A-Level Biology May/June 2025 Q3(c): Microarrays can be used to detect various forms of porphyria caused by mutant alleles.…

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  To detect a disease by analysing gene expression using a microarray, the following steps are taken:

  1. Messenger RNA (mRNA) is extracted from the patient's cells. Reverse transcriptase is used to synthesise single-stranded complementary DNA (cDNA) from this mRNA template.
  2. A fluorescent label is then attached to these cDNA molecules.
  3. The labelled cDNA is washed over the microarray. The cDNA molecules will hybridise, through complementary base pairing, to the specific DNA probes fixed on the microarray surface.
  4. The microarray is scanned for fluorescence. The presence and intensity of fluorescence at a particular spot indicate that the corresponding gene, such as a mutant allele for porphyria, is being expressed (transcribed) in the patient's cells.

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  How the marks are awarded

  • M1 — The first mark is awarded for stating that mRNA from the person's cells is used to create single-stranded complementary DNA (cDNA).
  • M1 — The second mark is for describing the addition of a fluorescent label or tag to this newly synthesised cDNA.
  • M1 — The third mark is for explaining that the labelled cDNA hybridises (binds via complementary base pairing) with the DNA probes on the microarray.
  • M1 — The final mark is for linking the detection of fluorescence to the expression or transcription of the specific disease-causing mutant gene or allele.

  Common mistakes

  • Stating that DNA is extracted from the cells instead of mRNA. Gene expression analysis requires measuring the products of transcription (mRNA), not the genomic DNA itself.
  • Forgetting to mention the fluorescent label/tag, which is essential for detecting which genes have been expressed.
  • Using vague language like 'the DNA sticks to the slide' instead of the precise term 'hybridisation' or describing 'complementary base pairing'.
  • Confusing gene presence with gene expression. The answer must state that fluorescence shows the gene is being expressed or transcribed, not just that the gene is present in the genome.

  Examiner tip: Practice breaking down complex biological processes into a clear, step-by-step sequence, ensuring you use the correct scientific term for each action (e.g., 'hybridisation' instead of 'binding').

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