A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Recombinant DNA Technology — mark scheme explained
The short answer
Recombinant DNA technology is a powerful tool in modern biology that involves the transfer of fragments of DNA from one organism to another.
The question
Explain the process of converting mRNA to cDNA using reverse transcriptase.
[Paraphrased for study — not reproduced from any exam paper.]
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 descriptive questions, marks are typically awarded for clear and concise explanations that demonstrate a deep understanding of the topic. For evaluative questions, marks are given for balanced assessments that consider both positive and negative aspects, as well as ethical implications. Ensure your answers are well-structured and supported by relevant examples.
What the command words demand
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Describe
- Give a clear account of the process or method, step-by-step.
- Evaluate
- Assess the advantages and disadvantages, ethical implications, and potential impacts of recombinant DNA technology.
- Identify
- Recognize and name specific components or steps in a process.
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, provide detailed explanations, and check your work.
- Isolate mRNA from the cell of interest.0 marks
- Use a primer to initiate the synthesis of a complementary DNA (cDNA) strand by reverse transcriptase.1 mark
- The reverse transcriptase enzyme reads the mRNA sequence and synthesizes a complementary DNA strand.2 marks
- The resulting cDNA is free from introns, as the mRNA has already been spliced in the cell.1 mark
Final answer: mRNA is isolated from the cell, and a primer initiates the synthesis of a complementary DNA (cDNA) strand by reverse transcriptase. The enzyme reads the mRNA sequence and synthesizes a cDNA strand that is free from introns.
Work through every step correctly and you earn all 4 marks.
Another worked example
Describe how restriction enzymes are used to cut a fragment containing the desired gene from DNA.
- Identify the specific recognition sites for the restriction enzyme within the DNA sequence of interest.1 mark
- Treat the DNA with the restriction enzyme, which cuts the DNA at these recognition sites.2 marks
- The resulting fragments can be separated using gel electrophoresis to isolate the desired gene fragment.0 marks
Final answer: Restriction enzymes are used to cut a fragment containing the desired gene from DNA by identifying specific recognition sites within the DNA sequence. The enzyme cuts the DNA at these sites, and the resulting fragments are isolated using gel electrophoresis.
Work through every step correctly and you earn all 3 marks.
Common mistakes
Confusing cDNA with genomic DNA.
Why it happens: Students may not fully understand the difference between cDNA, which is complementary to mRNA and lacks introns, and genomic DNA, which includes both exons and introns.
Fix: Remember that cDNA is synthesized from mRNA using reverse transcriptase and does not contain introns. Genomic DNA, on the other hand, contains both exons and introns.
Forgetting to mention the role of primers in PCR.
Why it happens: Students may overlook the importance of primers in initiating DNA synthesis during PCR cycles.
Fix: Always include the role of primers in your explanation of PCR. Primers are short sequences that anneal to the single-stranded DNA templates, allowing the DNA polymerase to extend them and synthesize new strands.
Confusing promoter and terminator regions with other regulatory elements.
Why it happens: Students may mix up the functions of different regulatory sequences, leading to incorrect answers in questions about gene expression.
Fix: Remember that promoters are located upstream of genes and initiate transcription, while terminators are downstream and signal the end of the gene. Other regulatory elements, such as enhancers, have distinct roles.
Failing to explain how marker genes work in transformation.
Why it happens: Students may not fully understand the mechanism by which marker genes allow for the selection of transformed cells.
Fix: Practice explaining that marker genes provide a selectable trait, such as antibiotic resistance. When host cells are cultured in the presence of the selective agent, only those that have taken up and expressed the marker gene will survive.
Incorrectly describing the role of restriction enzymes in DNA cutting.
Why it happens: Students may not clearly articulate how restriction enzymes recognize specific sequences and cut DNA at those sites.
Fix: Always specify that restriction enzymes recognize and bind to specific sequences within the DNA, cutting the molecule at these recognition sites. This allows for precise isolation of desired gene fragments.
Failing to mention the exponential nature of PCR amplification.
Why it happens: Students may not fully grasp how PCR cycles lead to exponential amplification of DNA sequences.
Fix: Emphasize that each cycle of PCR doubles the amount of target DNA, leading to exponential amplification over multiple cycles. This is why PCR is such a powerful tool for amplifying specific DNA sequences.
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 |
|---|---|---|
| Describe cDNA Synthesis | Describe how reverse transcriptase produces cDNA from an mRNA template. | 4 |
| Restriction Enzyme Cutting | Describe how restriction enzymes recognise and cut DNA to release the desired gene fragment. | 3 |
| Explain PCR Principle | Describe the stages and conditions used to amplify DNA fragments in vitro. | 4 |
| Describe Gene Technology | Explain how promoter and terminator regions are attached to DNA fragments for host cell expression. | 3 |
| Marker Gene Identification | Explain how marker genes reveal which cells have successfully taken up a modified gene. | 4 |
| Total across these question types | 18 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.