IB Biology A2.3 Viruses (AHL) - Comprehensive Exam
2026
150 Questions with Detailed Rationales
SECTION A: STRUCTURAL FEATURES COMMON TO ALL VIRUSES
(Questions 1-25)
1. Which of the following structural features is common to ALL viruses?
A. A cell membrane derived from the host cell
B. Cytoplasm containing ribosomes for protein synthesis
C. Genetic material consisting of either DNA or RNA
D. Enzymes for ATP production
Rationale: C. All viruses possess genetic material (either DNA or RNA) as a core feature.
The genetic material may be single-stranded or double-stranded. Viruses lack cytoplasm,
do not have ribosomes, and possess few or no enzymes .
2. Which of the following is NOT a structural feature common to all viruses?
A. Nucleic acid (DNA or RNA) as genetic material
B. A protein capsid
C. A phospholipid bilayer envelope
D. No cytoplasm
Rationale: C. A phospholipid bilayer envelope is NOT present in all viruses; only some
viruses are enveloped (e.g., coronaviruses, HIV) while others are non-enveloped (e.g.,
bacteriophage lambda). The nucleic acid genome, protein capsid, and lack of cytoplasm
are universal features .
3. What is the primary role of the protein capsid in a virus?
A. To carry out metabolic reactions
B. To protect the viral genetic material
,C. To synthesise viral proteins
D. To produce ATP for the virus
Rationale: B. The protein capsid protects the viral nucleic acid from degradation by
nucleases in the environment and facilitates attachment to host cells. Viruses do not carry
out metabolism, synthesise their own proteins, or produce ATP .
4. Why are viruses considered "obligate intracellular parasites"?
A. They can reproduce independently in any environment
B. They must rely on a host cell for energy, nutrition, and protein synthesis
C. They cause disease in all host cells they infect
D. They can only survive in specific temperature ranges
Rationale: B. Viruses are obligate intracellular parasites because they depend completely
on host cells for energy supply, nutrition, protein synthesis, and all other life functions.
Without a host cell, viruses cannot replicate or carry out any life processes .
5. What is the approximate size range of most viruses?
A. 1-10 nm
B. 20-300 nm
C. 500-1000 nm
D. 1-10 µm
Rationale: B. Most viruses are between 20 and 300 nanometres in diameter. This small size
allows them to pass through filters that trap bacteria and makes them invisible under light
microscopes .
6. A virus is described as having "no cytoplasm." What is the significance of this
feature?
A. It prevents the virus from being affected by antibiotics
B. It means the virus cannot carry out metabolic reactions independently
C. It allows the virus to be smaller than cells
D. All of the above
Rationale: D. The absence of cytoplasm means viruses cannot carry out metabolic
reactions, contributes to their small size, and helps explain why antibiotics are ineffective
against viruses. Viruses are acellular and rely entirely on host cell machinery .
7. Which type of genetic material can be found in viruses?
A. DNA only, always double-stranded
B. RNA only, always single-stranded
, C. Either DNA or RNA, which can be single-stranded or double-stranded
D. Both DNA and RNA simultaneously
Rationale: C. Viruses can have genomes of DNA or RNA, and these can be either single-
stranded or double-stranded. This diversity is a key feature of viruses; no virus contains
both DNA and RNA as genetic material .
8. Which of the following is a correct statement about viral enzymes?
A. All viruses contain numerous enzymes for replication
B. Viruses contain few or no enzymes of their own
C. Viruses have enzymes for ATP production
D. All viruses contain reverse transcriptase
Rationale: B. Viruses have few or no enzymes of their own. They rely on host cell enzymes
for most of their replication and biosynthesis. Retroviruses like HIV do contain reverse
transcriptase, but this is not universal .
9. What is the relationship between the viral genetic code and the host cell's
genetic code?
A. Viruses use a completely different genetic code
B. Viruses share the same genetic code as living organisms
C. Viruses do not use a genetic code
D. Viruses use a code that is only similar to bacteria
Rationale: B. Viruses share the same genetic code as all living organisms, which enables
them to hijack host cellular machinery for replication. This shared code allows viral nucleic
acid to be transcribed and translated by host cell ribosomes .
10. Which of the following is an example of a virus with an envelope?
A. Bacteriophage lambda
B. HIV
C. Both HIV and coronavirus
D. Only bacteriophage lambda
Rationale: C. Both HIV and coronaviruses have envelopes outside their capsids. HIV has an
envelope with spikes made of carbohydrates and proteins, and coronaviruses have a
spherical shape with an envelope and protein spikes .
