Les virus Flashcards
When and how were viruses initially discovered?
Initially described as “des agents filtrables” by Ivanovsky in 1892
What are viruses?
An agent that can be filtered
An obligatory intracellular parasite —> dependant on the machinery of the host for replication
Cannot produce energy or proteins with a host cell
Viral genome made of DNA or RNA
Infectious agent —> composantes individuelles sont assemblées, donc qui ne se reproduit pas par division cellulaire
Fun facts about viruses: (6+3)
- Viruses are not living
- Viruses must infect other cells to stay alive
- Viruses must be able to use cellular machinery to produce its own components
- Viruses must encode for all necessary processes that are not “provided” by the host cell
- Viral components must be self-assembling
- Not all viruses can infect all cells or hosts
- They must be able to enter
- Once entered, the cell must have the machinery the virus needs to replicate
- Once replicated, the virus must be liberated from the cell to transmit the infection
Structure et composition de la particule virale (viron)
Measured in nanometers (nm)
Smallest (18nm —> parvovirus)
Biggest (300nm —> poxvirus)
Rule of thumb: bigger viron = larger genome —> can encode for more proteins
How is genetic information contained in a viron?
Genome (nucleic acid) covered by a protective layer of protein (capside)
Genome + capside = nucleocapside
(Icosahédrique, Hélicoïdale, Complexe)
All virons have a nucleocapside with or without an envelope
DNA vs RNA in virus:
DNA: single or double strand, linear or circular
RNA: single strand (positive polarity or negative),
- double stranded (reovirus) —> polarité+/–ou double sens
- Segmented or not
What are the capsid and enveloppe?
The outermost layer of a virus (either one or the other)
- Constitutes the structure, protection and transmission mechanism of the virus.
- The structures present on the surface of the capsid or envelope allow interaction between the virus and the host cell via a viral attachment protein (VAP) or other structure
If the outermost layer is destroyed —> the virus is inactivated
What is the capsid?
Made of proteins
Rigid structure capable of resisting unfavourable environmental conditions
Naked (non-enveloped) viruses are generally resistant to dryness, acidity, detergents, etc (including bile and acid in GI tract)
Symmetric (icosaédrique ou hélicoïdale) or asymétrique (complexe)
What is the capside isosahédrique?
Les sous sous-unités protéiques virales s’unissent en protomères. Cinq protomères s’unissent pour former des capsomères (pentamères) qui s’assemblent en capside
Made of 12 capsomeres called pentamères/pentons
Information:
- The Icosahedral capsid resembles a sphere and is made up of an assembly of protein subunits.
- All the faces of the simple icosahedron are identical.
- The nucleic acid is contained in the centre of the capsid which protects it from the environment.
- Larger capsid virions are made by inserting capsomers (hexons) between the pentons.
- Ex: Herpes (12 pentons, 150 hexons), Adenovirus (12 pentons, 240 hexons)
What is the capside hélidoïdale?
The protein subunits interact with each other and with the nucleic acid to form a structure that resembles a hollow, rigid or flexible protein rod or cylinder
What is the enveloppe?
The envelope is a membrane composed of lipids, proteins and glycoproteins.
- Structure similar to cellular membranes.
- Viral glycoproteins are anchored to the envelope and project out of the surface of the virion, like spikes.
- Some act as viral attachment proteins (VAPs), capable of binding to target cells.
- Are major antigens that can elicit protective immunity
- Easily deactivated by environmental factors (dryness, acidity, detergents, solvents) —> survive in GI tract
-
The structure of the envelope can only be maintained in an aqueous solution (must remain wet).
- These viruses are usually transmitted by fluids such as blood, respiratory droplets and tissue.
What are the two ways in which viruses can be classified?
-
Nature of their genome:
- DNA or RNA
- Single or double stranded
- Segmented or not segmented
- Linear or circular
-
Structure (morphology):
- Symmetry of the nucleocapsid (icosahédrique, hélicoïdale, complexe)
- Envelopped or not envelopped
- Number of capsomers
What is the international classification of DNA viruses?
What is the international classification of RNA viruses?
What is the convention for naming viruses?