Exam 2 - Viruses Flashcards
Why are viruses considered “obligate intercellular parasites”?
They can only replicate inside of a host cell
Difference between a virus and a toxin
A virus cannot replicate itself even in a host cell
Filter experiment for tobacco mosaic viruses… Who? When?
Ivanofsky, 1892
Growth shown in tobacco for tobacco mosaic virus… Who? When?
Beijerinck. 1898
What are bacteriophage? Who discovered them? When?
Bacteria viruses, discovered by Twort in 1915
Animal viruses were first discovered with ________ disease. When was first human virus discovered? What virus was it?
Animal viruses were first discovered with ‘Foot and Mouth’ disease in 1898. First human virus discovered was ‘yellow fever virus’ in 1901
Cellular origin theory of viruses:
viruses were once cellular components but over time they evolved separately
Autopoitic theory of viruses
Viruses once auto poetic entities but become reliant on other cells over time
What is an envelope?
A virus modified cellular membrane that is picked up from a host cell as a virus exits. These can be easily identified because they are susceptible to lipid solvents and in the presence of them, the virus is non-infectious.
Advantage of being a small virus?
replicate genome multiple times befell cell recognizes that it is infected b/c can replicate faster
Advantage of being a large virus?
can carry more genes or proteins that can help the virus. Ex: can have immune modulators that can down-regulate the host inmate immune response or turn off the antibody response.
However, a larger genome requires more time to replicate
What is a (+)sRNA vs (-)ssRNA? Ambisense RNA?
(+)ssRNA can be used to make viral proteins immediately. (-)ssRNA needs to be used as a template to make (+)ssRNA that can be used for proteins.
Ambisense is part (+) and part (-)
Segmented genome?
for virus to be complete, all the segments of the viral genome must be packaged up and put into a single virus particle.
Smallest genome?
Picornovirus (polio)
How does ssDNA replicate?
ssDNA Relies on the host cell’s replication machinery but that machinery looks for a dsDNA. So, the genome will fold together to a double stranded “hairpin” that serves as signals for host machinery to sit down and start making the copies.
How does dsDNA replicate?
dsDNA has a nick-site where DNA repair mechanisms sit down and make repairs. As that repair is completed, a rolling cycle of replication similar to host cell normal dsDNA replication occurs.
How does (+)ssRNA replicate?
(+)ssRNA (ex: picornovirus) goes straight to the ribosomes once it is injected into the cell. It begins translation process immediately and makes RNA-dependant RNA polymerase that will be able to make a negative strand template that can be used to make additional copies of the genome.
NOTE: you don’t need any virus particles of picornavirus to get replication and production of virus particles for this reason. The (+)ssRNA is enough of a self-starter all by itself.
How does (-)ssRNA replicate?
(-)ssRNA must enter the host cell with a RNA-dependant RNA polymerase that can turn the (-) strand into a (+) strand that can subsequently make proteins. Eventually the (+) strands of genome will be used as templates to make (-) progeny genomes.
In general, what do DNA viruses need that RNA viruses don’t need?
DNA viruses need access to the nucleus. Often they are tumorigenic, meaning the keep the cell mitotically active so that there is access to the nucleus.
DNA viruses also need access to the host replication machinery
What do RNA viruses need that DNA viruses don’t need?
RNA need RNA-dependent RNA polymerase.
Sometimes one receptor is not enough to facilitate virus attachment and entry, and a _______ is needed.
Co-receptor (Ex: HIV)
What allows attachment and infection?
Attachment sequences
What is Tropism
By taking one type of virus and changing the surface proteins you can change what the virus can infect. What the virus can infect / types of cells / tissues is its tropism.
What is zoonosis?
Viruses that infect humans and inimals, but are not able to infect other more divergent hosts like plants, bacteria, and fungi.
What was the major reason that we were able to eradicate smallpox? (think tropism)
It is only a human pathogen. Yellow fever is in humans and mosquitoes so it is hard to get rid of.
What is receptor-mediated endocytosis?
