AQA AS Level Biology Paper 2, November 2021: Question 4

8 marks · Medium difficulty · Short Answer

Identify common cell structures, explain the effect of amino acid concentration and antibiotic X on plasmid and circular DNA replication, and interpret ultracentrifugation results.

Practise this question

Question

A four-part exam question about prokaryotic cells, plasmid replication, and ultracentrifugation results. Question 04.1 asks for two structures found in all prokaryotic and eukaryotic cells. Question 04.2 asks to suggest an explanation for a faster rate of plasmid replication in cells with high amino acid concentration. It includes a description of an experiment separating plasmids and circular DNA using ultracentrifugation, accompanied by Figure 6 which shows two centrifuge tubes labeled A (without antibiotic X) and B (with antibiotic X). Tubes A and B each show two distinct horizontal bands for plasmids and circular DNA molecules, with the band for plasmids appearing thicker in tube B. Questions 04.3 and 04.4 ask what can be concluded from Figure 6 about structural differences and the effect of antibiotic X.
Question text

04.1 Give two structures found in all prokaryotic cells and in all eukaryotic cells.

[2 marks]

All prokaryotic cells contain a circular DNA molecule and some prokaryotic cells

contain plasmids.

04.2 Scientists have found that the rate of plasmid replication is faster in cells growing in a

culture with a high concentration of amino acids than in a culture with a lower

concentration of amino acids.

Suggest one explanation for the faster rate of plasmid replication in cells growing in a

culture with a high amino acid concentration.

[2 marks]

A scientist prepared a culture of a bacterial species.

• She extracted the plasmids and the circular DNA molecules from a sample of cells

taken from this culture (A).

• She then added antibiotic X to the culture and let the cells divide for 4 hours.

• She then extracted the plasmids and the circular DNA molecules from a sample of

these cells (B).

• The scientist separated the plasmids from the circular DNA molecules in A and in B

using ultracentrifugation.

Figure 6 shows her results.

Figure 6

04.3 What can you conclude from Figure 6 about a structural difference between the

plasmids and the circular DNA? Explain your answer.

[2 marks]

04.4 What can you conclude from Figure 6 about the effect of antibiotic X on plasmid

replication and on circular DNA replication? Explain your answer.

[2 marks]

Mark scheme

Show the mark scheme The mark scheme for questions 04.1 through 04.4, providing marking guidance, allocated marks, and examiner comments. Question 04.1 awards marks for cell-surface membrane, ribosomes, cytoplasm, or DNA. Question 04.2 accepts paired points relating amino acids to protein/enzyme synthesis or respiration/ATP production for replication. Question 04.3 accepts that circular DNA is bigger, heavier, or denser because its band moved lower in the centrifuge tube. Question 04.4 states that plasmid replication continues or increases because the band is wider, while circular DNA replication stops or is not increased as the band remains identical.

Question Marking Guidance Mark Comments

04.1 1. Cell(-surface) membrane;

2. Ribosomes; 2. Ignore 70S

2 max

3. Cytoplasm;

4. DNA;

04.2 Mark in pairs: 1 and 2 OR 3 and 4 1. Accept for ‘protein

synthesis’,

1. (Amino acids used in) protein synthesis; translation

2. (So) more enzymes (for DNA/plasmid

replication)

OR 2

(So) more DNA polymerase;

3. (Amino acids used in) respiration;

4. (So) more energy/ATP (for DNA/plasmid

replication);

04.3 1. Circular DNA is bigger/heavier/denser; 1. and 2. Accept

converse for plasmids

2. (Because band) moved further/is lower (in

tube)/closer to bottom (of tube);

04.4 1. Plasmid replication continues/increases (with X) 1. Accept for

as band is wider; ‘replicate’, copy OR

reproduce

2. Circular DNA replication stops/not increased

(with X) as band is identical; 1. Accept for

2 ‘continues’, is not

affected

1. Accept for ‘wider’,

thicker OR ‘there

are more’

TOTAL 8

How to answer it

Prokaryotic Cells, Plasmids, and Ultracentrifugation

What this question tests

This exam question tests your core knowledge of cell ultrastructure (comparing prokaryotes and eukaryotes), factors affecting metabolic/replication rates, and your ability to interpret ultracentrifugation results (density gradients and band positions) in experimental biology.

Question 04.1

Shared Cell Structures

Give two structures found in all prokaryotic cells and in all eukaryotic cells. (2 marks)

✅ Correct Answers (Any two of):

  • Cell-surface membrane
  • Ribosomes
  • Cytoplasm
  • DNA
Mark breakdown: 2 marks total (1 mark per valid structure). Note: Ignoring references to specific sizes like 70S ribosomes is standard.

💡 Key Knowledge

Prokaryotic and eukaryotic cells share fundamental machinery required for basic survival, metabolic reactions, and holding genetic information, despite eukaryotes having membrane-bound organelles.

❌ Common Errors

  • Listing organelles unique to eukaryotes (e.g., mitochondria, nucleus, Golgi apparatus).
  • Listing structures unique to prokaryotes or specific kingdoms (e.g., cell wall, capsule, flagellum).
Question 04.2

Rate of Plasmid Replication

Suggest one explanation for the faster rate of plasmid replication in cells growing in a culture with a high amino acid concentration. (2 marks)

✅ Correct Answers (Marked in pairs 1+2 OR 3+4):

  • Pair 1 & 2: (Amino acids used in) protein synthesis ➔ (So) more enzymes (for DNA/plasmid replication) OR more DNA polymerase.
  • Pair 3 & 4: (Amino acids used in) respiration ➔ (So) more energy/ATP (for DNA/plasmid replication).
Mark breakdown: 2 marks. You must link the presence of amino acids to a consequence (protein/enzyme or respiration/ATP) and explicitly connect that consequence to plasmid replication.

🧠 Exam Technique

When an examiner asks for an explanation involving nutrient concentration, trace the mechanism step-by-step: Nutrient intake ➔ biochemical pathway ➔ final impact on the process asked about. Never leave the chain of reasoning incomplete!

Question 04.3

Ultracentrifugation & DNA Structure

What can you conclude from Figure 6 about a structural difference between the plasmids and the circular DNA? Explain your answer. (2 marks)

✅ Correct Answers:

  • 1. Conclusion: Circular DNA is bigger / heavier / denser than plasmids.
  • 2. Explanation: Because the band moved further / is lower down in the tube / closer to the bottom of the tube.
Mark breakdown: 1 mark for the correct structural conclusion and 1 mark for linking it to the visual evidence from the centrifuge tube diagram.

💡 Key Knowledge

Ultracentrifugation separates molecules based on their mass and density. In density gradient centrifugation, heavier, larger, or denser molecules pellet further down the tube compared to smaller molecules.

Question 04.4

Effect of Antibiotic X

What can you conclude from Figure 6 about the effect of antibiotic X on plasmid replication and on circular DNA replication? Explain your answer. (2 marks)

✅ Correct Answers:

  • 1. Plasmid replication: Continues / increases (with X) as the plasmid band is wider / thicker / more abundant.
  • 2. Circular DNA replication: Stops / is not increased (with X) as the circular DNA band is identical.
Mark breakdown: 1 mark for plasmid interpretation (citing band width/amount) and 1 mark for circular DNA interpretation (citing unchanged band appearance).

🧠 Exam Technique

Always compare the visual difference between control tubes (A) and treated tubes (B). Words like "wider", "thicker", or "unchanged/identical" are crucial observation terms that unlock marks in graphical or diagram-based questions.

Topics

Biology · Practical skills · 3.2 Cells · Data analysis

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.