What are viruses?
- Small, subcellular (smaller than cells) organisms with an obligate parasitic intracellular lifestyle (can’t survive outside cells for long)
- Composed of proteins, nucleic acids (DNA or RNA) and sometimes lipids (stealing cell membrane from host)
- From Latin; ‘poison/slime/vermin’
Are viruses alive?
- Cannot reproduce on their own (non-living criteria), but can do so in living cells and can also affect behaviour of host profoundly
- Plays a major role in shaping evolution, including humans (not just negative)
How large are viruses?
Small af:
- Viruses: 20-100 nm (Smallpox is big at 300nm)
- Bacteria: 1 - 10 μm (10,000nm = 10μm)
- Animal cells: 1 - 100 μm
Where are viruses found?
- Ubiquitous; everywhere (every living organism can host viruses; bacteria, fungi, animals, plants etc.), even other viruses (e.g. mamavirus infected with Sputnik virus)
How many viruses are humans on average infected with?
- At least two different viruses e.g. HSV-1/2, EBV, CMV
Can viruses be transmitted to future generations?
Yes, they can become part of the host genome and be transmitted to future generations
Are viruses always bad?
1) They play a key role in cycle of life in aquatic environments; maintaining equilibrium
2) Can be used to treat bacterial infections (bacteriophage therapy; confers no resistance)
3) 8% of human genome is of viral origin (retroviruses); shaped our evolution and made us into us (but the 8% has no activity)
WHat are the common features of viruses?
- Viral genomes (DNA or RNA) are packaged into particles (not stable on their own), necessary for transmission between hosts
- Viral genome contains information needed for replication within permissive host cell (one that allows viral replication)
- Viral survival esnured by establishing its genome in host cell population
- All viruses are obligate parasites; they NEED the cellular machinery (ribosomes, tRNA etc.) to replicate themselves
What is one way of classifying viruses? (Shapes)
Classified according to their structure and morphology, using the criteria:
- Absence OR presence of an envelope (enveloped vs. naked viruses; arising from hijacking cell membranes)
- Shape of the capsid; protein containing viral DNA or RNA, which is either helical or icosahedral
How many faces does an icosahedron have? Why is this shape preferred?
- 20 identical sides; nearest geometrical shape to sphere
- Spheres have the highest volume/surface ratio; can pack bare nucleic acids w/o too much protein for ‘packaging’
- Structure can be built from multiple repetitions of a single protein (only one gene required; gene economy, utilising space inside icosahedron capsid)
What are the rules for viral capsid self-assembly?
Capsids can self-assemble w/o need for additional proteins if:
1) Each subunit has identical bonding contacts w/its neighbouring proteins; usually achieved by symmetrical assemblies of oligomers (mainly pentamers/hexamers/dimers of a single protein)
2) Proteins are linked together by non-covalent bonds; allowing rapid assembly and disassembly of capsid to release nucleic acid
Why is the capsid needed?
1) Provides protection for nucleic acid outside the cell (BUT; needs to be released after entry to host cell too)
> Capsid made from multiple subunits non-covanlently linked (symmetry provides maximal contact between subunits)
> Capsid is metastable ‘spring-loaded’ during assembly; unfolds in cells like ‘bakugan’/Jack in the Box vibes, as conditions change (e.g pH/salt concentration)
2) Provides specific attachment to cell receptors (for naked viruses)
How does the enveloped/non-enveloped nature of viruses affect transmission?
Enveloped:
- Sensitive to: dryness, heat, detergents, acids (must stay wet to be transmitted); does not survive in GIT (thus not transmitted by food) e.g. HIV-1
Non-enveloped (much tougher/easier to transmit):
- Resistant to: dryness, heat, detergents, acids, proteases (can remain infective upon drying, transmitted on fomites, survive GIT, and in environment on surfaces) e.g. Adenovirus (cold), Rotavirus (stomach upset)
Define: capsid.
Protein shell containing the viral genome (RNA or DNA)
Define: nucleocapsid
Capsid containing nucleic acid
Define: capsomer
Individual protein molecules which together form the capsid (monomer of capsid)
Define: envelope
Lipid bilayer derived from host cells which surrounds the capsid
Define: tegument
Additional protein layer located between capsid and envelope in some viruses; same as matrix
Define: virion
A complete, mature infective viral particle outside the cell.
What are the distinct steps of the animal virus life cycle?
- Attachment (docking)
- Penetration
- Uncoating
- Transcription (or reverse transcription; generate copies)
- Biosynthesis
- Release & Maturation
How do viruses attach to host cells?
Bind to specific receptors on host cells:
- HIV-1 to CD4 and CXCR4 (co-receptor) on T-cells and macrophages
- Rhinovirus to ICAM-1 on nasal epithelium (Intracellular Adhesion Molecule 1)
- Influenza to sialic acid on respiratory epithelium (hence lung symptoms)
What is cell tropism?
Where the specific viral attachment to host cells results in a preference for specific cell types/tissues (as well as a specific host range)
What is host range? Give examples.
The ability of a virus to infect different species:
- Rabies; wide host range, can infect all mammals
- HIV; narrow host range, can only infect humans
What are zoonotic infections?
Diseases which normally affect animals but CAN infect humans; e.g. swine/bird flu, viral haemorrhagic fever.
