All Free Biology MCQs with Answers

3011. Bacteriophages kill:

• A. Fungi
• B. Parasites
• C. Bacteria
• D. Viruses

Explanation: Bacteriophages, or phages, specifically kill bacteria because they are viruses that are highly specialized to infect only bacterial cells. They recognize and attach to unique receptors found on the surface of bacteria, inject their genetic material, and use the bacterial machinery to replicate, eventually causing the bacterial cell to burst. Fungi, parasites, and viruses, however, do not have the same surface receptors or cellular structures that phages require to attach and replicate. Since phages are host-specific, their infection process cannot occur in these other organisms, which is why they are harmless to fungi, parasites, and viruses and target only bacteria.

3012. Which of the following are intracellular obligate parasites?

• A. Bacteria
• B. Viruses
• C. Slime moulds
• D. Cyanobacteria

Explanation: Viruses are obligate intracellular parasites because they lack the cellular structures and metabolic machinery needed to grow and reproduce independently. They cannot carry out essential life processes like energy production or protein synthesis on their own, so they must invade a host cell and hijack its machinery to replicate. In contrast, bacteria, slime moulds, and cyanobacteria are true living organisms with cellular organisation, enzymes, and metabolic pathways that allow them to survive, grow, and reproduce independently in suitable environments. This fundamental dependence on host cells for survival is what makes viruses obligate intracellular parasites, unlike bacteria, slime moulds, and cyanobacteria.

3013. All enzymes are present in HIV except:

• A. Reverse transcriptase
• B. Integrase
• C. Protease
• D. Lysozyme

Explanation: HIV contains reverse transcriptase, integrase, and protease because these enzymes are essential for its replication cycle: reverse transcriptase converts viral RNA into DNA, integrase inserts this DNA into the host genome, and protease processes viral proteins for assembly of new viruses. Lysozyme, on the other hand, is an enzyme that breaks down bacterial cell walls, which is unnecessary for HIV since it infects human cells, not bacteria.

3014. Where can viruses replicate?

• A. In dead tissue
• B. On the surface of the skin
• C. In nutrient-rich culture media
• D. Inside a living host cell

Explanation: Viruses can replicate only inside living host cells, as they depend on the host's metabolic and genetic machinery to make copies of themselves, since they cannot produce energy or proteins on their own. In contrast, they cannot multiply in nutrient-rich culture media, because unlike bacteria, they do not grow simply by using nutrients but require a living cell to reproduce. Similarly, viruses cannot replicate on the surface of the skin, as the skin does not provide the internal cellular environment necessary for viral multiplication, even though viruses may temporarily survive there. They also cannot reproduce in dead tissue, since the cellular activities needed for replication stop once the tissue is no longer alive.

3015. Which of these are the largest animal viruses?

• A. HIV
• B. Poxviruses
• C. COVID-19
• D. HBV

Explanation: HIV (Human Immunodeficiency Virus) measures about 100-120 nm in diameter. Poxvirus is the largest among them, with a size of about 200-400 nm. The COVID-19 virus (SARS-CoV-2) has a diameter of about 80-120 nm, while HBV (Hepatitis B virus) is the smallest, measuring only about 42 nm.

3016. Viruses do not have:

• A. Fossil record
• B. Traces in history
• C. Reproductive ability
• D. Nucleic acid

Explanation: Viruses do not have a fossil record because they lack hard structures such as bones, shells, or cell walls that can be preserved over time. They are made up only of nucleic acids and protein coats, which are too small and fragile to leave traces in rocks or sediments. In addition, their tiny size and dependence on living cells make it nearly impossible for them to be fossilized. Instead of fossils, scientists study the evolutionary history of viruses through their genetic material and by comparing it with that of their hosts.

3017. Which of the following has no nucleic acid?

• A. Bacteria
• B. Virus
• C. Prions
• D. Viroid

Explanation: Among bacteria, viruses, prions, and viroids, the only entity that does not contain nucleic acid is the prion. Bacteria have both DNA and RNA, viruses have either DNA or RNA, and viroids have only RNA. Prions, being just proteins without any genetic material, are the exception.

3018. Virus transmission is affected by:

• A. Biotic factors
• B. Chemical factors
• C. Physical factors
• D. All of these

Explanation: Chemical, physical, and biotic factors all directly influence virus transmission. Chemicals like disinfectants can inactivate viruses; physical conditions such as temperature, humidity, and radiation determine viral survival in the environment; and biotic factors such as vectors, immunity, and host density regulate how effectively viruses spread among living organisms. Therefore, these factors do significantly affect virus transmission, determining whether a virus persists, spreads quickly, or dies out in a given environment.

3019. Viruses are a _ entity between living and non-living.

• A. Balanced
• B. Transitional
• C. Threshold
• D. None of these

Explanation: Among the options, the most accurate term is "transitional," because viruses display features of both living and non-living entities: they are inert and crystalline outside the host but become active and reproduce inside it. The other terms ("balanced," "threshold," and "none of these") are less precise or incorrect in describing their unique status.

3020. When a virus attacks an unfamiliar organism, it mutates many times and comes as

• A. More virulent and dangerous
• B. More mutated
• C. More transmissible
• D. All of these

Explanation: When a virus attacks an unfamiliar organism, it undergoes repeated mutations. These mutations can lead to the virus becoming more virulent, more transmissible, or simply more mutated overall. Therefore, the most comprehensive answer is "All of these," since viruses can develop multiple new properties during adaptation to a new host.