A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Gene Mutations and Their Effects on Polypeptides — mark scheme explained
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.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the original and mutated codons.
- S2
Original codon: A-T (e.g., ATG, which codes for methionine)
- S3
Mutated codon: G-C (e.g., GTG, which codes for valine)
- S4
Step 2: Determine if the change results in a different amino acid.
- S5
ATG → GTG: Methionine → Valine
- S6
Step 3: Predict the effect on the polypeptide.
- S7
The substitution changes methionine to valine, which could alter the protein's structure and function.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the original and mutated codons.
- S2
Original codon: A-T (e.g., ATG, which codes for methionine)
- S3
Mutated codon: G-C (e.g., GTG, which codes for valine)
- S4
Step 2: Determine if the change results in a different amino acid.
- S5
ATG → GTG: Methionine → Valine
- S6
Step 3: Predict the effect on the polypeptide.
- S7
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.
Common mistakes
- Confusing silent mutations with mis-sense mutations. — 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. — 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. — 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. — Review the types of mutagenic agents (physical, chemical, biological) and their mechanisms for causing DNA changes.
- Incorrectly predicting the effect of a nonsense mutation. — 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. — 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
- Full worked solution (all marking points)4 marks