IB DP Biology: Origins of cells and viruses (HL) (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions cover IB Diploma Programme Biology sections A2.1 (Origins of cells) and A2.3 (Viruses) from the IB Biology guide, first assessment 2025. Every question is HL only. Read the study guide first and keep the revision notes beside you. Hub: IB DP Biology. Checklist: printable checklist. To find weak areas first, take the free 10-minute diagnostics.

Questions

1. State two ways in which conditions on early Earth differed from those today. [2]

2. Explain why viruses are classed as non-living. [3]

3. An electron micrograph of a virion has a scale bar of 25 mm that represents 100 nm. The virion measures 31 mm across.

(a) Calculate the magnification. [2] (b) Calculate the actual diameter of the virion in nm. [2]

4. Explain why the formation of a membrane-bound compartment was needed before the first cells could evolve. [3]

5. A research group runs three versions of a Miller-Urey type experiment and measures the amino acids collected after 5 days.

Set-up	Gases	Sparks	Amino acids (µmol)
A	methane, ammonia, hydrogen, water vapour	on	38
B	carbon dioxide, nitrogen, water vapour	on	3
C	methane, ammonia, hydrogen, water vapour	off	0.5

(a) Calculate how many times greater the yield in A is than in B, to 3 significant figures. [2] (b) Explain what set-up C shows. [2] (c) Using the data, evaluate the claim that the experiment shows how amino acids formed on early Earth. [3]

6. Explain why RNA, rather than DNA or protein, is presumed to have been the first genetic material. [4]

7. The last universal common ancestor (LUCA) is the most recent organism from which all living things descend.

(a) Outline two lines of evidence that LUCA existed. [3] (b) Explain why LUCA is not thought to have been the only form of life on early Earth. [2]

8. A rock sample contains an unstable isotope. The isotope has a half-life of 1.25 billion years, and 12.5% of the original amount remains.

(a) Calculate the age of the rock. [2] (b) Suggest two reasons why dating the first living cells is uncertain. [2]

9. Bacteriophage lambda infects a culture. After infection there are 4.0 × 10⁶ infected cells per mL. After lysis the culture holds 9.2 × 10⁸ phages per mL.

(a) Calculate the burst size. [2] (b) Compare the lytic and lysogenic cycles of lambda. [3]

10. A culture of 2000 bacterial cells is infected with lambda. After the infection, 340 cells carry a prophage. The culture is then exposed to ultraviolet light and 85% of these cells lyse.

(a) Calculate the percentage of cells that became lysogens. [1] (b) Calculate the number of cells that lyse after ultraviolet exposure. [2] (c) Explain why ultraviolet light causes lysis. [2] (d) Explain why these cells do not lyse without ultraviolet light. [2]

11. A retrovirus has a genome of 8400 nucleotides. The error rate of its reverse transcriptase is 4.0 × 10⁻⁵ per nucleotide per copy. One infected person produces 5.0 × 10⁹ new virions per day.

(a) Calculate the mean number of mutations per genome copy. [2] (b) Calculate the number of mutations per day in the new virions. [2] (c) Explain the consequences of this for drug treatment. [2]

12. Two influenza strains isolated three years apart differ at 9 of the 566 amino acids in a surface protein.

(a) Calculate the percentage difference, and the mean percentage change per year, to 3 significant figures. [2] (b) Explain how point mutation and reassortment of genome segments differ in their effect on influenza. [3] (c) Discuss the evidence that viruses have arisen more than once. [4]

Answers

1. No free oxygen, and so no ozone layer [1]; higher concentrations of carbon dioxide and methane, giving higher temperatures and more ultraviolet light reaching the surface [1]. [2] Examiner insight: Two distinct conditions are needed. “No oxygen” and “no ozone” are linked, so a second point should be separate, such as carbon dioxide, methane, temperature or ultraviolet light.

2. They have no cytoplasm, ribosomes or metabolism of their own, and few or no enzymes [1]; they cannot carry out functions such as respiration, growth or protein synthesis outside a host [1]; they reproduce only by using the host cell’s machinery [1]. [3] Examiner insight: Saying only “they need a host” gives one idea. Link each missing feature to a characteristic of life that a cell does for itself.

3. (a) Convert: 25 mm = 25 000 000 nm [1]; 25 000 000 ÷ 100 = ×250 000 [1] (b) Actual size = image ÷ magnification = 31 000 000 nm ÷ 250 000 [1] = 124 nm [1] Examiner insight: Write the unit conversion and the formula. A correct number with no working is less secure than a clear method, and a missing unit weakens the answer.

4. A boundary lets the inside chemistry differ from the outside [1]; molecules can be kept at high concentration so reactions are faster [1]; products are not lost by diffusion, and each compartment holds its own set of molecules, so selection can act on them [1]. [3] Examiner insight: “To protect the contents” is vague. State that the inside becomes different from the outside, then say what that allows.

