A-Level · Biology · AQA · Mark scheme decoded

AQA A-Level Biology: Gene Mutations and Their Effects on Polypeptides — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Biology specificationlast verified 3 July 2026

The short answer

In A-Level Biology, understanding gene mutations is crucial for grasping how genetic changes can affect the structure and function of proteins. This section delves into the types of gene mutations, their causes, and the resulting effects on the encoded polypeptide.

The question

A gene mutation results in the substitution of a base pair from A-T to G-C. Predict the effect on the encoded polypeptide if this change occurs within a codon.

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For identification questions, marks are typically awarded for correctly naming the type of mutation. For prediction and explanation questions, marks are given for clear and concise answers that demonstrate a deep understanding of the topic, including the effects on polypeptides and protein function.

What the command words demand

Identify: Recognize and name specific types of gene mutations based on given DNA sequence changes.
Predict: Forecast the effects of a mutation on the amino acid sequence and protein function.
Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Analyze: Examine and interpret data or scenarios to draw conclusions about the impact of mutations.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, analyze the DNA sequence changes, and provide well-reasoned answers.

Identify the original and mutated codons.1 mark
Original codon: A-T (e.g., ATG, which codes for methionine)Mutated codon: G-C (e.g., GTG, which codes for valine)
Determine if the change results in a different amino acid.1 mark
ATG → GTG: Methionine → Valine
Predict the effect on the polypeptide.2 marks
The substitution changes methionine to valine, which could alter the protein's structure and function.

Final answer: The mutation results in a different amino acid (valine) being incorporated into the polypeptide chain, potentially altering the protein's structure and function.

Work through every step correctly and you earn all 4 marks.

Another worked example

A gene mutation causes the deletion of one base pair. Predict the effect on the encoded polypeptide if this change occurs within a codon.

4 marks

Identify the original codon and the resulting frame shift.1 mark
Original codon: ATG (methionine)After deletion: TGA (stop codon) or other altered sequence
Determine if the change results in a different reading frame.1 mark
The deletion of one base pair shifts the reading frame, altering all downstream codons.
Predict the effect on the polypeptide.2 marks
A frame shift mutation typically leads to a completely different sequence of amino acids and often results in a non-functional protein.

Final answer: The deletion causes a frame shift, leading to a completely different sequence of amino acids and likely a non-functional protein.

Work through every step correctly and you earn all 4 marks.

Common mistakes

Confusing silent mutations with mis-sense mutations.
Why it happens: Students may not fully understand the difference between silent and mis-sense mutations, leading to incorrect predictions of their effects on polypeptides.
Fix: Remember that silent mutations do not change the encoded amino acid due to the degenerate nature of the genetic code, while mis-sense mutations result in a different amino acid being incorporated into the polypeptide chain.
Failing to recognize the severity of frame shift mutations.
Why it happens: Students might not realize that frame shift mutations can have severe consequences by altering the reading frame and leading to a completely different sequence of amino acids.
Fix: Understand that frame shift mutations, caused by additions or deletions of one or more bases, typically result in non-functional proteins due to the altered reading frame.
Misidentifying types of gene mutations.
Why it happens: Students may confuse different types of mutations, such as substitution and translocation, leading to incorrect answers in questions about mutation effects.
Fix: Practice identifying the specific type of mutation (addition, deletion, substitution, inversion, duplication, translocation) based on the given DNA sequence changes.
Overlooking the role of mutagenic agents in increasing mutation rates.
Why it happens: Students might not fully understand how physical, chemical, and biological agents can increase the rate of gene mutations.
Fix: Review the types of mutagenic agents (physical, chemical, biological) and their mechanisms for causing DNA changes.
Incorrectly predicting the effect of a nonsense mutation.
Why it happens: Students may not recognize that a nonsense mutation introduces a premature stop codon, leading to a truncated protein.
Fix: Understand that a nonsense mutation results in a premature stop codon, which truncates the polypeptide and often renders it non-functional.
Failing to explain the degenerate nature of the genetic code clearly.
Why it happens: Students may struggle to articulate why some mutations do not change the encoded amino acid, leading to vague or incorrect explanations.
Fix: Practice explaining that multiple codons can code for the same amino acid (degenerate nature), which allows some base pair changes to be silent and have no effect on the polypeptide sequence.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Predict Mutation Effect	Predict how a base-pair substitution affects the amino acid and resulting polypeptide.	4
Frame Shift Mutation	Predict how deleting one base pair alters the reading frame and resulting polypeptide.	4
Predict Mutation Effect	Predict how a base insertion within a codon alters the encoded polypeptide.	4
Predict Mutation Effect	Predict how a base-pair substitution within a codon affects the resulting polypeptide.	4
Total across these question types		16

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.