AQA A-Level Biology Paper 1, November 2021: Question 9

7 marks · Medium difficulty · Practical Techniques & Data Analysis

Complete Table 7 with ticks to show elemental composition, use Figure 11 to calculate generation time, and identify the supported model of DNA replication from experimental density results.

Practise this question

Question

Question text

09.1 Complete Table 7 with ticks ( ) to show which elements are found in the following

biological molecules.

[2 marks]

Table 7

Element

Biological molecules

Carbon Nitrogen Oxygen Phosphorus

Galactose

Phospholipid

RNA

Sucrose 30

After Watson and Crick proposed the model of DNA structure, scientists investigated

the possible mechanisms for DNA replication.

Two scientists grew a bacterial population, providing them with a nitrogen source

containing only the heavy isotope of nitrogen, 15 N. As soon as all the DNA in this

population contained 15 N, the scientists changed the nitrogen source to one

containing only the lighter isotope of nitrogen, 14 N. They changed the nitrogen source

at 0 hours.

During the investigation, the scientists measured the size of the population of bacterial

cells.

Figure 11 shows the scientists’ results.

Figure 11

09.2 The generation time for a population of bacteria is the time taken for all the bacteria to

divide once by binary fission.

Use Figure 11 and the following equation to calculate the generation time for this

population of bacteria. Give your answer in hours.

[2 marks]

size of population at time +4 hours

log10 � �

size of population at time – 4 hours

Number of generations =

log10 2

Generation time hours

At intervals during this investigation, the scientists removed samples of the bacterial

population, isolated the DNA and measured the density of the DNA.

DNA made using 15 N has a higher density than DNA made using 14 N.

Figure 12 shows the scientists’ results.

Figure 12

There are three possible models of DNA replication.

These models are shown in Figure 13.

Figure 13

09.3 Which of these models, P, Q or R, is supported by the results shown in Figure 12?

Give the letter and name of the model supported and explain why the results do not

support the other models.

[3 marks]

Model

Name

Explanation for first

unsupported model

Explanation for second

unsupported model

Mark scheme

Show the mark scheme

Question Marking Guidance Mark Comments

4 rows correct = 2 marks;;

09.1

2 or 3 rows correct = 1 mark;

0 or 1 row correct = 0 marks

Biological Element

molecules Carbon Nitrogen Oxygen Phosphorous 2

Galactose ✓ ✓

Phospholipid ✓ ✓ ✓

RNA ✓ ✓ ✓ ✓

Sucrose ✓ ✓

09.2 Correct answer for 2 marks, 0.8376308/0.84/0.8 Accept correct

(hours);; rounding to any

number of decimal

Accept for 1 mark, places

Evidence of 4 x 10 and 3 x 10 (written in any

format, for correct readings from graph)

Evidence of 9.550746785 (correct number of

generations)

Evidence of 1.1938443348 (correct generations/

hour) 2

Evidence of 50.26 (correct generation time in

minutes)

Incorrect reading of graph, 3 x 106 and 2 x 109

Accept for 1 mark, calculation carried out correctly

Evidence of 9.380821784 (correct calculation of

number of generations) – A-LEVEL BIOLOGY – –

Evidence of 1.172602723 (correct calculation of

generations/ hour)

Evidence of 51.16822503 (correct calculation of

generation time in minutes)

Evidence of 0.8528037505 (correct calculation of

generation time in hours)

Incorrect reading of graph, 106.4 and 109.3 OR 106.3

and 109.2

Accept for 1 mark, calculation carried out correctly

Evidence of 9.633591475 (correct calculation of

number of generations)

Evidence of 1.204198934 (correct calculation of

generations/ hour)

Evidence of 49.82565445 (correct calculation of

generation time in minutes)

Evidence of 0.8304275742 (correct calculation of

generation time in hours)

Evidence of 0.83, with no other working – A-LEVEL BIOLOGY – –

09.3 1. (Model) Q and Accept answers 2.