Trigger the cell to take the virus into the cell as if something good is on the source of the cell by confusing it for something else that the host cell does normally take inside.
Note: Direct penetration of plasma membrane is also possible
Non-enveloped viruses mode of entry VS enveloped viruses mode of entry:
Non-enveloped not as well understood:
- Picornavirus (causes polio) makes pores in the membrane
- Adenovirus and Reovirus cause membrane disruption. Proteins are present that can go in and make a large enough hole in the membrane to be able to pass the nucleocapsid through the hole.
Enveloped: viruses goes through membrane fusion. (best understood for influenza so use the Hemaaglutinen protein to show attachment and fusion…
Describe how Influenza virus enters the cell
Uses the hemaglutenin proteins to attach and fuse to the membrane of the host cell. Once the virus is taken in by endocytosis and when it is in the endosome the pH will start to drop. When the HA proteins is exposed to the pH drop it undergoes a conformational change and injects part of its receptor protein into the cellular membrane which will bring the cellular membrane and the viral membrane into close proximitity. This results in membrane intermixing. You then get a fusion state and a full pure forming and then finally once it is able to gain access to the cytoplasm it has to deal with the rest of the replication cycle.
What type of viral genome does not ever release its genome out to the host cell?
dsRNA
What type of viral genome usually rely upon cellular RNA polymerases.
DNA genomes
What type of viral genome must bring its own polymerase into the cell?
(-)ssRNA and dsRNA
What type of viruses must use host cell ribosomes?
All viruses must use host cell ribosomes!
Structural proteins are produced in high levels but non-structural proteins are only see inside the host cell. (T/F)
True
How does dsRNA replicate?
Virus particel include the viral polymerase dn the dsRNA does not leave the virus particle. mRNA gets produced and then is exported to the cytoplasm where the mRNA serves as a template for making proteins. Then the cell makes empty protein capsules. A (+) gets brought inside and as it is brought in, it is used as a template to make the (-) strand. Thus, yielding the dsRNA.
Why are poxviruses an exception?
They replicate in the cytoplasm and they have their own DNA polymerases needed to execute their transcription and replication process.
What are cellular viral factories?
Localized structural proteins to aid assembly. You get dense pockets of virus particle that are assembled in the same location. The genome will contain packaging signals and the proteins will recognize progeny genomes that will grab onto them and the capsid will assemble around it or the capsid will assemble empty and it will find genetic material to import inside the particle itself.
How does adenovirus package?
Adenovirus starts as an empty protein coat that grabs the genome and pulls it inside.
How does Reovirus package?
Reovirus starts with RNA packaged during capsid assembly and it will synthesize the (-)RNA strand on top of the (+) that is already in the cell
How do Retroviruses package?
Retrovirus has proteins that grab onto the genetic material itself and the capsid gets assembled around the progeny genomes
What is lysis
Ability to rupture the cell and dump contents out of the cell. This is likely to make a big immune response though because of all the cellular debris.
What is weak lysis?
The virus replicates and consumes all the raw materials inside the cell so that cell cannot maintain its membrane and its proteins. This leads to the cell deteriorating and then the cell eventually breaks open in a more passive way.
What is budding?
Budding for enveloped virus when the nucleocapsid comes near the cell membrane and induces the membrane to form around the capsid and then pinch off the virus into the extracellular space
(this is how HIV works! Influenza also does this, but Zanamivir/Tamiflu blocks the release of budding virions!)
Advantage to lytic vs non-lytic infections?
Progeny can more quickly be released to go infect more cells in lytic cycle. The lytic cycle also has a higher chance of triggering immune response because all the parts of the host cell are also getting ejected into the extracellular space
What is the normal multiplicity index for a 1-step growth curve?