11. What is the function of attachment proteins found on viruses?
A. To synthesise new viral proteins
B. To allow the virus to bind to specific host cells
2026
150 Questions with Detailed Rationales
SECTION A: STRUCTURAL FEATURES COMMON TO ALL VIRUSES
(Questions 1-25)
1. Which of the following structural features is common to ALL viruses?
A. A cell membrane derived from the host cell
B. Cytoplasm containing ribosomes for protein synthesis
C. Genetic material consisting of either DNA or RNA
D. Enzymes for ATP production
Rationale: C. All viruses possess genetic material (either DNA or RNA) as a core feature.
The genetic material may be single-stranded or double-stranded. Viruses lack cytoplasm,
do not have ribosomes, and possess few or no enzymes .
2. Which of the following is NOT a structural feature common to all viruses?
A. Nucleic acid (DNA or RNA) as genetic material
B. A protein capsid
C. A phospholipid bilayer envelope
D. No cytoplasm
Rationale: C. A phospholipid bilayer envelope is NOT present in all viruses; only some
viruses are enveloped (e.g., coronaviruses, HIV) while others are non-enveloped (e.g.,
bacteriophage lambda). The nucleic acid genome, protein capsid, and lack of cytoplasm
are universal features .
3. What is the primary role of the protein capsid in a virus?
A. To carry out metabolic reactions
B. To protect the viral genetic material
,C. To synthesise viral proteins
D. To produce ATP for the virus
Rationale: B. The protein capsid protects the viral nucleic acid from degradation by
nucleases in the environment and facilitates attachment to host cells. Viruses do not carry
out metabolism, synthesise their own proteins, or produce ATP .
4. Why are viruses considered "obligate intracellular parasites"?
A. They can reproduce independently in any environment
B. They must rely on a host cell for energy, nutrition, and protein synthesis
C. They cause disease in all host cells they infect
D. They can only survive in specific temperature ranges
Rationale: B. Viruses are obligate intracellular parasites because they depend completely
on host cells for energy supply, nutrition, protein synthesis, and all other life functions.
Without a host cell, viruses cannot replicate or carry out any life processes .
5. What is the approximate size range of most viruses?
A. 1-10 nm
B. 20-300 nm
C. 500-1000 nm
D. 1-10 µm
Rationale: B. Most viruses are between 20 and 300 nanometres in diameter. This small size
allows them to pass through filters that trap bacteria and makes them invisible under light
microscopes .
6. A virus is described as having "no cytoplasm." What is the significance of this
feature?
A. It prevents the virus from being affected by antibiotics
B. It means the virus cannot carry out metabolic reactions independently
C. It allows the virus to be smaller than cells
D. All of the above
Rationale: D. The absence of cytoplasm means viruses cannot carry out metabolic
reactions, contributes to their small size, and helps explain why antibiotics are ineffective
against viruses. Viruses are acellular and rely entirely on host cell machinery .
7. Which type of genetic material can be found in viruses?
A. DNA only, always double-stranded
B. RNA only, always single-stranded
, C. Either DNA or RNA, which can be single-stranded or double-stranded
D. Both DNA and RNA simultaneously
Rationale: C. Viruses can have genomes of DNA or RNA, and these can be either single-
stranded or double-stranded. This diversity is a key feature of viruses; no virus contains
both DNA and RNA as genetic material .
8. Which of the following is a correct statement about viral enzymes?
A. All viruses contain numerous enzymes for replication
B. Viruses contain few or no enzymes of their own
C. Viruses have enzymes for ATP production
D. All viruses contain reverse transcriptase
Rationale: B. Viruses have few or no enzymes of their own. They rely on host cell enzymes
for most of their replication and biosynthesis. Retroviruses like HIV do contain reverse
transcriptase, but this is not universal .
9. What is the relationship between the viral genetic code and the host cell's
genetic code?
A. Viruses use a completely different genetic code
B. Viruses share the same genetic code as living organisms
C. Viruses do not use a genetic code
D. Viruses use a code that is only similar to bacteria
Rationale: B. Viruses share the same genetic code as all living organisms, which enables
them to hijack host cellular machinery for replication. This shared code allows viral nucleic
acid to be transcribed and translated by host cell ribosomes .
10. Which of the following is an example of a virus with an envelope?
A. Bacteriophage lambda
B. HIV
C. Both HIV and coronavirus
D. Only bacteriophage lambda
Rationale: C. Both HIV and coronaviruses have envelopes outside their capsids. HIV has an
envelope with spikes made of carbohydrates and proteins, and coronaviruses have a
spherical shape with an envelope and protein spikes .
11. What is the function of attachment proteins found on viruses?
A. To synthesise new viral proteins
B. To allow the virus to bind to specific host cells
