Free Acellular Life MCQs with Answers

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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.

28. New influenza vaccines are needed every few years because

• A. immunity produced by the vaccine lasts only days
• B. influenza virus infects only elderly people
• C. antibiotics destroy the vaccine
• D. the surface antigens of the virus mutate frequently

Explanation: Influenza's RNA genome mutates rapidly and its haemagglutinin and neuraminidase surface proteins keep changing, so last year's antibodies may not recognise this year's strain. This antigenic drift is the reason vaccination is reviewed every season.

29. Dengue fever is transmitted to humans by

• A. contaminated drinking water
• B. the bite of Aedes mosquitoes
• C. coughing and sneezing
• D. eating contaminated food

Explanation: Aedes aegypti and Aedes albopictus mosquitoes carry the dengue virus between people, which is why outbreaks follow the mosquito season and stagnant water. Water and food routes spread gut infections such as cholera, not dengue.

30. Antibiotics are useless against viral diseases because viruses

• A. lack their own metabolism and the structures that antibiotics target
• B. are too small for drugs to reach
• C. live only in the soil
• D. are destroyed by stomach acid

Explanation: Antibiotics attack bacterial machinery such as cell walls and 70S ribosomes, and viruses possess neither, borrowing the host's instead. Antivirals must therefore target viral enzymes like reverse transcriptase, which is why far fewer of them exist.