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5 > 3. Molecular Virology and Infection > Flashcards

3. Molecular Virology and Infection Flashcards

1
Q

What is a virus?

A
  • Infectious, obligate intracellular parasite
  • Not a living cell; does not grow or undergo division
  • Cannot make energy or proteins independent of a host cell
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2
Q

What is the difference between an extracellular and intracellular phase virus?

A

Intracellular:

  • called a virus
  • re-programming host cells, virus replication occurs, production of new virus components

Extracellular:

  • A virus particle (virion)
  • No biosynthetic function
  • Structure in which the virus genome is carried from the cell in which it has been produced to a new target cell
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3
Q

What is the structure of a virus particle?

A

Virus particles consist of:

  1. Nucleic acid (DNA or RNA)
  2. Capsid (protective shell made up of proteins)
  3. Sometimes a membranous (lipid) envelope- with viral membrane proteins
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4
Q

What is the origin of viruses?

A
  • Likely that mobile genetic elements provided the raw material for the construction of viruses
  • Maybe an unavoidable consequence of rapid gene evolution
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5
Q

How are viruses classified?

A
  • Historically by the properties of the cells/organisms they infect
  • Now classified by:
    1. Nature of nucleic acid
    2. Presence or absence of cell envelope
    3. Dimensions of virion and capsid
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6
Q

What are the general features of a viral reproductive cycle?

A
  1. Attachment:
    - Virus interacts with host cells via receptors
    - Enters host cell
    - Un-coating (release of viral genome)
  2. Replication and amplification of viral genome
  3. Viral gene expression:
    - Synthesis of viral mRNA
    - Synthesis of viral proteins
  4. Assembly of capsid and packaging of viral nucleic acid into virus particles
  5. Virus is released from host cells
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7
Q

What is a virus particle with helical symmetry?

A
  • Rod shaped coat of repeating units
  • Promoters association with nucleic acid in spiral or helical arrangement
    e. g. TMV
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8
Q

What is a virus particle with icosahedral symmetry?

A
  • A solid shape with 20 triangular sites
  • Most economicall way to build a symmetric shell of maximal internal volume with non-symmetric molecules
    e. g. Poliovirus
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9
Q

What is a virus particle envelope?

A
  • An outer structure that surrounds the icosahedral or helical capsid
  • Derived from host-derived lipid from cell nuclear or plasma membrane
  • Contains virus-encoded proteins or glycoproteins that project as spikes
    e. g. Papillomavirus, many RNA viruses
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10
Q

What does the term: virus of complex structure mean?

A
  • A virus that has a capsid and thus overall shape with a mix of shapes with no consistent symmetry
    e. g. Poxvirus, giant virus such as mimivirus, many bacteriophages
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11
Q

What is a primary cell culture?

A
  • Used to propegate viruses
  • Directly prepared from human/animal tissue, dissociated into single-cell suspension
  • consist of several cell types
  • have a limited life span (5-10 cell divisions)
  • used when state of cell differentiation is important
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12
Q

What is a continuous cell line?

A
  • Used to propagate viruses
  • Consists of a single cell type
  • Can be propagated indefinitely (immortal)
  • Derived from tumour tissue
  • Uniform population of cells
  • Different from the cell of origin
  • Often abnormal in chromosome morphology and number
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13
Q

What is the cytopathic effect?

A
  • Viruses can cause microscopic or macroscopic abnormalities in host cells and in tissue
    e. g. cell lysis, swelling of nuclei, formation of fused cells etc.
  • Evidence of viral growth
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14
Q

How are viruses detected?

A
  1. Measurement of infectious units:
    - Plaque assay
    - Transformation assay
    - Animal infectivity experiments
    - Note: only measures infectious viruses
  2. Measurement of viral particles and their components:
    - Electron microscopy
    - Hemagglutination
    - Measurement of viral enzyme activity
    - Serological methods e.g. ELISA
    - Note: measures all virus particles- both infectious and non-infectious
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15
Q

What are the phases of a one step growth curve for viruses? What is measured?

