AQA A-Level Biology Paper 1, June 2024: Question 6

8 marks · Medium difficulty · Short Answer

Compare the structure of mRNA and tRNA, use the genetic code table to determine amino acids from mRNA codons, and analyze the effect of a base substitution mutation on enzyme structure and function.

Practise this question

Question

Question 06 consists of three parts. Question 06.1 asks for three structural differences between an mRNA molecule and a tRNA molecule using a provided comparison table worth 3 marks. Question 06.2 provides a genetic code table (Table 3) and Figure 5 showing mRNA codons UUU, CGG, and GCG, asking students to give the corresponding amino acids in a 3-box table worth 1 mark. Question 06.3 shows Figure 6 with a mutated mRNA sequence (UUU, AGG, GCG) and asks students to name the type of mutation, give the change in DNA, and explain its effect on enzyme function, worth 4 marks with answer lines provided.
Question text

06.1 Give three structural differences between an mRNA molecule and a tRNA molecule.

[3 marks]

mRNA tRNA

06.2 Table 3 shows mRNA codons and the amino acid coded by each codon.

Table 3

Figure 5 shows the mRNA base sequence produced when part of a gene coding for

an enzyme is transcribed.

Figure 5

UUU CGG GCG

Use Table 3 to give the amino acids coded by the mRNA base sequence in Figure 5.

[1 mark]

06.3 A mutation occurred in the part of the gene transcribed in Question 06.2.

Figure 6 shows the mRNA base sequence produced when the identical part in the

mutated gene is transcribed.

*16* Figure 6

UUU AGG GCG

The amino acids coded by this mRNA base sequence form part of the enzyme’s

active site.

Use all the information in this question to:

• name the type of mutation that occurred to produce the mutated gene

• give the change in DNA caused by this mutation

• explain the effect this mutation will have on the function of the enzyme.

[4 marks]

Type of mutation

Change in DNA

Explanation of effect on function of enzyme

Mark scheme

Show the mark scheme The mark scheme provides answers for questions 06.1, 06.2, and 06.3. For 06.1, it lists valid comparative differences such as codons vs anticodons, presence/absence of hydrogen bonds and amino acid binding sites, shape, and length (3 max). For 06.2, it gives the amino acid sequence Phe, Arg, Ala (1 mark). For 06.3, it accepts single base substitution, guanine to thymine (or G to T, GCC to TCC) change in DNA, and explains that since Arg is still coded for due to degeneracy, there is no change in primary/tertiary structure or the active site (4 marks).

Question Marking Guidance Mark Comments

Must be comparisons

mRNA tRNA

1. (Has) codon(s) (Has) anticodon; 3. Accept description

of binding site, eg

amino acid only

2. No hydrogen/H Has hydrogen/H

3 max bound to tRNA

bonds/base pairs bonds/base pairs;

06.1 (3 x 3. Accept mRNA

3. No amino acid Has amino acid AO1) cannot carry an amino

binding site binding site; acid, tRNA can

4. Linear/straight/not ‘Clover (leaf’ shape)/

folded folded;

5. Long/many Short/few nucleotides/

nucleotides/bases bases;

Phe, Arg, Ala; 1 Reject if order is

06.2 different

(1 x

AO2)

(Name of mutation) Marks can be

achieved in any

1. (Single base) substitution; section

(Change in DNA)

2. Guanine to thymine

OR 2. Reject thiamine

G to T

OR

GCC to TCC;

06.3 (4 x

(Explanation)

AO2)

3. (So) Arg (still) present 3. Reject amino acids

OR are formed

No change in amino acid;

4. (So) no change in primary structure

OR

(So) no change in tertiary structure

– A-LEVEL BIOLOGY – –

OR

(So) no change in active site (shape); 13

How to answer it

Nucleic Acids, Translation, and Gene Mutations

What this question tests

This question assesses your knowledge of the structural differences between RNA types (mRNA vs tRNA), your ability to use a genetic code lookup table (codon to amino acid translation), and your understanding of how gene mutations (specifically substitution) alter DNA sequences, primary structures, and final protein functions.

Question 06.1 • 3 Marks

Structural Differences Between mRNA and tRNA

✅ Acceptable Comparisons (Any 3)

  • Codons vs Anticodons: mRNA has codons; tRNA has an anticodon.
  • Hydrogen Bonding: mRNA has no hydrogen bonds / base pairs; tRNA has hydrogen bonds / base pairs.
  • Binding Sites: mRNA has no amino acid binding site; tRNA has an amino acid binding site.
  • Shape: mRNA is linear / straight / not folded; tRNA is cloverleaf shaped / folded.
  • Length: mRNA has more nucleotides / bases; tRNA has fewer nucleotides / bases.

🧠 Exam Technique

This is a comparison question. Examiners heavily penalize answers that write isolated facts about mRNA and tRNA without linking them directly. Always frame your points as direct contrasts (e.g., "mRNA has codons, whereas tRNA has anticodons").

Mark breakdown: 3 marks total (3 × AO1 recall and comparison).
Question 06.2 • 1 Mark

Translating mRNA Codons to Amino Acids

✅ Correct Answer

Phe, Arg, Ala

(Derived from reading the table for UUU → Phe, CGG → Arg, GCG → Ala)

❌ Common Errors

  • Writing amino acids out of order. The sequence must match the exact left-to-right reading frame of the mRNA strand.
  • Spelling mistakes that alter meaning (though minor phonetic spellings are sometimes accepted, use standard abbreviations from the table to be safe).
Mark breakdown: 1 mark for correct sequence (1 × AO2 application).
Question 06.3 • 4 Marks

Mutations and Enzyme Function

💡 Key Knowledge & Answers

  • Type of mutation: (Single base) substitution
  • Change in DNA: Guanine to thymine (or G to T, or GCC to TCC)
  • Explanation of effect: Arg is still present / no change in amino acid encoded; therefore, no change in primary or tertiary structure, and no change in the active site shape. Function remains normal.

🧠 Examiner Insights & Top-Level Responses

To secure all 4 marks, top-level responses systematically step through the causal chain: DNA change → Codon change → Amino acid consequence → Protein structure → Enzyme function. Due to the degeneracy of the genetic code, not all base substitutions change the resulting amino acid (silent mutation).

Mark breakdown: 4 marks available (4 × AO2 application). Common pitfall: writing "amino acids are formed" instead of focusing on the specific amino acid or primary structure.

Topics

Biology · 3.4 Genetic information, variation and relationships between organisms · 3.8 The control of gene expression (A-level only) · 3.5 Energy transfers in and between organisms (A-level only)

Question and mark scheme from the AQA A-Level Biology examination, Paper 1, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.