1. Reminder: Actinomycin D is a compound used in the laboratory to inhibit mRNA
ID: 260549 • Letter: 1
Question
1. Reminder: Actinomycin D is a compound used in the laboratory to inhibit mRNA transcription of host cells. (a) If this drug was added to cells infected with a rhabdovirus (e.g., rabies virus), what do you suppose would happen to viral replication? Explain your reasoning. [3 pts.] (b) If this drug was added to cells infected with Influenza virus, what do you suppose would happen to viral replication? Explain your reasoning. [3 pts.] 2. Describe the difference between a protein called a "capsid" compared to a "nucleocapsid." [4 pts.] 3. Consider the information shown in the table below: Influenza A Influenza B Humans only genetic drift only Host range marine mammals Source of genetic variation genetic shift and drift (a) Compare and contrast the terms genetic shift and drift. [3 pts.] (b) Why do Influenza A strains demonstrate both shift and drift while B strains usually experience only genetic drift? [4 pts.] 4. (a) Explain why Influenza viruses were forced to develop the process of "cap-snatching" during their replication cycles? [4 pts.] (b) Pretend that you were working in the research lab that initially discovered the "cap- snatching" mechanism. Describe two experiments or lines of evidence that would have supported this model. Note: I am not asking you to find the original paper(s) and describe what the researchers did. I am asking you to think about what pieces of evidence would have suggested that cap-snatching actually occurred within infected cells. [4 pts.]Explanation / Answer
1 -
( a )
VSV and rabies virus are only poorly sensitive to cytosine arabinoside at doses as high as 250 ?g/ml which completely inhibit host DNA syntheses.
Rabies virus is more sensitive than VSV to doses of actinomycin D higher than 0.01 gm/ml but this is probably due to the duration of the viral cycle.
( b )
Evidence has been presented to show that actinomycin D inhibits the replication of influenza virus when added during the first 1:30 hours of infection. After 1:30 hours the inhibitory effect progressively decreased.
Actinomycin D has an effect on the replicative form (RF) of the virus.
When the drug was added at 1:30 hours the RF already made disappeared, but when added at 3:30 hours the RF already present remained.
In the absence of the drug, unlabeled uridine added at either time did not chase the uridine-3H out of the RF but, in fact, the total amount of labeled RF increased.
2- Difference – Capsid vs neurocapsid :
The capsid is the protein shell surrounding the genome.
The nucleocapsid is the nucleic acid/protein assembly within the virion.
3 - Compare and contrast : genetic shift and drift :
DRIFT ;
Virus can reolicate in the bird - it can pick up its own genes and never seen the other virsu - Drifts ( happens year to year)
SHIFT :
If they see the other virus - as they are packaged - viruses can pack the other virus gene - Shift ( the big outbreaks - pandemics)
( b )
Drift :
The gradual alteration of the HA and NA within a subype resulting in the reduced ability of antibody to previrous strains to inhibit the HA or N of the mutant virus.
Antigenic drift occurs in both influenza A and B and causes preriodic epidemics.
occurs at a rate of <1% per year at the amino acid level.
Shift :
Appearnce in human population of a new subtype of influenza A virus (HA) and neuraminidase (NA) that is immunologically different from that of isolates circulating previously.
Responsible for world-wide pandemics
Antigenic shift reflects an abrupt change where the new HA may differ by >50% from previously circulating strains.
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