IB Biology 2025 · Theme A · Unity and diversity

A2.3 Viruses (HL). Practice questions with markscheme.

67 original IB-style questions on A2.3, written from the 2025 guide: 29 multiple-choice, 23 short-answer, 9 data-based, 3 extended-response parts, 2 drawing, 1 labelling. This subtopic is additional higher level in its entirety, so below is a 22-mark higher-level practice paper built from them. Hand one out as a class quiz or homework, or sit it yourself and mark against the scheme. Print it, project it, or build a fresh one on the same topic.

What the guide asks for

All 6 statements are additional higher level, so this subtopic is HL only.

A2.3.1HL Structural features common to viruses
A2.3.2HL Diversity of structure in viruses
A2.3.3HL Lytic cycle of a virus
A2.3.4HL Lysogenic cycle of a virus
A2.3.5HL Evidence for several origins of viruses from other organisms
A2.3.6HL Rapid evolution in viruses

In the bank for A2.3

29 multiple-choice
23 short-answer
9 data-based
3 extended-response parts
2 drawing
1 labelling
67 higher level only

Every question is original and tagged to a guide statement. See the whole bank →

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The practice paper

Additional higher level only · 22 marks

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Biology · topic quiz

Higher level · topic practice, not an exam format

35 minutes22 marks

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Covers A2.3 Viruses · HL

Name:

The table shows the genetic material of six viruses and whether each has an envelope. Which conclusion is supported by these data?

[1]

Virus	Genetic material	Envelope
Herpes simplex virus	double-stranded DNA	present
Adenovirus	double-stranded DNA	absent
Bacteriophage φX174	single-stranded DNA	absent
Rotavirus	double-stranded RNA	absent
Rabies virus	single-stranded RNA	present
Norovirus	single-stranded RNA	absent

All enveloped viruses have genetic material made of RNA
Viruses with a double-stranded genome never have an envelope
Single-stranded genetic material is found only in RNA viruses
DNA viruses and RNA viruses can each be enveloped or not enveloped

A culture of Escherichia coli carrying the DNA of bacteriophage lambda as a prophage grows normally for many generations. When the culture is briefly exposed to ultraviolet light, most of the cells lyse within an hour and release phage particles. Which explains this?

[1]

Ultraviolet light damages the bacterial cell wall, allowing phages that were already assembled inside the cells to escape
Ultraviolet light kills the bacteria, and the prophage DNA is then released and assembles into phage particles on its own
Ultraviolet light converts the bacterial chromosome into phage DNA, so the cell produces phages instead of dividing
Ultraviolet light damages the host DNA, which triggers the prophage to leave the chromosome and enter the lytic cycle

Giant viruses such as the mimiviruses have genomes larger than those of some bacteria and carry genes for proteins involved in translation, although they still have no ribosomes.

(a)Comment on what this discovery implies for the regressive hypothesis, which proposes that viruses evolved from cells by loss of structures.[2]

(b)Suggest why the fossil record cannot be used to investigate the origin of viruses.[1]

(c)Suggest two observations that would support the idea that viruses have arisen more than once in evolution.[2]

HIV patients were treated either with a single antiviral drug (monotherapy) or with three drugs at once (combination therapy). The concentration of virus in the blood (viral load) was followed for 24 weeks. This question links viral biology (A) to natural selection (Theme D).

Week	Viral load: one drug / copies mL⁻¹	Viral load: three drugs / copies mL⁻¹
0	100 000	100 000
4	500	400
12	9 000	300
24	70 000	200

(a)Analyse the difference between the two treatments over the 24 weeks.[2]

(b)Explain the rebound in viral load under monotherapy in terms of mutation and natural selection.[3]

(c)Predict why using three drugs at once makes resistance far less likely to emerge, and justify your prediction.[2]

A culture of bacteria was mixed with bacteriophage T4 for two minutes to allow the viruses to attach to the cells, then diluted so that no further infections could occur. At intervals a sample was taken and the number of plaque-forming units (PFU) was counted. Each PFU is either a free virus particle or an infected bacterium that will release viruses. The graph shows the results.

