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PID > Virus structure and classification > Flashcards

Virus structure and classification Flashcards

1
Q

What is a virus?

A

Obligate intracellular parasite
* Needs a host cell to survive
* Lacks organelles
* Extremely small – filterable agents
− Range from 18 – 230 nm
* Need electron microscope to visualise

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2
Q

What is virus nomenclature?

A

Example:
Family name ends in -viridae
Genus name ends in -virus
Rhabdoviridae
Lyssavirus
Rabies Lyssavirus

Virus species is a group of viruses sharing the same genetic information and ecological niche
Can be further divided into types and
subtypes

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3
Q

What is an enveloped virus?

A

Viruses can be unenveloped (naked) or enveloped.
Host derived lipid bilayer
Virus encoded glycoproteins; often form spikes that protrude from the virus surface

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4
Q

How are viruses classified?

A
  • Nature of the viral genome– DNA or RNA; Polarity of nucleic acid– Structure of nucleic acid (ss or ds); linear or circular;
    segmentation
  • Structure and symmetry of the viral nucleocapsid– Icosahedral; Helical; Complex
  • Presence or absence of an envelope
  • Size and Morphology
  • Genome organisation and different coding
    strategies
  • Tissue and cell tropism
  • Varying pathogenicity
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5
Q

DNA vs RNA viruses

A

Can be ss or ds DNA, or ss or ds RNA.

Can be positive sense ss RNA, which is the same as mRNA, translated directly to protein.

Or negative sense ss RNA, where +ve sense RNA needs to be synthesised before viral proteins can be translated.

Some ds and ss RNA can be segmented, and some ds DNA can be circular.

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6
Q

What are characteristics of viruses with a DNA genome?

A
  • All monopartite (all viral genes on a single segment)
  • Mostly ds (except parvo and circo)
  • Few are circular
  • Many linear DNA virus have characteristics that enable them to
    adopt a circular configuration
  • Little diversity in structure
  • More stable, show little variation
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7
Q

What are characteristics of RNA viruses?

A
  • Mostly single stranded (except Reoviridae and Birnaviridae)
  • All have linear genomes
  • Can have more than one segment
  • Segments of Reoviridae and Orthomyxoviridae code for single genes
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8
Q

Why are RNA viruses so variable?

A
  • Need a 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
  • Consequence of this
    − RNA viruses are more variable
  • Within a species of virus are more subtypes/serotypes
    − Can evolve rapidly if need
  • If a virus jumps from one species to another, RNA viruses can more readily adapt
  • Often zoonotic (jump from animals to humans)
  • Segmentation of RNA viruses
    − Allows virus to increase its diversity very rapidly (reassortment)
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9
Q

What is the structure of the virus capsid?

A

The capsid encloses the nucleic acid. The 3 capsid types are icosahedral, helical and complex.

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10
Q

What is an icosahedral capsid?

A

Has 12 vertices and 20 triangular sides (facets). Composed of capsomers, penton or hexon capsomers.

Examples: parvoviridae or adenoviridae

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11
Q

Penton vs hexon capsomers

A

Penton capsomers:
* Found at the vertex of the capsid
* 12 present – one at each vertex
* Has 5 neighbouring capsomers

Hexon capsomers:
as 6 neighbouring capsomers–
Number present in capsid varies
Found in spaces between the
penton capsomers

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12
Q

What is an helical capsid?

A
  • Capsid protein are arranged in a “spiral”
    configuration around a single axis
  • Structural unit is one capsid protein
  • Single capsid proteins are arranged as a
    helix around the genome

All animal viruses with helical symmetry are enveloped

Examples: Paramyxoviridae (measles, Nipah), Rhabdoviridae (rabies)

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13
Q

What is a complex capsid?

A
  • Some of the large viruses have capsid structures that are more
    complex
  • Example: Poxviridae
  • > 100 proteins
  • Neither helical or icosahedral structure
  • Enveloped, brick-shaped or
    ovoid virion,
  • 220-450nm long and 140
    260nm wide.
  • Surface membrane displays
    surface tubules or surface
    filaments.
    Examples: Smallpox, Mouse pox
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14
Q

Which types of virus have an envelope?

A

−Few viruses with icosahedral capsid
−All viruses with helical capsid
−Complex capsid

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15
Q

Enveloped vs unenveloped viruses

A

Enveloped:
* Acquire envelope as
they bud through the host cell
membrane
* Viral envelope contains host cell lipid
bilayer as well as viral proteins
* Viral proteins contain receptors
needed for virus entry

Naked:
* Viruses are released by
lysis of the infected cell
* Viral receptors are present on
the capsid surface
* Can be transmitted as virus clusters inside vesicles, such as Rotavirus and Norovirus; vesicles remain intact and pass through GI tract to intestines

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16
Q

What are biological properties of enveloped viruses?

A

*Components are lipids, proteins and glycoproteins
* More fragile than viruses with just a capsid
* More easily destroyed by
−Detergents
−Disinfectants
−Outside environment
* If the envelope is destroyed, then the virus is not infectious
−Destroys the receptors needed for entry

17
Q

What are biological properties of unenveloped viruses?

A
  • Components are protein
  • More stable to temp, pH etc. can survive adverse conditions in the gut
  • Lyse cell to release so has to kill the cell, usually cause acute infections
18
Q

Structural vs non-structural proteins?

A

Structural proteins:
− Capsid proteins
− Envelope proteins
− Matrix protein (layer inside the envelope and outside capsid)
− Virion associated enzymes

Non structural proteins
− Proteins that are not structural components of the virus
− Often enzymes (but some enzymes can be structural)
− Also some viruses encode regulatory proteins, oncoproteins, etc.

19
Q

What is the function of the virus capsid?

A
  • Protect the viral nucleic acid and deliver the viral nucleic acid to the cell
  • Capsids of naked viruses contain receptors that attach to the host membrane to
    allow entry
  • Contains sites that will induce an antibody response
20
Q

What are virus envelope proteins?

A
  • Contain receptors that allows the virus to attach and then
    enter the host cell
  • Are targets of the humoral and cellular immune response
    −Antibodies will recognise these surface exposed viral proteins
  • Interact with the capsid during virus assembly
  • Often integral membrane proteins
21
Q

What are non structural viral proteins?

A
  • Are not structural components of the virus particle
  • Made in the virus-infected cell following infection:
  • Often enzymes involved in viral replication
    − Proteases, Helicases, Polymerase (which can be a structural protein!), protein primers for nucleic acid replication
  • Can be proteins that help the virus avoid the host immune response
  • Targets of the host cellular response (T cell epitopes)

PID (15 decks)

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