AQA A-Level Biology Paper 1, November 2021: Question 3
7 marks · Medium difficulty · Short Answer
Describe translation, explain how an amino acid replacement changes protein properties using a genetic code table, and complete codons/triplets for a substitution mutation.
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Question text
03.1 Describe how one amino acid is added to a polypeptide that is being formed at a
ribosome during translation.
[3 marks]
Table 2 shows:
• mRNA codons and the amino acid coded for by each codon
• the type of bond formed by the R group of some of the amino acids.
Table 2
03.2 Crystallin is a structural protein found in the human eye. An inherited disease that
leads to blindness is caused by changes in properties of crystallin. The replacement
of the amino acid Arg with the amino acid Gly causes these changes.
Use information in Table 2 to suggest why this amino acid replacement changes the
properties of crystallin.
[2 marks]
03.3 The amino acid replacement of Arg with Gly is caused by a single base substitution
mutation in the DNA. The non-mutant DNA triplet is TCC.
Complete Table 3.
Give:
• the mRNA codon complementary to the non-mutant DNA triplet
• the mutated mRNA codon that could cause the change from Arg to Gly in the
crystallin protein
• the DNA triplet complementary to this mutated mRNA codon.
[2 marks]
Table 3
mRNA codon for the non-mutant triplet
Mutated mRNA codon
Mutated DNA triplet
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
03.1 1. tRNA brings specific amino acid (to ribosome);
2. Anticodon (on tRNA) binds to codon (on
mRNA);
3. Amino acids join by condensation reaction
(using ATP) 3
OR
Amino acids join to form a peptide bond (using
ATP);
03.2 1. Hydrogen bonds form instead of ionic bonds; 2. Ignore reference to
2 active site
2. Changes the tertiary structure (of the crystallin);
03.3
3 correct = 2 marks;;
2 correct = 1 mark;
0 or 1 correct = 0 marks
mRNA codon for the non-mutant triplet AGG 2
Mutated mRNA codon GGG
Mutated DNA triplet CCC
How to answer it
A-Level Biology Study Guide: Translation & Mutations
What this question tests
This question assesses your understanding of protein synthesis (specifically translation at the ribosome), how changes in amino acid R-groups impact tertiary protein structure, and how DNA base substitution mutations alter genetic codes. You will need to apply data from a genetic code table and demonstrate accurate complementary base pairing rules.
Describing Translation and Peptide Bond Formation
✅ Correct Answer & Mark Scheme
- Point 1: tRNA brings a specific amino acid to the ribosome.
- Point 2: The anticodon on tRNA binds to the complementary codon on mRNA.
- Point 3: Amino acids join together via a condensation reaction to form a peptide bond (requiring ATP).
💡 Key Knowledge
- Translation relies heavily on tRNA molecules acting as adapters with specific anticodons matching mRNA codons.
- Peptide bond formation is an endergonic process that couples with the hydrolysis of ATP.
🧠 Exam Technique
Keep your sequence chronological: first the tRNA arrives, then base-pairing alignment happens, and finally bond formation occurs. Mentioning ATP or condensation secures the third mark.
❌ Common Errors
- Confusing transcription (nucleus) with translation (ribosome).
- Stating that DNA enters the ribosome rather than mRNA.
- Forgetting to state that a condensation reaction occurs when forming peptide bonds.
Amino Acid Substitution & Tertiary Structure
✅ Correct Answer & Mark Scheme
- Mark 1: Hydrogen bonds form instead of ionic bonds (referencing Table 2 shading for Arg vs Gly).
- Mark 2: This changes the tertiary structure of the crystallin protein.
💡 Key Knowledge
- Arginine (Arg) has an R-group involved in ionic bonding, while Glycine (Gly) forms hydrogen bonds.
- Tertiary structure is maintained by interactions between R-groups (hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions). Changing an amino acid alters these forces and folds the protein differently.
🧠 Exam Technique
Always cross-reference visual keys and tables provided in the exam. Look directly at the shading key to see what bond types are associated with each amino acid mentioned in the stem.
❌ Common Errors
- Discussing "active sites" inappropriately (crystallin is a structural protein, not an enzyme!). Examiners explicitly instruct to ignore references to active sites here.
- Vaguely stating "the shape changes" without specifying tertiary structure.
Base Substitution Mutation & Table Completion
✅ Correct Answer Table
| mRNA codon for non-mutant triplet | AGG |
| Mutated mRNA codon | GGG |
| Mutated DNA triplet | CCC |
💡 Key Knowledge & Step-by-Step Logic
- Non-mutant DNA: Given as TCC . Using base-pairing rules (A-U, C-G for transcription), the complementary mRNA codon is AGG .
- Check Table 2: AGG codes for Arginine (Arg).
- Mutation requirement: The question asks to change Arg to Glycine (Gly). Looking at Table 2, Gly is coded by GGN (specifically GGG , GGU , GGC , GGA ).
- Single substitution: Changing the first base of AGG (A) to G yields the mutated mRNA codon GGG .
- Mutated DNA template triplet: Transcribing back from mRNA GGG gives the complementary DNA triplet CCC .
🧠 Exam Technique
Double-check your directionality. Ensure you clearly distinguish between DNA triplets and mRNA codons (remembering that RNA uses Uracil U instead of Thymine T ).
❌ Common Errors
- Mixing up T and U when translating back and forth between DNA and mRNA.
- Introducing a multi-base insertion or deletion instead of adhering to a single base substitution constraint.
Topics
Biology · 3.4 Genetic information, variation and relationships between organisms · 3.8 The control of gene expression (A-level only)
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.