(a)Calculate the mean rate of increase in PFU between 20 and 40 minutes after infection.[1]

(b)The experiment was repeated with bacteria that had been starved of nutrients, so that they made ATP and synthesized proteins much more slowly. Predict, with reasons, how the graph would differ.[2]

(c)The number of PFU rises over about 20 minutes rather than all at once. Suggest reasons for this.[2]

(d)Outline the events of the lytic cycle that occur between infection and the release of new viruses.[3]

Original practice questions © Biology by Bradford · CC BY-NC-SA 4.0 · Not affiliated with or endorsed by the International Baccalaureate Organization.
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Markscheme BbB-EAEQAAAAABQAOiLe

One mark per point; / separates alternative wording within a point, OR separates alternative answers, words in brackets are not required, underlined words are essential. OWTTE = or words to that effect.

1. A2.3.2 [1]

D: herpes simplex virus (DNA) and rabies virus (RNA) are enveloped, while adenovirus (DNA) and norovirus (RNA) are not, so the type of nucleic acid does not decide whether a virus has an envelope; A and B are contradicted by herpes simplex virus, an enveloped virus with double-stranded DNA; C is contradicted by bacteriophage φX174, which has single-stranded DNA;

2. A2.3.4 [1]

D: DNA damage is the signal that induces the prophage to excise and switch to the lytic cycle, producing new phages that lyse the cell; A is wrong because no phage particles exist in a lysogenic cell; B is wrong because dead cells cannot synthesize phage components; C describes a process that does not occur;

3. A2.3.5

(a) [2 max]
- genes for translation proteins are what would be expected in a lineage descended from a cell that has lost most of its own machinery, so the finding is consistent with the regressive hypothesis;
- however, the same genes could have been acquired from hosts by horizontal gene transfer, so the evidence is not conclusive;
- one hypothesis need not explain all viruses, since different viral groups may have arisen in different ways, OWTTE;
(b) [1]
- virus particles are far too small and have no hard parts, so they do not fossilize, OWTTE;
(c) [2 max]
- different viral groups share no genes in common, so no single ancestral virus can be reconstructed;
- some groups use DNA and others RNA, single- or double-stranded, which is hard to derive from one ancestor;
- viruses of bacteria, plants and animals show quite different structures and replication strategies, OWTTE;

4. A2.3.6

(a) [2]
- both fall sharply by week 4;
- under one drug the viral load rebounds/rises again after week 4, whereas under three drugs it stays low/keeps falling, OWTTE;
(b) [3]
- HIV mutates rapidly (reverse transcriptase lacks proofreading), so variant viruses arise, some resistant to the drug;
- the drug kills non-resistant viruses but resistant variants survive and reproduce (selection);
- the resistant variants increase in frequency, so the population recovers / viral load rises again, OWTTE;
(c) [2]
- a virus would need to carry mutations giving resistance to all three drugs simultaneously;
- the chance of these independent mutations occurring together in one virus is very small (product of the separate probabilities), so replication stays suppressed, OWTTE;

5. A2.3.3

(a) [1]
- (98 − 3) ÷ 20 = 4.75 × 10³ PFU cm⁻³ min⁻¹;
Accept 4.5 to 5.0 × 10³ PFU cm⁻³ min⁻¹. Units required.
(b) [2 max]
- the latent period would be longer / the rise would start later, because the phage relies on the host for energy/ATP and (host) ribosomes to make its proteins;
- the plateau / burst size would be lower, because the host supplies the nucleotides and amino acids (nutrition) from which new phages are built;
- the rise would be less steep / slower, because replication and assembly inside each cell would take longer, OWTTE;
(c) [2 max]
- the infected cells do not all lyse at the same moment / lysis is not synchronized;
- cells were infected at slightly different times during the two minutes of mixing;
- cells differ in size / stage / metabolic state, so phages are made faster in some cells than in others;
- different cells were infected by different numbers of phages, OWTTE;
(d) [3 max]
- the phage attaches to receptors on the bacterial cell wall / surface;
- the phage DNA is injected into the cell (the capsid stays outside);
- host enzymes / ribosomes are used to transcribe and translate viral genes and to replicate the viral DNA;
- new capsids are assembled around copies of the viral genome;
- the cell is lysed (by a virus-coded enzyme / lysozyme) and the new viruses are released;
Content pivot. Accept any three.

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