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M101 > W21 Virus structure, viral replication cycle and prions > Flashcards

W21 Virus structure, viral replication cycle and prions Flashcards

1
Q

Virus general characteristics:

A
  • NOT CELLS – Acellular Infectious particles
  • After prions, viruses are the smallest infectious agents (10 to 400 nm in diameter)
    -Different sizes and shapes
  • Do not grow or undergo division outside a cell
  • Obligate intracellular parasites - Requires a host for replication
    (They need the machinery and metabolism of a host cell)
  • Are produced by replication from the assembly of pre-formed viral components
  • Carry genetic material either DNA or RNA (not both)
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2
Q

What are the misconceptions about viruses?

A

Viruses are NOT bacteria
* Antibiotics are not effective against viruses
* Specific virus infections should be treated with antiviral agents

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3
Q

How can viruses be seen?

A

Virus particles can only be seen through electron microscopy (not by a light microscope)

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4
Q

Virus nomenclature/taxonomy: (for info)

A
  • Classification:
  • Family names end in –viridae
  • Genus and species names end in -virus.
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5
Q

Virus composition:
What is a virion?
What does it comprise of? (3)

A

= Complete, fully developed viral particle composed of:
-Nucleic acid
-Capsid
-Envelope

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6
Q

Virion composition:
What is a nucleic acid?

A

-Nucleic acid: DNA or RNA can be single or double-stranded; complexed with matrix proteins or enzymes

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7
Q

Virion composition: What is a capsid?

A

Protein coat/shell to protect the nucleic acid

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8
Q

Virion composition: What is the envelope?

A

-Envelope—lipid bilayer membrane (Host cell derived) on some viruses, in which some peripherical glycoproteins (e.g. Spikes) projects from outer surface

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9
Q

Virus particle/virion composition:

A
  1. Covering:
    -Capsid structure
    -Envelope (not always present)
  2. Central core:
    -Nucleic acids (DNA/RNA)
    -Matrix proteins/ enzymes (depends on virus)
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10
Q

Presence/absence envelope

A

Naked or enveloped viruses
* External layer of a virion
* Not always present
-Not present in rhinoviruses, adenoviruses, etc. (naked viruses)
-SARS-CoV-2, HIV, herpesviruses, influenza viruses possess it (enveloped)

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11
Q

What is a Viral envelope?
Are enveloped viruses strong?

A
  • External layer of a virion
  • Envelope is a lipid bilayer derived from host cell
    (infection’s final step)
    -Viral genome does not encode lipid synthetic machinery
  • Envelope acquired by budding of capsid when
    viruses leaves the cell

Enveloped viruses are more susceptible to
chemical disinfectants

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12
Q

Features of Envelope glycoproteins :
What are they?
Where are they synthesised?
What are they involved in?

A
  • Transmembrane glycoproteins (encoded by the viral genome) that protrudes outside the envelope
    -Synthesised through the secretory pathway – destined to the plasma membrane
  • Involved in binding specific protein receptors on the external surface of the host’s cell (viral absorption, the first step)
  • Sometimes involved in viral-cell fusion
  • In some viruses, those external proteins have other names e.g. hemagglutinin and neuraminidase in influenza viruses
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13
Q

Viral capsid features:

A
  • Large and protective shell that surrounds/protects viral nucleic acid –
  • Also called nucleocapsid
  • Composed of many capsomers (identical subunits - like the pieces of a puzzle)
  • Each one has ‘identical’ and reversible bonding contacts with its neighbours
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14
Q

What are the shapes of different capsids?

A
  1. Helical- hollow, cylindrical
  2. Polyhedral- Many-sided
  3. Complex- complicated structures
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15
Q

What are Viral genomes?

A
  • Viruses contain either DNA or RNA
  • Can be double-stranded (ds) or single-stranded (ss), linear or circular
  • Low size genomes - Hepatitis B (4 genes), herpesviruses (100 genes)
  • No viral metabolic genes, as the virus uses the host’s metabolic resources
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16
Q

Viral genomes possess only the genes to encode proteins involved in:

A

 The structure of the virion (capside protein, envelope glycoproteins)
 Invading the host cells and hijacking/regulating its metabolic activity to
produce more viral proteins and quench the production of cellular proteins
 Enzymes to reproduce/multiply their genomes (e.g. DNA/RNA polymerases)

17
Q

Spectrum of a viral infection:

How do viruses bind to receptors?
What are bacteriophages?