5x the virus particles / cells to infect
Eclipse phase of 1-step growth curve:
Attachment and uptake causes a decrease in initial concentration of infectious particles. Particles are inside the host cells and are replicating
Exponential growth phase of 1-step growth curve:
Replication and assembly is completed and causes a great increase in the number of virus particle that are available for infection of new cells
Plateau phase of 1-step growth curve:
All the cells resources are used up and there is no more ability to make more virus particles and/or all the host cells have been lysed
Time of 1-step growth curve is variable:
bacteriaphage: 30 min
Vesicular somatitis virus ~6 hours
Vaccina (smallpox virus) ~24 hours
Productivity of 1-step growth curve is variable
Vesicular somatitis virus 1:1,000
Vaccina produces 1:100
Confirmation of the virus / disease causing agent: Huff’s Principle
Putting the virus back into eggs, cell culture, etc. to show that the virus makes more of the virus.
Hierarchy of numerical detection/quantification
Physical (counting particles)
Infectivity
Genomic (PCR)
Serological (antibody reaction)
Ways to look for infection assays:
Look at Cytopathic effect (CPE) - look for rounding, syncytial formation and/or inclusion bodies. This happens in cells infected with virus.
Flourescence assay
Plaque assay
Infectious dose assay
Particle assays / Hemagglutinin assay
Use plates with RBCs and virus. Where not enough virus the RBCs will pelt. Where there is enough virus to make a matrix then you will get a fuzzy appearance and there will not be a pellet. The concentration where it changes between polluting and non-pelletting you can calculate a value.
Different assays give different numerical values!
Direct electron microscope count highest then infectivity then plaque formation then hemagglutination is less still. Plaque forming units can range from 1:1 to 1:10,000 virus particles.
Viruses are __________ and use the host cell’s replication processes to duplicate themselves.
Obligate intercellular parasites
Human viruses range in size from ____ to ____
30nm - 300nm
Virus are classified by the ________ , ______ , and the _______
Genome, particle structure, and replication strategy.
What viruses are primarily associated with children?
Measles virus Respiratory Syncytial Virus Varicella Zoster Virus Rotavirus Poliovirus
What family is Measles virus? What else is in this family?
Paramyxovirus (also Measles and also mumps virus)
What is measles genome?
(-)ssRNA
What is measles genome shape? Enveloped?
Helical ; Yes, enveloped (same as RSV, b/c same family… paramyxovirus)
Where is measles replicated?
cytoplasm
How does measles virus exit host cell?
budding
What causes syncytial formation? (especially in measles virus)
Fusion protein
How is measles spread?
inhalation of aerosolized droplets from coughing/sneezing
What is the characteristic symptom of measles?
“Koplik spots” which are red spots with a bluish middle to them. It is caused by the virus and immune response to damage to epithelial and endothelial cells
Describe the process of measles infection/spread
Measels virus first infects respiratory epithelial cell. It then enters the blood stream from the lungs and causes the primary viremia and spread to many locations of the body and begins to cause disease symptoms.
Once it spreads to the spleen, it can cause a secondary viremia b/c blood is passed through the spleen.
What is incubation period for measles? Recovery period?
Incubation period is 10-14 days
Recovery is 20 days after infection
10% mortality
What complications does measles virus cause?
Opportunistic infections (Strep pneumoniae, Staph aureus, or Haemophilus influenzae)
Blindness in children with Vit A deficiency
Neurological manifestations:
Acute Demyelinated Encephalomyelitis (ADEM) 1:1000
Subacute Sclerosing Panecephalitis (SSPE) 1:1,000,000 after 7-10 years. It is a progressive neurological deterioration
What is ADEM?
Acute Demyelinated Encephalomyelitis (ADEM) 1:1000 ; it is a rare demyelinating disease that results from measles virus
What is SSPE?
Subacute Sclerosing Panecephalitis (SSPE) 1:1,000,000 after 7-10 years. It is a progressive neurological deterioration Caused by measles virus
How is measles diagnosed?
Fever and cough for 2-3 days then Koplik Spots
What are the hosts for measles virus?
Only humans, but very contagious with Ro = 15-20. People are able to spread 2-3 days before rash.
How long does the measles vaccine convey immunity?
life-long
How is measles treated?
Vit A can reduce severity but there are no antivirals. Use supportive care for symptoms (ex: benedryl for rash…)