A
  1. Latent period: time in which viruses dock onto host cell, inject their DNA and produce enzymes, nucleic acids and protein coats
  2. Rise period: newly assembled virions are released out of bacterial host cells
  • Uses plaque-forming units to measure the growth of a phage in a population of bacteria
  • Burst size = number of infectious viral particles released per infected cell
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16
Q

How does a DNA virus replicate its genome?

A
  • Genome of DNA virus is transported into nucleus
  • Translation of early viral mRNA is required
  • Early gene products include:
    DNA polymerases
    Proteins that bind origin of replication
    Proteins that stimulate cell to enter S phase
17
Q

What is a Polymavirus?

A
  • A type of virus that causes very serious disease immunocomprimised people e.g. BK virus (causes liver transplant failure) and JC virus (causes destruction of CNS and death)
  • Virions are non-enveloped with icosahedral capsid
  • dsDNA viruses
  • Has a multifunctional protein called T antigen
  • T antigen binds to host cellular growth control products and causes the cell to enter S-phase
18
Q

What are Papillomaviruses?

A
  • A type of dsDNA virus
  • Non-enveloped
  • Icosahedral capsid
  • Cause warts or can cause oncogenic transformations and cancer if the papillomavirus DNA is integrated into the host cell DNA
  • Oncogenic Transformation due to expression of E6 and E7
  • Vaccine available
19
Q

How do DNA viruses replicate?

A
  • Genome is transported to nucleus (exception is poxvirus- has its own polymerases)
  • Translation of early viral mRNA required such as DNA polymerases, proteins that bind origin of replication and proteins that stimulate cell to enter S-phase
  • Some of these cause cancer by inhibiting the Rb gene which controls the cell cycle (SV virus by T antigen, HPV with E7)
20
Q

How do positive strand single stranded RNA viruses replicate?

A
  1. Viral ssRNA enters host cell
  2. This +ssRNA can be directly translated into proteins (used as mRNA)
  3. RNA dependent RNA polymerase is made by viral genes
  4. RNA dep. RNA pol. Uses +ssRNA as a template to made –ssRNA (complementary)
  5. –ssRNA is then used as a template to replicate the original +ssRNA
  6. The extra proteins are synthesised
  7. Virus is assembled
  8. Progeny virions are released
21
Q

How do negative strand single stranded RNA viruses replicate?

A
  1. Viral (-)ssRNA enters host cell
  2. This viral (-)ssRNA cannot be used as a template for protein synthesis
  3. The RNA dep. RNA pol. Is co-packaged inside the virion
  4. The RNA dep. RNA pol. Converts the (-)ssRNA into (+)ssRNA which can be translated into proteins (including more RNA dep. RNA pol.)
  5. The (-)ssRNA is replicated using the RNA dep. RNA pol.
  6. The virion is then assembled with packaged RNA dep. RNA pol.
  7. Progeny virions are released
22
Q

How do retroviruses replicate?

A
  1. Retrovirus enters the cell
  2. Capsid opens slightly to allow flow of nutrients
  3. Capsid contains necessary enzymes packaged (reverse transcriptase)
  4. dsDNA is generated from virus RNA genome
  5. This DNA then is integrated into normal host chromosome
  6. When in the host chromosome the viral encoding dsDNA encodes viral mRNA and proteins
  7. The virion particles are then packaged (with necessary enzymes)
  8. Progeny virions are released
23
Q

What are the two main types of polio immunisations?

A
  1. Oral polio vaccine (OPV):
    - Produces antibodies in the blood
    - Produces local immune response in lining of intestines
    - Short term shedding of OPV in the stools of recently immunised which spreads immunity and prevents the spread of wildtype poliovirus
    - Disadvantage: can cause paralysis
    - OPV consists of a live attenuated virus
  2. Inactivated polio vaccine (IPV)
    - Injected (killed) polio vaccine- injection means no mucosal immunity
    - Produces antibodies in the blood
    - Produces only a very low level of immunity inside gut (only provides individual immunity)

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