Free Viruses MCQs with Answers

11. In an enveloped animal virus, the envelope is derived from

• A. protein synthesised by the viral genome alone
• B. the membrane of the host cell it budded from
• C. the nucleic acid core of the virus
• D. capsomeres rearranged after assembly

Explanation: As the assembled particle buds out of an infected cell it wraps itself in a patch of the host's plasma membrane or nuclear membrane, so the envelope is host lipid carrying virally coded glycoprotein spikes. This is why enveloped viruses such as HIV and influenza are easily destroyed by soap and alcohol, which dissolve lipid. The spikes are viral, but the lipid bilayer itself is not.

12. Interferons help the body against viral infection because they are

• A. antibodies secreted by plasma cells that neutralise virus particles
• B. antibiotics that block viral protein synthesis
• C. glycoproteins released by infected cells that make neighbouring cells resistant
• D. enzymes that digest the viral capsid in the bloodstream

Explanation: An infected cell secretes interferon, which binds receptors on nearby uninfected cells and switches on genes whose products block viral replication in those cells. Interferon therefore protects the neighbours rather than saving the cell that made it, and it is not virus specific. Antibodies are the tempting answer, but they are made by plasma cells, are highly specific and act on the virus itself, not on host cells.

13. Which viral disease has been declared completely eradicated worldwide by vaccination?

• A. Poliomyelitis
• B. Measles
• C. Hepatitis B
• D. Smallpox

Explanation: Smallpox was declared eradicated by the WHO in 1980 after a global vaccination campaign, helped by the fact that the virus had no animal reservoir and every infection was visibly obvious. Polio is the tempting choice because it is close to eradication and Pakistan is one of the last countries still reporting cases, but it is not eradicated. Measles and hepatitis B remain widespread despite effective vaccines.

14. The first scientist to show that the cause of tobacco mosaic disease passes through filters that retain bacteria was

• A. Dmitri Ivanovsky
• B. Louis Pasteur
• C. Alexander Fleming
• D. Edward Jenner

Explanation: In 1892 Ivanovsky filtered sap from diseased tobacco plants and found the filtrate still infectious, proving the agent was smaller than any bacterium. Beijerinck later called it a contagious living fluid, and the word virus followed.

15. Viruses can only be grown in the laboratory in

• A. sterile nutrient agar plates
• B. living cells, such as chick embryos or cell cultures
• C. rich broth shaken with oxygen
• D. extracts of sterile soil

Explanation: Because viruses have no metabolism of their own, they multiply only inside living host cells, so laboratories use cell cultures, chick embryos or live animals. This obligate dependence on cells is exactly why artificial media fail.

16. Viruses that infect bacteria are called

• A. viroids
• B. plasmids
• C. bacteriophages
• D. prions

Explanation: Bacteriophages attach to specific receptors on the bacterial surface and inject their nucleic acid, making them powerful tools of genetics and, increasingly, alternatives to antibiotics. Viroids and prions are quite different infectious agents.

17. The first virus to be crystallised, proving that infectious agents could behave like chemicals, was

• A. influenza virus
• B. poliovirus
• C. HIV
• D. tobacco mosaic virus

Explanation: Wendell Stanley crystallised tobacco mosaic virus in 1935 and the crystals remained infectious, blurring the line between living and non living. The achievement earned him a Nobel Prize and started modern virology.

18. A typical bacteriophage consists of

• A. a head containing nucleic acid attached to a protein tail with tail fibres
• B. an RNA core surrounded by a lipid envelope only
• C. a single infectious protein molecule
• D. DNA enclosed in a polysaccharide capsule

Explanation: The phage's icosahedral head packs the DNA, and the hollow tail with base plate and fibres anchors to the bacterium and injects the genome. Descriptions involving envelopes or capsules belong to animal viruses and bacteria instead.

19. The lytic cycle of a bacteriophage ends with

• A. integration of viral DNA into the host chromosome
• B. bursting of the bacterium and release of new phage particles
• C. the virus becoming permanently dormant
• D. the bacterium turning into a virus

Explanation: After multiplication and assembly, lysozyme made by the phage dissolves the bacterial wall and the progeny escape to infect fresh cells. Integration into the chromosome describes the lysogenic route instead, a distinction examiners test repeatedly.

20. Which of the following diseases is caused by a virus?

• A. Cholera
• B. Tuberculosis
• C. Poliomyelitis
• D. Typhoid

Explanation: Polio is caused by the poliovirus, which attacks motor neurones and can cause paralysis. Cholera and typhoid are bacterial diseases of the gut and tuberculosis is caused by Mycobacterium, so only poliomyelitis in the list is viral.