Q1. The “information” contained in PrpSc must be contained in
Correct Answer: (d)
Prions do not contain nucleic acids. The 'normal' protein (PrPC) and the infectious form (PrPSc) have identical amino acid sequences. The infectious property of the prion is derived entirely from its abnormal three-dimensional folding pattern, which it can impose on normal proteins.
Q2. In very rare cases, HIV can infect T cells that revert to what is called a memory state. In this state, HIV genes are not expressed. At a later time, the T cell may become permissive for HIV gene expression and new virions are made. What kind of viral infection does this behavior of HIV exemplify?
Correct Answer: (d)
Latent infections are characterized by the persistence of the viral genome within host cells without active viral replication or disease symptoms. The virus can remain 'hidden' for long periods until it is triggered to reactivate and enter the lytic cycle.
Q3. The tail of certain bacteriophage contracts to inject the DNA genome of the phage into the host cell. Neither the tail nor the baseplate proteins hydrolyze ATP. Based on this, what kind of energy likely drives tail contraction?
Correct Answer: (c)
The tail assembly of these phages is often 'primed' during assembly, effectively acting like a loaded spring. The potential energy is stored in the metastable conformation of the proteins. Binding to the host triggers a conformational change that releases this energy to drive the injection process.
Q4. Why is a drug that blocks HIV binding to one of its cell surface receptors not effective at treating influenza?
Correct Answer: (a)
Drugs that block entry are highly specific to the shape and chemistry of the specific receptor-ligand interaction. HIV uses the CD4 receptor and CCR5/CXCR4 co-receptors, while influenza uses sialic acid receptors. A drug blocking one will not match the structural requirements of the other.
Q5. What is the most reasonable explanation for why a bacteriophage capable of infecting E. coli would be unable to infect a human lung epithelial cell?
Correct Answer: (c)
Viral infection begins with the specific binding of viral surface proteins to specific receptor molecules on the host cell surface. Since human lung cells do not express the specific receptors that a bacteriophage evolved to recognize on E. coli, the virus cannot attach or enter.
Q6. Which approach would be best to identify novel viruses in a wastewater treatment plant?
Correct Answer: (b)
Metagenomics involves extracting all genetic material from an environmental sample and sequencing it. This allows scientists to identify the presence of any virus, including novel ones that cannot be grown in a lab or those that do not match known PCR primers.
Q7. Why is HAART more effective than a single drug in reducing HIV levels in infected people?
Correct Answer: (b)
HIV has a high mutation rate. While a single mutation might allow a virus to become resistant to one drug, the probability of a single virus simultaneously acquiring multiple mutations to bypass three or more different drug mechanisms (as in HAART) is extremely low.
Q8. How does the use of one or only a few proteins to make a viral capsid help to keep viral genomes small?
Correct Answer: (a)
By using a large number of identical protein subunits (capsomeres) to build the capsid, the virus only needs to carry the genetic information (genes) for one or two types of proteins, thereby minimizing the total amount of DNA or RNA required.
Q9. Which of the following kinds of proteins would be least likely to be expressed during the lysogenic cycle of a temperate phage?
Correct Answer: (d)
The lysogenic cycle is characterized by the integration of the viral genome into the host's DNA, where it remains dormant. Proteins that cause cell lysis (rupturing the cell) are only expressed during the lytic cycle, as they would kill the host and terminate the lysogenic state.
Q10. Why do retroviruses need to contain the enzyme reverse transcriptase?
Correct Answer: (b)
Host cells follow the central dogma where DNA acts as the template for RNA. They do not naturally possess an RNA-dependent DNA polymerase (reverse transcriptase). Therefore, retroviruses must provide this enzyme to convert their RNA genome into DNA for integration into the host genome.
Q11. Phage conversion is an example of
Correct Answer: (a)
Phage conversion (or lysogenic conversion) occurs when a bacteriophage integrates its DNA into a host bacterium's genome and introduces new phenotypic traits, such as toxin production. This represents a transfer of genetic material between different organisms, which is horizontal gene transfer.
Q12. What characteristic of the enzyme used by influenza viruses to replicate their genomes is responsible for genetic drift and is also a characteristic shared by the enzyme used in the replication of the HIV genome?
Correct Answer: (c)
Both the RNA polymerase used by influenza and the reverse transcriptase used by HIV lack proofreading capabilities. This 'low fidelity' leads to a high frequency of errors (mutations) during replication, resulting in the rapid evolution known as genetic drift.
Q13. Why might the ability of a virus to convert a benign bacterium into a pathogen be selected for by natural selection?
Correct Answer: (b)
Pathogenicity often results in symptoms like coughing, sneezing, or diarrhea. These symptoms are mechanisms that physically expel the virus or bacteria into the environment, facilitating transmission to new hosts and thereby increasing the evolutionary fitness of the virus.
Q14. What viral strain could be created by the coinfection of an H2N2 human influenza A virus and an avian H3N8 human influenza A virus into a pig?
Correct Answer: (b)
When two different strains of influenza infect the same host (reassortment), they can swap genomic segments. A pig acting as a mixing vessel could produce a reassortant virus containing the H3 segment from the avian strain and the N2 segment from the human strain, resulting in H3N2.
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