Lecture 2 Viruses Flashcards
(21 cards)
Describe viral capsid assembly
(Usually) spontaneous process in which proteins associate into progressively larger units and assemble into capsid.
Viruses can exist in 2 phases. What are they?
- Extra cellular phase (virion)
2. Intracellular phase
What are the 2 principle capsid shapes? Describe them.
HELICAL: hollow protein cylinders which may be flexible or rigid
ICOSOHEDRAL: regular polyhedron with 20 faces.
Also atypical shapes with tail (phages)
Describe 4 characteristics of the viral capsid.
May posses distinctive antigenic structure
Involved in attachment and entry into host cell
Resistant to desiccation, pH change and detergents
Protects viral nucleic acid
What is the viral envelope? Describe it’s composition.
Outer membranous layer surrounding the capsid.
Consists of host-encoded lipids and carbohydrates, and virus-encoded proteins. May have spikes or pep lovers projecting from surface which are involved in attachment and haemagglutination.
Envelope determines antigenic characteristics.
Under what conditions can the viral envelope be disrupted?
Easily disrupted by chemicals, solvents and extremes of pH.
What are the 4 classes of viral genome? Which are the most common?
dsDNA, ssDNA, dsRNA and ssRNA.
Most DNA genomes are double stranded.
Most RNA genomes are single stranded, and may be positive or negative sense, or segmented.
How are viruses classified?
May be classified into major groups or families based on morphology, type of nucleic acid or mode of replication.
What are the 6 principle steps of viral replication?
- ADSORPTION and attachment to host cell receptor
- PENETRATION: direct, phagocytosis or membrane fusion
- UNCOATING: nucleic acid is released into host cell cytoplasm
- SYNTHESIS: nucleic acid replication
- ASSEMBLY of viruses
- RELEASE: naked viruses usually lose cell. Enveloped viruses bud
What necessary things for viral replication are provided by the host? What is provided by the virus?
The host cell provides substrates, hardware and enzymes.
The virus provides some enzymes.
Describe positive sense RNA viruses.
Look like mRNA to the host cell and can bind to ribosomes to direct protein synthesis.
Encode RNA-dependent RNA polymerase because host cells cannot replicate RNA.
using RNA polymerase, a negative sense RNA template is formed, and used to generate more mRNA to replicate the genome.
Describe negative sense RNA viruses.
Need to be converted to positive sense before replication. The negative sense strand provides the template for the production of mRNA.
Unlike positive sense RNA, negative sense RNA is not infectious by itself.
RNA dependent RNA polymerase generates mRNA for viral proteins, and full length positive sense RNA as a template for genome replication.
What are 8 types of viral infections?
- acute
- Sub clinical
- Chronic
- Latent
- Slow
- Oncogenic
- Oncolytic
- Persistent
Describe ACUTE viral infections.
Also known as LYTIC infections. Release of visions from host cell causes lysis. Host immune system resolves infection and develops “memory”.
Have well-defined symptoms, eg common cold.
Mutations in antigenic profile can compromise memory of acute viral infections (eg influenza)
Describe SUB CLINICAL viral infections.
No recognisable symptoms. Some general, mild symptoms such as malaise, mild fever or lymphadaema.
May be serological evidence of infection.
Eg glandular fever, hep B
Describe CHRONIC viral infections.
“Carrier state” of the viral infection.
May cause acute infection, recovery or death in 1-2% of patients.
80-90% chance of chronic infection after vertical transmission between mother and unborn baby.
5-10% of hep B patients have a mild or sub clinical infection resulting in carrier status.
Describe PERSISTENT viral infections.
Viral replication is ongoing, but the virus adjusts it’s replication and pathogenicity so as not to kill the host.
The virus may be present and continue to replicate in the host for its entire lifetime.
Describe LATENT infections.
Infection may produce disease and apparent recovery, but the virus remains dormant (often in atypical cells).
Virus may be re activated when host physiology is altered, for example in immunosuppresion.
Eg herpes
Describe SLOW viral infections.
Some viruses remain dormant for many years and manifest as a different disease upon reactivation.
For example, measles virus may persist in brain cells and become reactivated to cause subacute sclerosing panencephalitis which is fatal.
Describe ONCOGENIC viral infections.
Virus is associated with the development of malignant disease.
Examples include hepatitis B and C, papilloma virus.
Describe ONCOLYTIC viral infections.
Some viruses may be used in the treatment of disease (virotherapy).
ONCOLYTIC viruses are used for the selection and destruction of cancer cells, while leaving normal cells intact.
Phage therapy: use of bacteriophages to control bacterial infections
Gene virotherapy: use of viruses to deliver genes to cells
