Pellett: Virology - Molecular Aspects Flashcards

1
Q

Virus

Definition

Genome consists of:

Metabolism on their own:
In host cell:

A

Infectious, obligate intracellular parasite

Genome consists of DNA or RNA

Metabolically inert on their own (outside the context of a living cell)

Viral genome directs its own replication using cellular systems in a host cell

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2
Q

What are virions?

What is a progeny virion?

A

Virus particles (virions) are formed by assembly from newly synthesized components within the host cell

A progeny virion is the vehicle of transmission of the genome to the next host cell/organism (disassembly initiates the beginning of the next infectious cycle)
- This disassembly stage is a vulnerable state for the virus

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3
Q

What is a minimal virus?

A

Minimal Virus: genome with origin of replication and a coat

o DNA or RNA genome
o Proteinaceous coat: also called a capsid

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4
Q

Nucleocapsid:

Envelope:

Virion:

Translation Parasites:

A

Nucleocapsid: genome + capsid

Envelope: viruses can be enveloped or not

Virion: a complete virus particle with its DNA or RNA core and protein coat as it exists outside the cell; also called a viral particle

Translation Parasites: must use cellular ribosomes in a host in order to replicate

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5
Q

How do we classify viruses?

A

Nature of genetic material
Capsid Structure
Enveloped or non-enveloped (naked)

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6
Q

Classification
Nature of Genetic Material:

Types of RNA viruses (3):

Types of DNA viruses:

A

RNA:

  1. single- and double-stranded
  2. positive or negative sense
  3. non-segmented or segmented

DNA:
ss, ds, linear, circular

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7
Q

Positive or negative sense (Baltimore system)

What must all viruses adopt at some point in their replication cycle?

What does it require?

Possible scenarios:
Retrovirus:

A

At some point in their replication cycle, all viruses must adopt a form that allows transcription of mRNA that can be translated by cellular ribosomes

Requires –ssRNA or dsDNA/dsRNA in order to translate into +mRNA

Possible Scenarios:
o +ssRNA Virus –> –ssRNA –> +mRNA
o +ssRNA Virus –> –ssDNA –> dsDNA –> +mRNA (Retrovirus)

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8
Q

Capsid Structure (3):

A

o Helical
o Icosahedral
o Complex

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9
Q

Other methods of virus classification:

A

o Gene organization
o Mode of replication
o Susceptibility to physical and chemical agents (for example, chloroform makes enveloped viruses non infectious)
o Cell tropism (what cells and tissues they infect)
o Pathogenic effects

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10
Q

Examples of RNA and DNA viruses:

A

RNA Virus Examples: polio virus, influenza virus

DNA Virus Examples: adenovirus, herpesvirus

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11
Q

Virus vs bacteria size

A

In general: most viruses smaller than most bacteria

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12
Q

How can you distinguish herpesviruses?

A

Individual virus species may share many properties with their relatives, but yet still have readily distinguished genomes:

Example of differences between herpesviruses shown via Southern blot

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13
Q

Picornaviruses:

Size:
Enveloped?
DNA:
Segmented?
What does the genome encode?
A

Small

Non-enveloped

+RNA genome (needs to be transcribed to negative sense)

Non-segmented: genome encodes single open reading frame that is translated into a large polypeptide and then proteolytically processed to a set of individually functioning proteins

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14
Q

Influenza Viruses:

Enveloped?
DNA:
Segmented?
What does the genome encode?

A

Enveloped

-RNA genome

Segmented: each gene encoded on individual “chromosomes”;
Genome segmentation is the basis for re-assortment (new strains of virus emerge from infections with 2 different strains)

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15
Q

Adenovirus:

Enveloped?
Size:
DNA:
Transcribed by:

A

Non-enveloped

Medium sized dsDNA genome

Transcribed by cellular RNA polymerase II (studying this virus led to discovery of mRNA splicing)

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16
Q

Herpesvirus:

Tegument:
Important in helping:

Number of genes:

A

Enveloped

Tegument: layers of proteins between envelope and capsid

Important in helping the virus control the cell following infection

dsDNA genome (may encode for as many as 200 genes; tightly controlled expression)

17
Q

The interaction between the virus and the cell (5)

A
  1. Get into the cell
    - receptors and co-receptors
    - what tissues
  2. Deal with host defenses
    - innate immunity
    - acquired immunity
  3. Control cellular processes
    - macromolecular synthesis and stability
    - transport
    - cell cycle
  4. Replicate
  5. Get out
18
Q

Properties of lytic infections

Lytic Infection definition:

Eclipse period:

Burst Size:

Time Scale:

A

Lytic Infection: making new infectious virions; can be initiated by a single virion

Eclipse Period: interval between disassembly of input virion and production of new infectious progeny

Burst Size: number of infectious virions released per infected cell (100s-1000s)

Time Scale: can be hours or days from time of infection to lytic burst

19
Q

Steps in lytic infection

A
Entry
Gene Expression
Genome replication
Assembly/packaging
Egress/Release
20
Q

Lytic Replication

Entry:
Mechanisms with examples:

A

Entry: attachment to receptor, penetration and uncoating

Mechanisms:

  1. Endocytosis (poliovirus, influenza virus, adenovirus)
  2. Genome injection (poliovirus)
  3. Fusion at cell membrane (HIV, vaccinia virus, herpesvirus)
21
Q

Lytic Replication

Gene expression:

A

mRNA synthesis

Protein translation

22
Q

Lytic Replication

Sites of geneome replication:
Examples:

A

Cytoplasm: most RNA viruses + poxviruses (DNA)

Nucleus: most DNA viruses + orthomyxoviruses (RNA)

Both: retroviruses, hepadnaviruses

23
Q

Lytic Replication

Egress/release
Mechanisms (4):
Examples:

A

Cell lysis (poliovirus, adenovirus)

Budding from cytoplasmic membrane (influenza, HIV)

Cell associated (vaccinia virus)

Vesicle fusion (herpes virus)

24
Q

Consequences of Infection:

Transformation (4):

A

Immortalization

Serum independent cell replication

Loss of contact inhibition

Ability to form tumors

25
Q

Consequences of Infection:

Cytopathic Effect (CPE) (5):

A

Rounding (lose defined shaped)

Fusion (syncytia formation- giant infected cell)

Intracellular vacuoles and inclusion bodies

Lysis

Cell death

26
Q

Examples of cytopathic effects in cell culture

A

Herpes simplex virus - Plaque formation

CMV

RSV - Fusion

27
Q

Polio replication

A

Positive-strand RNA genome encoding a single polypeptide that is processed to functioning proteins.

28
Q

Herpesvirus replication

Type of genome:
Encodes:
# of genes required for lytic replication:

A

Large double-stranded DNA genome encoding ~80 to ~200 individually transcribed genes that are expressed as one of three major kinetic classes.

Only ~50 of the genes are required for lytic replication in cell culture.