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
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
3
Q
What is the structure of a virus particle?
A
Virus particles consist of:
- Nucleic acid (DNA or RNA)
- Capsid (protective shell made up of proteins)
- Sometimes a membranous (lipid) envelope- with viral membrane proteins
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
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
6
Q
What are the general features of a viral reproductive cycle?
A
- Attachment:
- Virus interacts with host cells via receptors
- Enters host cell
- Un-coating (release of viral genome) - Replication and amplification of viral genome
- Viral gene expression:
- Synthesis of viral mRNA
- Synthesis of viral proteins - Assembly of capsid and packaging of viral nucleic acid into virus particles
- Virus is released from host cells
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
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
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
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
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
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
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
14
Q
How are viruses detected?
A
- Measurement of infectious units:
- Plaque assay
- Transformation assay
- Animal infectivity experiments
- Note: only measures infectious viruses - 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
15
Q
What are the phases of a one step growth curve for viruses? What is measured?
A
- Latent period: time in which viruses dock onto host cell, inject their DNA and produce enzymes, nucleic acids and protein coats
- 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
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
- Viral ssRNA enters host cell
- This +ssRNA can be directly translated into proteins (used as mRNA)
- RNA dependent RNA polymerase is made by viral genes
- RNA dep. RNA pol. Uses +ssRNA as a template to made –ssRNA (complementary)
- –ssRNA is then used as a template to replicate the original +ssRNA
- The extra proteins are synthesised
- Virus is assembled
- Progeny virions are released
21
Q
How do negative strand single stranded RNA viruses replicate?
A
- Viral (-)ssRNA enters host cell
- This viral (-)ssRNA cannot be used as a template for protein synthesis
- The RNA dep. RNA pol. Is co-packaged inside the virion
- The RNA dep. RNA pol. Converts the (-)ssRNA into (+)ssRNA which can be translated into proteins (including more RNA dep. RNA pol.)
- The (-)ssRNA is replicated using the RNA dep. RNA pol.
- The virion is then assembled with packaged RNA dep. RNA pol.
- Progeny virions are released
22
Q
How do retroviruses replicate?
A
- Retrovirus enters the cell
- Capsid opens slightly to allow flow of nutrients
- Capsid contains necessary enzymes packaged (reverse transcriptase)
- dsDNA is generated from virus RNA genome
- This DNA then is integrated into normal host chromosome
- When in the host chromosome the viral encoding dsDNA encodes viral mRNA and proteins
- The virion particles are then packaged (with necessary enzymes)
- Progeny virions are released
23
Q
What are the two main types of polio immunisations?
A
- 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 - 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)