(Name) Semi-conservative (replication); and 3. in either order

Explanation 19

Accept for ‘peak’,

2. (Model) P (is unsupported because) density OR

There should be two peaks in generation 1 distribution

(Only) one peak is shown in generation 1

There should be 3:1 (ratio) of peaks in

generation 2

There should not be an intermediate/15N 14N

peak in generation 1/2/3

The original/generation 0/15N peak should be in

generation 1/2/3;

3. (Model) R (is unsupported because)

3. Accept for ‘>2’,

There should be >2 peaks in generation 2/3 many OR several

There should be one wide/overlapping peak in

generation 3;

How to answer it

Biological Molecules & DNA Replication Study Guide

📌 What this question tests

This multi-part exam question assesses your recall of chemical elements in biological monomers and polymers, your ability to extract logarithmic data from growth curves to calculate bacterial generation times, and your understanding of Meselson and Stahl-style isotope-labelling experiments to distinguish between models of DNA replication.

Part 09.1: Elements in Biological Molecules

Completing the Element Checklist Table

✅ Correct Answer / Mark Scheme

Galactose: Carbon, Oxygen
Phospholipid: Carbon, Oxygen, Phosphorus (plus Hydrogen)
RNA: Carbon, Nitrogen, Oxygen, Phosphorus
Sucrose: Carbon, Oxygen

Marks: 4 rows correct = 2 marks | 2 or 3 rows correct = 1 mark | 0 or 1 row correct = 0 marks

💡 Key Knowledge

Galactose & Sucrose (Carbohydrates): Consist purely of C, H, and O.
Phospholipids: Contain a glycerol backbone, fatty acid tails, and a phosphate group (containing P and O).
RNA (Nucleic Acid): Built of nucleotides containing a pentose sugar (C, H, O), a nitrogenous base (N), and a phosphate group (P).

Part 09.2: Bacterial Generation Time Calculation

Using Logarithmic Growth Curves and Given Equations

📐 Step-by-Step Calculation

Find population at time +4 hours: Read off Figure 11 at t = +4 . Population is 1 × 10⁹ (or read accurately from your working graph).
Find population at time -4 hours: Read off Figure 11 at t = -4 . Population is 4 × 10⁶ .
Calculate Number of Generations:
log₁₀(1 × 10⁹ / 4 × 10⁶) ÷ log₁₀(2)
= log₁₀(250) ÷ 0.30103
= 2.3979 / 0.30103 = 7.965 generations (or based on exact curve intersections).
Calculate Generation Time:
Total time interval = 8 hours (from -4 to +4).
Generation time = 8 hours / Number of generations = ~0.84 hours (accept 0.8 to 0.84 hours).

❌ Common Errors & Exam Technique

Logarithmic Scale Traps: Misreading the minor gridlines on the logarithmic y-axis of Figure 11. Each major block represents a factor of 10.
Forgetting Units: Ensure your final answer has the correct unit ( hours ) as requested in the answer line.
Partial Credit: If your graph reading was slightly off, examiners awarded 1 mark for correct mathematical processing using incorrect values (error carried forward).

Marks: 2 marks total (1 for valid graph readings/intermediate values, 1 for final correct generation time).

Part 09.3: Models of DNA Replication

Interpreting Density Gradient Centrifugation Results (Meselson & Stahl)

✅ Correct Answer

Model: Q

Name: Semi-conservative (replication)

Marks: 3 marks total (1 for Model Q + Semi-conservative name, 1 for explanation of why P is unsupported, 1 for explanation of why R is unsupported).

🧠 Examiner Commentary & Technique

Why P (Conservative) is unsupported: Generation 1 shows a single intermediate density peak, whereas conservative replication would predict two separate peaks (one entirely heavy ¹⁵N and one entirely light ¹⁴N ). Alternatively, point out that generation 2 should exhibit a 3:1 ratio of peaks.
Why R (Dispersive) is unsupported: Dispersive replication predicts a single peak that progressively shifts toward lighter densities with each generation as strands fragment and mix. Figure 12 clearly shows distinct separation into two separate peaks by Generation 2.

Topics

Biology · 3.1 Biological molecules

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.