The use of labelled DNA probes that can be used to screen patients for…

The idea

A DNA probe is a short, single-stranded DNA sequence complementary to a specific allele, labelled with a fluorescent or radioactive tag. To screen a patient, their DNA is extracted, amplified by PCR, made single-stranded, and fixed to a nylon membrane. Labelled probes are washed over the membrane and bind only where complementary to the patient's DNA, by complementary base pairing. Unbound probes are washed away; a signal (e.g. autoradiography for a radioactive label, or fluorescence) shows the patient carries the allele. Screening can reveal carrier status for a heritable condition such as cystic fibrosis, drug-response alleles such as CYP2C19 variants affecting clopidogrel activation, or health-risk alleles such as BRCA1 mutations. Each allele needs its own complementary probe, so a negative result shows only that the tested alleles were not detected.

What earns the marks

• State that a DNA probe is a short, single-stranded DNA sequence labelled with a fluorescent or radioactive tag.
• Explain that a probe is complementary to a specific allele and binds to it by complementary base pairing.
• Describe that patient DNA is amplified by PCR, made single-stranded, and fixed to a membrane before labelled probes are added.
• Explain that unbound probes are washed off, and a signal shows the patient carries the allele detected by the bound probe.
• Explain that a separate complementary probe is needed for each allele, so a negative result shows only that the tested alleles were not detected.

Common mistakes and how to put them right

• Stating that the patient DNA is labelled when describing the use of labelled DNA probes.

  Put it right: The specification refers to labelled DNA probes, so describe the probes as carrying the label and binding to fixed patient DNA.

• Treating a positive result for a health-risk allele such as a BRCA1 mutation as a definite diagnosis of cancer.

  Put it right: State that it indicates increased probability, not certainty.

• Assuming one probe detects all mutations.

  Put it right: State that each allele requires its own complementary probe, so a negative result only rules out the alleles tested.

• Confusing a drug-response allele with a health-risk allele.

  Put it right: A drug-response allele affects how a patient metabolises or responds to a medicine, whereas a health-risk allele is associated with increased probability of a condition.

Exam tips

• Use the specification wording 'labelled DNA probes': state that the probe carries the label and binds to fixed patient DNA.
• Always link the screening result to its application, such as choosing the correct drug dose or identifying a carrier.
• For a negative result, state what it does and does not prove: only the alleles tested were not detected, not that no mutation exists.
• When asked for a drug-response example, name a specific gene variant and the drug it affects, rather than describing drug response in general terms.