5. (a) 38 ÷ 3 [1] = 12.7 times [1] (b) Set-up C is a control with no energy input [1]; the tiny yield shows that the sparks, not the gases alone, drive formation of most of the amino acids [1]. (c) Support: amino acids formed from simple gases only when energy was supplied [1]. Limitation: set-up B, without methane or ammonia, gave little, so the result depends on a strongly reducing mixture that may not match early Earth [1]; only small molecules formed, so the experiment shows what is possible, not what happened, and does not explain polymers or cells [1]. [7] Examiner insight: An evaluation must end in a verdict, not just a set of points. Finish with a sentence such as “plausible for amino acids, unproven for early Earth”.

6. RNA can be replicated by complementary base pairing, so it can carry heritable information [1]; RNA can also act as a catalyst, so it can function as both gene and enzyme [1]; this avoids the problem that DNA needs protein enzymes to replicate while protein needs DNA to be made [1]; ribozymes in the ribosome still catalyse peptide bond formation, which supports the idea [1]. [4] Examiner insight: Naming both roles, genetic material and catalyst, is the core of the answer. Saying only “RNA is simpler” gives no checkable reason.

7. (a) The genetic code is almost universal, so the same codons specify the same amino acids across life [1]; many genes, such as those for ribosome components, are shared by all organisms [1]; these similarities are better explained by common descent than by separate origins [1]. (b) Other forms of life may have evolved [1], but probably became extinct through competition with LUCA and its descendants [1]. [5] Examiner insight: For “outline two lines of evidence”, name the evidence and say what it suggests. Leaving the inference out leaves a bare fact.

8. (a) 12.5% is one eighth, which is three half-lives [1]; 3 × 1.25 = 3.75 billion years [1] (b) Very old rocks are rare or have been altered [1]; single-cell fossils are small and hard to identify as evidence of life, and the first protocells did not fossilise [1]. [4] Examiner insight: Give two different reasons. Repeating “the evidence is old” in two ways adds little credit.

9. (a) Burst size = phages released ÷ infected cells [1] = 9.2 × 10⁸ ÷ 4.0 × 10⁶ = 230 [1] (b) Both begin with attachment and injection of viral DNA [1]; in the lytic cycle the DNA is copied at once, new phages assemble and the cell bursts [1]; in the lysogenic cycle the DNA integrates as a prophage and passes to daughter cells, with the host surviving until induction [1]. [5] Examiner insight: A comparison needs similarities or differences stated side by side. Describing each cycle in separate paragraphs, without a link, does not compare.

10. (a) 340 ÷ 2000 × 100 = 17% [1] (b) 340 × 0.85 [1] = 289 cells [1] (c) Ultraviolet light damages DNA [1]; this triggers induction, so the prophage leaves the chromosome and the lytic cycle begins, ending in lysis [1]. (d) A repressor protein keeps lytic genes switched off [1]; the prophage is copied with the host chromosome at each division, so the host survives [1]. [7] Examiner insight: In part (b) show the multiplication. The answer is a whole number of cells, so give 289 and not a rounded 290.

11. (a) 8400 × 4.0 × 10⁻⁵ [1] = 0.34 (0.336) [1] (b) 5.0 × 10⁹ × 0.336 [1] = 1.7 × 10⁹ mutations per day [1] (c) Many different variants arise each day, so some are resistant to a drug [1]; resistant variants are selected and increase while a single drug is used, so combinations of drugs are needed [1]. [6] Examiner insight: A consequence should link cause to effect: more variants, then selection under the drug, then resistance. Stating only “HIV mutates” is a description.

12. (a) 9 ÷ 566 × 100 = 1.59% [1]; 1.59 ÷ 3 = 0.530% per year [1] (b) Point mutations change the surface protein slightly and gradually, so immunity from earlier infection or vaccine fits less well over time [1]; reassortment mixes whole segments between strains, which can produce a sudden large change [1]; either way vaccines must be updated, because memory cells made against the old protein respond less well [1]. (c) Viruses have very different genomes, shapes and replication methods, which suggests several origins [1]; they share an extreme form of obligate parasitism, so common features such as a capsid and small size may be convergent evolution [1]; the genetic code is shared with living organisms, which fits origins from or alongside cells [1]; the evidence is indirect, so a single origin cannot be completely ruled out [1]. [9] Examiner insight: A discuss answer should include both evidence for and a point of caution. Convergent evolution is a reason that shared features do not prove one origin.

Where marks are usually lost

• Describing viruses as living because they carry genes or reproduce.
• Giving amino acid formation as proof of the origin of life instead of evidence that carbon compounds can form spontaneously.
• Writing “to protect the contents” for a membrane’s role, with no mention of different internal chemistry.
• Missing the conversion between mm and nm in magnification.
• Confusing LUCA with the first living cell.
• Describing the lysogenic cycle without naming the prophage or the repressor.
• Using “mutation” for influenza reassortment, which is a mixing of segments.
• Giving a list for evaluate and discuss questions with no judgement.

Next steps

• Revise with the revision notes and re-read the study guide.
• Go to the IB DP Biology hub or tick off topics on the printable checklist.
• Take all free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Sections A2.1 and A2.3 (additional higher level).