give the structure of the hep B virus
Give some epidemiological factors of it
Spherical virus, DNA virus, icosahedral shape, enveloped, only 42nm, double stranded
Blood Bourne disease, worldwide distribution, causes acute hepatitis, vaccine and treatment available.
What is the structure of the COVID 2 virus?
Its an enveloped virus with a helical capsid. Has a positive single stranded genome.
How do we diagnose viruses laboratory?
Detection of virus and viral components:
- Nucleic acid amplification technology
- Antigen detection
- Culture
- Electron microscopy
Detection of virus-specific antibody responses by serological technique. No IgG shows infection of vaccination from virus in past. IgM can be used to detect if someone has an acute infection.
What is viral tropism?
The need for a specific cell receptor narrows the species and the type of cells the virus can enter.
(between the proteins on the surface of virus and the cognate molecules expressed on cell surface)
How do non-enveloped viruses mediate attachment?
using external capsomere proteins
What happens in an acute viral infection?
- Infect the host
- Establish a focus of infection (at site of infection or inoculation)
- Overcome host defences
- Spread from initial focus
- Further replication
- Exit from the host (in sufficiently large numbers or infect other hosts)
What are the two sites of acute viral infection?
- Superficial
- Systemic
What are systemic and superficial infections?
Systemic = complicated traffic of virus during long incubation period. Replication of virus at multiple sites.
Natural infection - lifelong immunity
Superficial = replication in epithelium at portal of entry e.g. rhino virus, influenza .
Short incubation period, acute infection of short duration.
What is the outcome after viral infection of cell?
either …
Clearance following acute phase of cell death.
Persistence of infection with continued production of infection virus and immune evasion.
Latent infection with virus persistence after initial clearance and asymptomatic for symptomatic reactivation.
What are latent viral infections?
Viral DNA persists but does not 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.
Why are viruses obligate intracellular parasites?
They can only replicate by entering a cell.
Cannot survive outside cell for very long.
