JM - viral pathogenesis Flashcards
If a virus has a low number of genes, how does this affect the host cell?
There is a higher percentage of the genome that codes for proteins. The host cell is more likely to supply more of the raw materials.
What does the viral genome not code for?
- Ribosomes, tRNA or protein synthesis enzymes.
- Genes for lipid or AA synthesis or energy metabolism.
- (Less) non-coding DNA, introns and control sequences.
What can +ve ssRNA bind to?
Genome can bind to the ribosome and translate to a protein.
What can -ve ssRNA bind to?
The genome cannot bind directly to the ribosome
What is a capsid?
Protein that encloses the viral nucleic acid
What is a capsomer?
Multiple protein subunits within the virus
What is a nucelocapsid?
Both the nucleic acid and the capsid
What is the function of proteins within the virus?
- Packaging of the genome and enzymes.
- Protection of the nucleic acid from UV light.
- Protection from nucleases.
- Provides specificity for the attachment of the virus.
Describe the 3 structures that proteins can take within a virus.
- Can form a hexagonal array, which is flat sheet. This can be rolled to form a cylinder, which has helical symmetry. new molecules can be added like steps in a staircase.
- Pentagon can be surrounded by hexagons to form a spherical shape. This requires 12 pentagons, and the size of the sphere can increase, if the number of hexagons are increased.
- Icosahedron. Triangles are arranged as overlapping pentamers. 5, 3 and 2 fold symmetry allows for space to be saved within the genome.
Where is the virus’ envelope derived from?
The host cell
Which viruses have an envelope?
Helical viruses and many icosahedral viruses.
What is a peplomer?
Membrane anchored glycoprotein sugar added by host cell enzymes in the golgi apparatus.
What are the 5 ways that a virus can enter the host?
- Respiratory route
- Oral route
- Cutaneous localised infection
- Percutaneous injection
- Sexually transmitted infection
Why is HIV and FIV restricted to its own species?
HIV binds with a high affinity to CD4 receptors on human T-cells. This causes a conformational change to allow a low affinity interaction with chemokine receptor CCRS/CXCR4.
FIV binds with a high affinity to CD134 receptors on cat T-cells. This causes a conformational change to allow a low affinity interaction with chemokine receptor CXCR4.
Human CD134 cannot replace cat CD134.
Human CXCR4 can replace cst CD134.
The fact that HIV must bind to CD4 to causes a conformational change initially, means that it is restricted to its species.
Why can influenza cross species?
Influenza hemagglutinin binds to sialic acid, which is a widespread ligand, and so increases the chance for a cross species infection.
How does a lytic infection affect the cells?
Releases viral particles into the environment.
This causes an extensive loss of function because many more cells can be infected.
Many acute viral infections end in cell death.
How does a latent infection affect the cells?
Persists within a cell and does not cause cell death.
New viral particles are released without killing the cell.
Can persist for the lifetime of the host.
Acts as a source of infection for others.
How does a persistent infection affect the host?
Associated with the failure of the immune system to clear the virus.
How does enhanced cell growth affect the host?
Viruses can manipulate cell replication to increase the number of cell divisions and expand the number of infected cells.
During a viral infection, what causes an inflammatory response?
Poly U tails initiate an inflammatory response, as these are found on -ve ssRNA.
Interferons allows for neighbouring cells to prepare for the viral infection.
Increased MHC expression
Activated macrophages and dendrites.
Recruited and activated NK cells.
What is a plaque assay used for? (In terms of viruses)
To show the replication competence.
Holes are where the cells have died
How does a virus replicate within a host cell?
- Attachment to plasma membrane
- Entry into the cytoplasm/nucleus
- Uncoating of the viral particle
- Synthesis of mRNA and proteins
- Replication of nucleic acid
- Virus assembly
- Exit of virus from the cell
Describe the reading frame of a virus.
Start codon - AUG
Ribosome reads the genome until it reaches AUG, and then the polypeptide is assembled.
Until the ribosome reaches a stop codon.
Why must all viruses produce mRNA?
So that the genome can bind to a ribosome and be encoded to make proteins.
What reaction does virus RNA polymerase catalyse?
-ve sense RNA into +ve sense RNA. This is so that the genome can bind to the ribosome.
The virus must carry this with them when they infect a host cell.
How do retroviruses make proteins?
They have a diploid +ve ssRNA, the only virus to have this.
Reverse transcriptase - this forms dsDNA.
Integrase - integrates dsDNA into the host genome.
mRNA+ - formed from using the host cell RNA polymerase.
What does the FMD virus bind to?
It binds to the RGD motif on molecules that are found in the basement membrane.
This is found in high density on basal epithelial cells.
How does FMD enter cells?
Receptor mediated endocytosis via clarthyrin coated pits.
What happens during the uncoating of the FMD virus?
H+ ions are pumped into the endosome, which causes the pH in the vesicles to drop.
The drop in pH causes a pore to form in the endosome membrane. The virus exits the cell before it is digested.
How is mRNA and protein synthesised with FMD virus?
- In the cytoplasm, the VPG is dissociated from the 5’ end.
- +ve sense RNA already has a polyA tail, without a cap.
- 5’ end has an internal ribosome entry site. This allows ribosomes to bind and initiate translation.
- This makes 1 large polyprotein.
What is the function of the polyprotein produced by the FMD virus?
To acts as a proteolytic enzyme. So it can cut itself into pieces.
How does the FMD replicate within the cell?
- RNA polymerase uses VPG as a primer.
- This copies the +ve sense RNA to a -ve sense RNA.
How is the FMD virus assembled?
The genome binds to a VPG protein and are packaged within self-assembling capsids.
How does FMD destroy host cell enzymes?
1 virus required 60 polypeptide chains. This means that large amounts of RNA polymerase and RNA virus proteases to destroy host capping enzymes.
How is the FMD virus released from it’s cell?
- There is a loss of host cell protein synthesis. So all cell vital functions are compromised.
- Cell swells and bursts due to a lack of ion pumps. This releases the newly synthesised viral particles.
