Intro Virology and Replication Flashcards
1
Q
What is a Virus?
A
- Obligate intracellular parasite- needs host cell to survive (in order to replicate & complete its life cycle)
- Lacks organelles- no mitochondria/ energy source etc
- Extremely small - ‘filterable agents’- Need electron microscope to visualise
2
Q
Virion
(an entire virus particle)
A
- In its most simple form comprises of:
- Nucleic acid- RNA/DNA (confers infectivitity)
- A capsid (confers specificity)- structure & symmetry of this is important.
- Additional features may include:
- An envelope- Host derived lipid bilayer contains virus encoded glycoproteins
-Viral enzymes- incorporated into the envelope/ within the capsid
3
Q
DNA Viruses
A
- All monopartite (all viral genes on a single molecule)
- All ds (except parvoviridae and circoviridae)
4
Q
RNA Viruses
A
- Mostly single stranded & can be segmented
- Segmentation- allows virus to ↑ its diversity very rapidly (reassortment)
- Need an RNA polymerase to copy their RNA genome (no equivalent enzyme in the host)
- RNA dependent RNA polymerase
- RNA polymerases are error prone- No proof reading capability, as a consequence of this:
- RNA viruses are more variable:
- Within a species of virus are more subtypes/serotypes
- Often zoonotic (jump from animals to humans and cause disease)
- Can evolve rapidly if needed
- If a virus jumps from one species to another, RNA viruses can more readily adapt
5
Q
Capsids
(Structure)
A
Viral capsids: enclose the nucleic acid & have 3 forms of symmetry
1. Icosahedral-e.g.parvoviridae
- 12 vertices, 20 facets, 5:3:2 fold symmetry
- Composed of capsomers:
- *-Penton Capsomer**- 12 present, one at each vertex,has 5 capsid proteins
- *-Hexon Capsomer**- Composed of 6 capsid proteins
2. Helical Capsid-e.g.paramyxoviridae & rhabdoviridae
- Structural unit is one capsid protein- arranged as a helix
- All animal viruses with helical symmetry are enveloped
3. Complex Capsid-e.g.poxviruses
- Some of the large viruses have structures that are more complex
- >100 proteins -encode more proteins than other viruses
- Neither helical or icosahedral structure
6
Q
Enveloped VIrus
A
- Few viruses with icosahedral capsid
- All viruses with helical capsid
- Complex capsid
7
Q
Naked Virus
A
- Icosahedral Capsid
8
Q
Viral Envelope
A
Many acquire an envelope as they bud through the host cell membranes
- Host membrane = lipid bilayer- ‘coating ‘is effectively inert
- BUT would not permit recognition of receptor molecules on the host cell
- SO viruses modify the envelope by synthesis of viral encoded proteins which are associated with the envelope
9
Q
Biological Properties of Enveloped Viruses
A
- More pleomorphic (not a regular shape)
- More fragile than viruses with just a capsid
- More easily destroyed by: detergents, disinfectant & outside environment
10
Q
Naked Capsids
A
- Components: Protein
- Properties: Environmentally stable to:
- Temperature, pH, Porteases, Detergents, Drying, Released by cell lysis
- Consequences:
- Can be spread easily
- Can dry out and retain infectivity
- Can survive adverse conditions in the gut
- Resisitant to detergents
- Lyses cell to release; usually cause acute infections
11
Q
Enveloped Viruses
A
- Components: lipids, porteins, glyoproteins
- Prorperties: evironmentally labi
- destroyed by: acid, detergents, drying, heat, released by budding
- Consequences:
- Not easily spread (large droplets, secretions, transplants/transfusions)
- Must stay wet
- Cannot survive in the GIT
- Easilty destroyed by detergent
- Does not need to kill the cell to spread; can cause persistent infections
12
Q
Baltimore Classification System
A
classified by the mechanism of generating positive strand mRNAs
- Seven fundamentally different groups
13
Q
Hierarchical VIrus Classification System
A
classified according to characteristics:
- Presence or absence of viral envelope
- Capsid symmetry
- Size and shape
- Genome composition, polarity and structure
- Virus Taxonomy:
- Order (-virales)
- Example: family (-viridae)- Flaviridae, genus (- virus)- Pestivirus, species – Bovine viral diarrhoea virus 1
- Divided into genera containing species that further subdivide into serotypes and then subtypes
14
Q
Antigenicity
A
- Viruses can be differentiated on basis of antigenic sites on their surface = “serotypes”
- Viruses can be divided into “serotypes” and further sub grouped into subtypes
- Classified on the basis of their reactivity with antibody (serological reactivity)
15
Q
What viral proteins determine “serotype”
A
- Generally proteins on the virus surface that are involved in Virus entry & Antibody reactions
- In different serotypes, these proteins tend to vary in their precise amino acid composition–>immune system recognizes these proteins as slightly different
22
Q
Viral Attachment
A
- Viral surface proteins bind to receptor on cell surface
- Receptor is cellular protein that happens to fit viral protein
- Virus-receptor interaction determines specificity of viruses for cells and tissues (no receptor–> no entry)
23
Q
Viral Entry
A
Two possible routes:
- Endocytosis: virus is released from endosome by pH change or fusion of viral envelope with endosomal membrane
- Some enveloped viruses fuse directly with the plasma Membrane
28
Q
Viral Release
A
Three different mechanisms of release:
- Budding from plasma membrane
- Enveloped viruses need to acquire envelope by budding
- Occurs at plasma membrane or at internal membrane (nucleus/ E.R.)
- Exocytosis
- Lysis of cells due to toxic effect- Naked viruses are released by this method- can lead to some acute effects
32
Q
Cytopathic Effects
(CPE)
A
Visible changes to cells following virus infection
- Cells round up and detach from the flask –> holes appear in cell layer
- Syncytia formation: cells start to fuse creating very large cells with several nuclei, eventually cell death
- Inclusion bodies (masses of viral proteins within the cell)- within nucleus/ cytoplasm
34
Q
Infective Assays
A
- To quantify number of infectious virus particles-made in cell culture/ egg/ sample from infected animal
- 10 fold dilutions of sample/virus stock are prepared
- Inoculate cell cultures (several replicates of each dilution)
- TCID50 (50% Tissue Culture Infective Dose) –endpoint dilution assay
- Plaque Assays: Count plaques –calculate pfu/ml = virus titre
- Plaque forming units (pfu): using an agar overlay over infected plates, each virus forms a plaque (of lysed cells)
