AQA A-Level Biology: Recombinant DNA Technology — mark scheme explained

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

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Isolate mRNA from the cell of interest.

• S2

  Step 2: Use a primer to initiate the synthesis of a complementary DNA (cDNA) strand by reverse transcriptase.

• S3

  Step 3: The reverse transcriptase enzyme reads the mRNA sequence and synthesizes a complementary DNA strand.

• S4

  Step 4: The resulting cDNA is free from introns, as the mRNA has already been spliced in the cell.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Isolate mRNA from the cell of interest.

2. S2

   Step 2: Use a primer to initiate the synthesis of a complementary DNA (cDNA) strand by reverse transcriptase.

3. S3

   Step 3: The reverse transcriptase enzyme reads the mRNA sequence and synthesizes a complementary DNA strand.

4. S4

   Step 4: The resulting cDNA is free from introns, as the mRNA has already been spliced in the cell.

5. 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.

Common mistakes

• Confusing cDNA with genomic DNA. — 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. — 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. — 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. — 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. — 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. — 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

• Full worked solution (all marking points)4 marks

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