1.4 Exercise: Viruses, AIDS and HIV Quiz-2
1. During the infection cycle of a T4 bacteriophage, which structure is responsible for the enzymatic degradation of the bacterial cell wall to allow DNA entry?
Tail fibers
Base plate and lysozyme-like enzymes
Viral RNA polymerase
Check Answer
Explanation: The tail fibers first attach, but the base plate settles on the surface and releases enzymes to digest a hole in the peptidoglycan layer.
2. Which statement best explains why viruses are described as “obligate intracellular parasites”?
They lack the metabolic machinery and ribosomes for independent protein synthesis.
They are always transmitted by biological vectors like mosquitoes.
They have a circular DNA that only activates in low pH environments.
Check Answer
Explanation: Viruses must use the host’s ribosomes, enzymes, and energy (ATP) to replicate, as they cannot perform these functions alone.
3. In the lysogenic cycle, what event triggers the transition of a prophage into the lytic cycle?
Environmental stress or “Induction”
The fusion of two bacterial cells
Depletion of the host’s DNA
Check Answer
Explanation: Factors such as UV radiation or chemical changes can cause the prophage to excise itself from the bacterial chromosome and enter the lytic phase.
4. A pathogen that consists purely of a single-stranded, circular RNA molecule without a protein coat is most likely a:
Prion
Viroid
Bacteriophage
Check Answer
Explanation: Viroids are infectious RNA particles that lack capsids, distinguishing them from viruses.
5. The enzyme Reverse Transcriptase, found in HIV, performs which specific catalytic function?
RNA $rightarrow$ DNA
RNA $rightarrow$ Protein
DNA $rightarrow$ Protein
Check Answer
Explanation: This enzyme allows retroviruses to integrate their genetic information into the host’s DNA by first converting their viral RNA into DNA.
6. Why are prions considered uniquely dangerous compared to standard viruses?
They are resistant to standard sterilization methods like heat and radiation.
They contain double-stranded RNA that cannot be detected by the immune system.
They only infect plants, destroying the food supply.
Check Answer
Explanation: Since they are misfolded proteins (not living and no nucleic acid), they are incredibly difficult to deactivate or “kill.”
7. Which form of Hepatitis is primarily spread through blood transfusions and sexual contact, rather than the fecal-oral route?
Hepatitis B
Hepatitis E
All of the above
Check Answer
Explanation: Hepatitis B and C are serum-based viruses transmitted through body fluids, unlike Hepatitis A and E which are enteric (fecal-oral).
8. What is the primary composition of a viral envelope?
Chitin and proteins
Lipids derived from the host’s cell membrane
Purely viral-encoded glycoproteins
Check Answer
Explanation: When enveloped viruses “bud” out of a host cell, they wrap themselves in a piece of the host’s plasma membrane.
9. The “Eclipse Period” in a viral growth curve is defined as the time when:
No infectious viruses can be detected as they are being disassembled and replicated.
The host immune system is actively destroying the viruses.
The virus is integrating into the host genome.
Check Answer
Explanation: During this phase, the virus has uncoated its genome and is synthesizing parts, so no complete, infectious virions exist yet.
10. Bacteriophages are highly valued in Genetic Engineering primarily because:
They act as vectors to transport foreign DNA into bacterial cells.
They kill harmful bacteria in the human gut.
They can turn inorganic carbon into organic sugar.
Check Answer
Explanation: Their natural ability to inject DNA makes them perfect tools for inserting specific genes into bacteria for study or production.
11. What distinguishes HIV (Human Immunodeficiency Virus) from TMV (Tobacco Mosaic Virus) structurally?
HIV is enveloped and spherical, while TMV is non-enveloped and helical.
TMV possesses reverse transcriptase, but HIV does not.
HIV infects plants, while TMV infects humans.
Check Answer
Explanation: Structure and host range are key differences; HIV is a complex enveloped retrovirus, whereas TMV is a naked helical virus.
12. Tobacco Mosaic Disease symptoms include mottled leaves and stunted growth. These symptoms result from the virus interfering with which host process?
Photosynthesis and chlorophyll production
Nitrogen fixation
Water absorption through roots
Check Answer
Explanation: The “mosaic” pattern is caused by the loss of chlorophyll in infected leaf cells, which severely reduces the plant’s ability to produce food.
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