AQA A-Level Biology Paper 1, June 2025: Question 2
8 marks · Medium difficulty · Extended Answer
Describe mRNA splicing and RNA polymerase action, identify activation energy from a reaction profile, and compare historical enzyme conclusions with the induced-fit model.
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Question text
02.1 Describe how mRNA is obtained from pre-mRNA.
[1 mark]
02.2 Describe how RNA polymerase joins RNA nucleotides.
[2 marks]
02.3 Figure 1 shows the energy contained in molecules during an enzyme-controlled
reaction.
Figure 1
Which expression represents the activation energy (Ea) of this reaction?
Tick ( ) one box.
[1 mark]
A – C
D – C
*03D – C*
B
D 5
02.4 In 1914, a group of scientists investigated enzyme action.
After studying their results, the scientists reached the following conclusions:
An enzyme is a substance which causes a reaction to occur at “ordinary”
temperatures by being present, but not by its ability to combine with another
substance. An enzyme changes the rate of a reaction without itself being changed
by that reaction.
Our current understanding of enzyme action is based on the induced-fit model.
Describe similarities and differences between the conclusions from 1914 and our
current understanding of enzyme action.
[4 marks]
Similarities
Differences
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
Introns removed (and exons joined); Accept description of
1 non-coding DNA for
02.1 (1 x ‘introns’
AO1)
1. Reject if
1. Phosphodiester bond;
phosphodiester bond is
between incorrect
2. (By) condensation (reaction); 2
molecules, eg joins
02.2 (2 x
AU/GC OR
AO1)
complementary
nucleotides/bases
(D – C); 1
02.3 (1 x
AO1)
Similarities
1. Enzymes are not changed/can be
reused (in reactions);
2. Increase rate of reaction at low/ordinary/ normal/room
temperature
OR
(Causes/allows) reaction(s) at
low/ordinary/normal/room temperature
02.4 (4 x
OR AO2)
(Both) lower activation energy;
Differences
3. Active site changes shape (in induced fit model);
4. Enzyme-substrate complex forms 4. Accept ‘E-S complex’
4. Ignore ‘ESC’
OR
Enzyme/active site and substrate bind/touch;
How to answer it
RNA Splicing, Transcription & Enzyme Action
- Post-transcriptional modification: Removal of non-coding introns to yield functional mRNA (splicing).
- Enzyme mechanics in transcription: The specific role of RNA polymerase in catalyzing condensation reactions that form phosphodiester bonds.
- Reaction coordinate energy diagrams: Calculating activation energy (Ea) from graphical coordinates.
- Models of enzyme action: Evaluating historical beliefs against modern knowledge (induced-fit model and enzyme-substrate complexes).
Post-Transcriptional Modification of mRNA
Describe how mRNA is obtained from pre-mRNA. [1 mark]
✅ Mark Scheme Answer
- Introns are removed (and exons are joined / spliced together). 1 mark
💡 Key Knowledge
- In eukaryotes, transcription produces pre-mRNA containing both coding sequences (exons) and non-coding sequences (introns).
- Splicing takes place inside the nucleus before mRNA exits via nuclear pores to the ribosomes.
🧠 Exam Technique
The question asks "how mRNA is obtained", meaning you need the process of modification. Mentioning that non-coding sections (introns) are removed is essential. You can also gain credit by explaining that remaining exons are joined together.
❌ Common Errors
- Confusing introns with exons (e.g., stating exons are cut out).
- Just saying "splicing" without stating what is actually removed or joined.
- Referring to DNA replication or translation instead of RNA processing.
Mechanism of RNA Polymerase
Describe how RNA polymerase joins RNA nucleotides. [2 marks]
✅ Mark Scheme Answer
- Forms phosphodiester bonds (between adjacent RNA nucleotides). Mark 1
- By a condensation reaction. Mark 2
💡 Key Knowledge
- RNA polymerase joins adjacent ribose nucleotides along the sugar-phosphate backbone (5' carbon of one ribose to the 3' carbon of another).
- A molecule of water (H₂O) is eliminated in every condensation step.
🧠 Exam Technique
Always distinguish between phosphodiester bonds (within the sugar-phosphate backbone) and hydrogen bonds (between complementary nitrogenous base pairs). RNA polymerase catalyses phosphodiester bond formation, NOT hydrogen bond formation.
❌ Examiner Pitfalls to Avoid
- Immediate Reject: Stating RNA polymerase joins "A to U" or "G to C" or forms bonds between complementary bases. Hydrogen bonds form spontaneously between bases without enzyme action.
- Forgetting to state the reaction type: always state condensation when polymerisation is discussed.
Determining Activation Energy from a Reaction Profile
Which expression represents the activation energy (Ea) of this reaction? [1 mark]
✅ Correct Answer
Tick box: D – C 1 mark
📐 Step-by-Step Graphical Breakdown
- Definition of Ea: The minimum energy required to start the reaction = Peak Energy – Energy of Reactants (ER).
- Identify arrow D: Extends from the transition state (peak) all the way down to the Energy of Products (EP).
- Identify arrow C: Extends from the Energy of Reactants (ER) down to the Energy of Products (EP) (overall ΔH).
- Deduct C from D:
(Peak – EP) – (ER – EP) = Peak – ER = Ea .
🧠 Exam Technique
Always draw dashed construction lines across the energy axes to trace what the ends of each arrow line align with. Since no single arrow spanned from the ER line to the peak, you must find a difference between two arrows.
❌ Common Distractors
- A – C: Incorrect because A represents the time coordinate line, not an energy distance.
- (D – C) / B: Confuses rate calculation (energy / time) with activation energy itself.
- D: Represents the reverse reaction's activation energy, not the forward reaction.
Historical Science vs Modern Induced-Fit Model
Describe similarities and differences between the conclusions from 1914 and our current understanding of enzyme action. [4 marks]
| Aspect | 1914 Conclusion | Current Understanding (Induced-Fit) |
|---|---|---|
| Enzyme recovery | "without itself being changed" | Enzymes are unchanged at the end of the reaction / can be reused. |
| Temperature / Energy | "causes a reaction to occur at 'ordinary' temperatures" | Enzymes increase rate at room/body temp by lowering activation energy. |
| Substrate binding | "not by its ability to combine with another substance" | Substrate binds to active site to form an enzyme-substrate complex. |
| Active site conformation | (Implicitly static/no interaction) | The active site changes shape to mould snugly around the substrate. |
✅ Mark Scheme Points (Any 4 of the following)
Similarities (Max 2 marks):
- Enzymes are not changed / can be reused (in reactions). Mark 1
- Increase rate of reaction at low / ordinary / normal / room temperature OR lower activation energy. Mark 2
Differences (Max 2 marks):
- Active site changes shape (in induced-fit model). Mark 3
- Enzyme-substrate complex forms (accept 'E-S complex') OR enzyme/active site and substrate bind/touch. Mark 4
🧠 Exam Technique & Examiner Tips
- Extract directly from the prompt: The 1914 text explicitly stated enzymes act "not by its ability to combine with another substance". That is your biggest clue to contrast with modern enzyme-substrate complex (E-S complex) formation.
- Address both headings: Divide your answer clearly into "Similarities" and "Differences" as structured on the exam paper to ensure you hit both AO2 assessment objectives.
- Avoid lazy shorthand: Mark scheme states "Ignore ESC". Always write out enzyme-substrate complex or E-S complex fully!
Topics
Biology · 3.1 Biological molecules · 3.4 Genetic information, variation and relationships between organisms
Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.