A
  • The spectrum of host cells a virus can infect is usually narrow
  • Most viruses infect only (or a few) specific types of
    cells in one host (defined as viral tropism)
  • Mainly determined by the presence of specific
    receptors on the surface
  • Also, cells should have cellular factors capable
    of replicating virus
  • Different viruses can bind the same receptor
  • Viruses of the same family may bind different receptors

Bacteriophages or phages - viruses that infect only bacteria

18
Q

Viral replication cycle (generic)

A
  • Human viruses undergo a series of events to enable the replication (multiplication) inside a host’s cell
  • Steps change from virus to virus
  • Some viruses have additional steps
    (e.g. HIV – genome integration)
19
Q

What are the 6 viral replication steps? (brief)

A
  1. ATTACHMENT- A generic virus becomes attached to a target epithelial cell
  2. PENETRATION- The cell engulfs the virus by endocytosis
  3. UNCOATING
20
Q

What occurs in Viral attachment?

A
  • Virus adheres to cell surface
     (random collisions & electrostatics)
     No specificity required
  • Attachment to specific receptors
    on cell surface by envelope
    glycoprotein or protruding capsid
    protein (naked viruses)
21
Q

What occurs in STEP 2 – Virus penetration – Membranes fusion? (2 steps)

A
  • After attachment, viruses need to penetrate the cell (2 ways)
  1. Enveloped viruses: by fusion of viral envolope and host cell membrane
    -mediated by viral fusion proteins (on the envelope)
  2. Naked viruses (e.g. Adenovirus, Adv): enter cells
    by endocytosis
     Receptor binding triggers the process of endocytosis
     Cell membrane is pinched off
     The virus is engulfed
     A vesicle is formed (Endocytosis)
22
Q

What occurs in STEP 3 – Virion uncoating - Genome release?

A
  • Virion uncoating: stepwise process of disassembly of the virion to release the genetic material in the nucleus of the cell
23
Q

What occurs in STEP 4: Virus genome replication and protein synthesis?

A
  • Viruses hijack the host cell synthesis machinery to:
  • Replicate their genome
  • Produce their proteins (Transcription of viral genes into mRNA and protein translation)
  • This step depends on the type of nucleic acid carried by the virus
24
Q

What occurs in STEP 5 – Assembly of NEW virions?

A
  • New genome copies and many capsid subunits are synthesised
     Through the endomembranous system, envelope glycoproteins are synthesised
    by the cell and sent to the plasma membrane
  • This happens in proximity to the plasma membrane
  • Capsid subunits assembled together to form a capsid
  • The replicated viral genome and some viral proteins become packaged within
  • From a single infected cell – 104 / 106 new virions are produced
25
Q

What occurs in STEP 6 – Release of new virions?

A

Release. Completed viral particles exit the cell and can infect other cells.
* Mechanisms of virion release differ between naked and enveloped viruses
a) Budding / exocytosis – enveloped viruses - new
capsid particles bind the membrane, the membrane
pinches off and viruses acquire the envelope and
there are released. Cells are not destroyed

b) Lysis – Naked viruses - Viruses are released only when the cell dies

26
Q

What are Prions?

A
  • Proteinaceous infectious particles – misfolded proteins
  • Prions do not contain nucleic acids
  • Cause transmissible spongiform encephalophaties fatal neurodegenerative diseases
    -Mad cow disease“ (BSE)
    -Creutzfeldt-Jakob disease (CJD)
    -Sheep scrapie

Prions
* Inherited and transmissible by ingestion
(common in animals), transplant, and surgical instruments

  • Extremely resistant to standard sterilization
    techniques:
    -Such as standard autoclave programs of 121o C
    for 15 minutes (more than bacterial endospores)
  • Prion inactivation requires autoclave cycle at
    134°C for 18 minutes or more

M101 (41 decks)

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