Explain how microarrays analyse gene expression
Use labelled cDNA hybridisation to interpret transcript patterns on a microarray.
Section 19.1 · Principles of genetic technology
A Level; Cambridge International Biology 9700, 2028–2030. This is curriculum teaching, not medical advice or a claim that a treatment is suitable for an individual. Written responses are self-checked
Before you begin
- DNA stores genetic information; genes can be expressed to produce RNA and protein.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What molecule is commonly converted to labelled cDNA for expression analysis?
What you will learn
- Explain how microarrays can analyse genomes and detect mRNA
- Interpret complementary binding and fluorescent signals
Build the explanation
A microarray contains many DNA probes fixed at known positions. mRNA from a sample can be converted to labelled cDNA, which hybridises to complementary probes. Fluorescence at a spot indicates that complementary transcript sequence was present in the sample; comparing samples can reveal differences in gene expression.
Use the evidence carefully
A signal depends on sequence complementarity and experimental conditions. Microarrays detect sequences represented by their probes and do not directly measure protein abundance or every gene in a genome; controls and normalisation matter when comparing samples.
Pause and explain
A microarray signal is higher in sample A than B. What conclusion is reasonable?
Worked example
Labelled cDNA from a tissue binds strongly to a probe for gene X. What does this support?
Show the worked solution
- The sample contained complementary transcript sequence.
- The gene’s mRNA was present.
Answer The tissue sample contained mRNA complementary to the gene X probe, supporting expression of gene X in that sample.
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What does a fluorescent spot on a microarray indicate?
Give me a hint
Identify the relevant material or technology and explain each step.
Compare my reasoning
- Labelled cDNA has hybridised to a complementary probe.
- The sample contained a matching transcript sequence.
A fluorescent spot supports the presence of a transcript sequence complementary to the probe.
Look for these in your work
- Method identified
- Steps linked to function
Why may a microarray fail to detect a gene not represented by a probe?
Give me a hint
Use the stated example to justify a conclusion.
Compare my reasoning
- The array only tests its designed probe set.
- No matching spot is available.
The gene may not be represented by a probe on the array, so it cannot be detected by that array.
Look for these in your work
- Evidence interpreted
- Limits stated
A microarray signal is higher in sample A than B. What conclusion is reasonable?
Give me a hint
Apply the idea to a new application or trade-off.
Compare my reasoning
- Compare the relative signal under matched conditions.
- Do not infer protein amount directly.
The transcript represented by the probe is relatively more abundant in sample A under the matched assay conditions; this does not directly establish protein abundance or causation.
Look for these in your work
- Application explained
- Claim appropriately bounded
Common mistakes
- Saying a microarray directly measures protein
- Assuming no signal proves a gene is absent
What can you explain now?
What molecule is commonly converted to labelled cDNA for expression analysis?
Compare with the explanation
mRNA
The cDNA can hybridise to complementary DNA probes.
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Make it stick
How does labelled cDNA produce a microarray signal?
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