AQA AS Level Biology Paper 1, November 2020: Question 6
7 marks · Medium difficulty · Short Answer
Describe transcription of mRNA, define an exon, translate a DNA sequence into amino acids using a provided codon table, and evaluate a mutation type.
Practise this questionQuestion
Question text
06.1 Describe how mRNA is produced from an exposed template strand of DNA.
Do not include DNA helicase or splicing in your answer.
[3 marks]
06.2 Define the term exon.
[1 mark]
Table 3 shows mRNA codons for some amino acids.
Table 3
Serine Proline Glycine Threonine Alanine
UCU CCU GGA ACU GCA
UCC CCA GGG ACC GCG
06.3 Figure 6 shows the DNA template nucleotide base sequence that determines the
sequence of four amino acids.
Figure 6
AGG CGT CCT GGA
Use information from Table 3 and Figure 6 to give the amino acid sequence
determined by this sequence of nucleotides.
[1 mark]
06.4 A mutation in the nucleotide sequence shown in Figure 6 resulted in the following
amino acid sequence.
Serine Glycine Glycine Proline
A student concluded that the mutation involved the addition of one nucleotide within
the sequence shown in Figure 6. Does information in this question support the
student’s conclusion? Give reasons for your answer.
[2 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
06.1 1. (Free RNA) nucleotides form complementary 3 1. Accept A-U, G-C
base pairs; OR combination of
those pairs
2. Phosphodiester bonds form;
3. By (action of) RNA polymerase; 2. Accept linkages
for ‘bonds’
06.2 Base/nucleotide/triplet sequence coding for 1
polypeptide/sequence of amino acids/primary
structure;
06.3 Serine Alanine Glycine Proline; 1 Must be in this order
Accept Ser Ala Gly
Pro / S A G P
06.4 Mark as pairs. 1 and 2 or 3 and 4. 2
(No)
1. G to C (in the second codon/for Glycine)
OR
CGT to CCT (in second codon/for Glycine); 2. Reject if
2. (So), substitution (not addition mutation); substitution of Glycine
(for Alanine)
OR
3. (If addition) frameshift / all triplets moved;
4. (So), affects more than one amino acid;
TOTAL 7
How to answer it
Transcription, Exons, and Gene Mutations
What this question tests
This sequence assesses your core understanding of protein synthesis, specifically transcription (mRNA production), the definition of coding regions (exons), translating DNA triplets into amino acid sequences using genetic code tables, and analyzing the structural impact of gene mutations on reading frames.
Question 06.1
Describe how mRNA is produced from an exposed template strand of DNA. (3 marks)
✅ Correct Answer (Mark Scheme)
- (Free RNA) nucleotides form complementary base pairs with the DNA template strand.
- Phosphodiester bonds form between adjacent nucleotides.
- Catalysed by the action of RNA polymerase.
💡 Key Knowledge
- Remember base pairing rules during transcription: Adenine pairs with Uracil (A-U) and Cytosine pairs with Guanine (G-C).
- RNA polymerase joins nucleotides together via condensation reactions, creating phosphodiester bonds.
🧠 Exam Technique
Read the negative constraints carefully! The question explicitly states: "Do not include DNA helicase or splicing in your answer." Mentioning helicase here will immediately lose you marks as it describes DNA replication or initial strand separation, not the transcription process requested.
❌ Common Errors
- Stating that Thymine (T) pairs with Adenine (A) in RNA (RNA contains Uracil instead of Thymine).
- Confusing phosphodiester bonds with hydrogen bonds. Hydrogen bonds form between bases, but phosphodiester bonds form the backbone.
Question 06.2
Define the term exon. (1 mark)
✅ Correct Answer
A base/nucleotide/triplet sequence coding for a polypeptide / sequence of amino acids / primary structure.
💡 Key Knowledge
Exons are the coding regions of a gene (unlike introns, which are non-coding sequences removed during pre-mRNA splicing).
❌ Common Errors
Vague definitions such as "a section of DNA" without mentioning that it codes for amino acids/polypeptides will not gain credit.
Question 06.3
Use information from Table 3 and Figure 6 to determine the amino acid sequence. (1 mark)
✅ Correct Answer
Serine Alanine Glycine Proline
🧠 Exam Technique & Working
First, transcribe the DNA template strand sequence ( AGG CGT CCT GGA ) into complementary mRNA codons, remembering A pairs with U, T with A, G with C:
- DNA: AGG → mRNA: UCC → Serine
- DNA: CGT → mRNA: GCA → Alanine
- DNA: CCT → mRNA: GGA → Glycine
- DNA: GGA → mRNA: CCU → Proline
Question 06.4
Evaluate the student's conclusion regarding the mutation. (2 marks)
✅ Correct Answer (Mark Scheme Pairs)
No, the conclusion is not supported. Award marks in pairs (1 & 2 OR 3 & 4):
- Pair 1 & 2: There is a G to C mutation (or CGT to CCT) in the second codon for glycine, therefore it is a substitution mutation, not an addition.
- Pair 3 & 4: An addition mutation would cause a frameshift / shift all triplets downstream, which would affect more than one amino acid.
🧠 Examiner Commentary
Top-level responses systematically compared the original amino acid sequence with the mutated sequence. They identified that only one amino acid changed (Alanine became Glycine), which proves a substitution occurred. An addition would have altered every subsequent triplet due to a frameshift.
❌ Common Errors
Failing to state "No" at the beginning of the answer, or failing to explicitly explain why an addition mutation produces a different outcome (frameshift affecting multiple amino acids).
Topics
Biology · 3.4 Genetic information, variation and relationships between organisms
Question and mark scheme from the AQA AS Level Biology examination, Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.