Free Viruses MCQs with Answers

1. Viruses are described as obligate intracellular parasites because they

• A. are smaller than any known bacterium
• B. carry both DNA and RNA in the same particle
• C. have a cell wall made of peptidoglycan
• D. lack the ribosomes and enzymes needed to make their own proteins and ATP

Explanation: A virus particle has nucleic acid and protein but no ribosomes, no respiratory enzymes and no ATP generating system, so it can only replicate by taking over a living host cell's machinery. Small size alone does not force parasitism, since many free living bacteria are smaller than the largest viruses. A virus also never carries both DNA and RNA as its genome, and it has no cell wall of any kind.

2. The protein coat that encloses the genetic material of a virus is called the

• A. capsid
• B. capsomere
• C. envelope
• D. nucleoid

Explanation: The capsid is the complete protein shell around the viral nucleic acid, and it protects the genome and helps the virus attach to the host cell. A capsomere is only one of the repeating protein subunits from which that shell is assembled, so it is a part and not the whole. The envelope is an extra lipid layer present only in some viruses, and a nucleoid is the DNA region of a prokaryotic cell.

3. The genetic material of tobacco mosaic virus is

• A. double stranded DNA
• B. single stranded DNA
• C. single stranded RNA
• D. double stranded RNA

Explanation: TMV is a rod shaped plant virus whose genome is a single strand of RNA wound in a helix inside about 2,130 identical capsomeres. It is the classic example used to show that RNA alone can carry hereditary information, proved when reconstituted TMV made new virus of the type its RNA came from. Double stranded DNA is the genome of bacteriophages such as T4, not of TMV.

4. A bacteriophage recognises and attaches to a specific bacterial cell by means of its

• A. head
• B. tail fibres
• C. collar
• D. sheath

Explanation: The tail fibres carry the binding proteins that fit specific receptor molecules on the bacterial cell wall, which is why a phage infects only one species or strain. The head is simply the polyhedral capsid holding the DNA, and the sheath contracts after attachment to drive the core through the wall. Attachment must come first, so a structure involved in penetration cannot be the one that provides recognition.

5. Which event occurs in the lysogenic cycle but never in the lytic cycle?

• A. Synthesis of viral coat proteins
• B. Injection of viral nucleic acid into the host
• C. Attachment of the virus to the host cell
• D. Integration of viral DNA into the host chromosome

Explanation: In lysogeny the phage DNA becomes part of the bacterial chromosome as a prophage and is copied silently every time the bacterium divides, which is the one step the lytic cycle has no equivalent of. Attachment and injection begin both cycles, and coat protein synthesis happens in the lytic cycle and in a lysogenic cell only after induction switches it to the lytic pathway.

6. Viral DNA that has become integrated into the chromosome of a bacterium is called a

• A. prophage
• B. provirus
• C. prion
• D. viroid

Explanation: Prophage is the term reserved for phage DNA sitting integrated and inactive in a bacterial chromosome. Provirus describes the same situation in an animal cell, as with HIV DNA in a human T lymphocyte, so it is the tempting wrong answer here because the host is bacterial. A prion is an infectious protein and a viroid is a naked infectious RNA, neither of which integrates in this way.

7. Which statement about viruses is NOT correct?

• A. They can be crystallised like a chemical substance
• B. They are destroyed by antibiotics such as penicillin
• C. They contain either DNA or RNA but never both
• D. They multiply only inside a living host cell

Explanation: Antibiotics act on structures and pathways that only cells have, such as peptidoglycan wall synthesis or bacterial ribosomes, and a virus has none of these, so penicillin has no effect on a viral infection. Viruses genuinely can be crystallised, which Stanley first did with TMV in 1935, and they genuinely carry only one type of nucleic acid and replicate only inside a host.

8. Naked infectious RNA molecules that lack a protein coat and cause diseases of plants are called

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

Explanation: Viroids are short circular RNA molecules with no capsid at all, and they cause plant diseases such as potato spindle tuber disease. The word prion is the tempting confusion because both are sub viral agents, but a prion is the opposite case: infectious protein with no nucleic acid. A bacteriophage is a complete virus with a capsid and a tail.

9. Prions differ from every other infectious agent because they contain

• A. RNA only
• B. DNA only
• C. an envelope of host lipid
• D. no nucleic acid at all

Explanation: A prion is a misfolded form of a normal cellular protein that converts healthy copies of that protein into the same abnormal shape, so infection spreads without any genetic material being transmitted. This is why prions survive boiling, formalin and radiation doses that destroy viruses. Diseases such as scrapie, mad cow disease and Creutzfeldt Jakob disease are caused this way.

10. The size of most viruses lies in the range

• A. 20 to 300 nm
• B. 20 to 300 um
• C. 1 to 10 mm
• D. 0.5 to 5 um

Explanation: Viruses are measured in nanometres, from about 20 nm for the smallest to roughly 300 nm for the largest such as poxviruses, which is why they pass through filters that hold back bacteria and are visible only under an electron microscope. The range 0.5 to 5 um is the size of typical bacteria, about a thousand times larger by volume.