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DNA Probes

January 16, 2026
This page covers AQA spec point 3.8.4.2 Differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions

DNA probes & hybridisation

Spec Point

The use of labelled DNA probes and DNA hybridisation to locate specific alleles of genes.

  • DNA probes and DNA hybridisation allow us to diagnose genetic diseases by helping us to identify specific disease-causing alleles in DNA sequences.

What are DNA probes & hybridisation?

  • DNA probe = Short single-stranded length of DNA with a label, which makes it identifiable. Examples of DNA probes:

    • Radioactively labelled probes
      • Detected using X-ray film or autoradiography
    • Fluorescently labelled probes
      • Emit light when exposed to UV light

  • DNA hybridisation = The process where a single-stranded DNA probe binds to a complementary single-stranded DNA sequence.

- DNA hybridisation: checking for a mutant allele

  1. Identify the DNA sequence of the mutant allele. The base sequence of the harmful mutation is known from DNA sequencing.
  2. Produce a DNA probe. A short single-stranded DNA sequence with complementary bases to the mutant allele is made and a fluorescent label is attached.
  3. Obtain DNA from the person being tested. DNA is extracted from cells (e.g. saliva).
  4. Amplify the person’s DNA if necessary (PCR). PCR is used to produce enough copies of the target DNA region.
  5. Heat is used to separate the person’s double-stranded DNA into single strands.
  6. Mix the DNA probes with the person’s single-stranded DNA.
  7. DNA hybridisation occurs if the mutant allele is present. The probe binds to the complementary DNA sequence by base pairing.
  8. Detect the probe. Fluorescence is detected under UV light, indicating the presence of the mutant allele.

In addition to the process above, we can use microarrays which allows the screening of multiple genes using different DNA probes. This saves the person having to do multiple different screenings for different genes.

DNA Probe & Hybridisation Applications

Spec Point

The use of labelled DNA probes that can be used to screen patients for heritable conditions, drug responses or health risks. The use of this information in genetic counselling and personalised medicine.

Genetic Screening

  • DNA probes and DNA hybridisation can be used in genetic screening to identify disease-causing alleles in an individual’s DNA. Genetic screening has many uses, including:
  • Detecting whether an individual is at a higher risk of developing certain genetic diseases caused by known mutations, for example:
    • Cystic fibrosis (mutations in the CFTR gene)
    • Huntington’s disease (expanded repeat in the HTT gene)
  • Detecting whether an individual is a carrier of a recessive genetic disease
    • Carriers do not show symptoms but can pass the allele to their offspring.
  • Detecting oncogenes or mutated tumour-suppressor genes, allowing identification of individuals at a higher risk of developing certain cancers, for example:
    • BRCA1 and BRCA2 mutations (increased risk of breast and ovarian cancer)

Genetic Counselling

The results from this genetic screening can be used in genetic counselling. This involves providing advice to help individuals:

  • understand their risk of inheriting or passing on genetic conditions
  • make informed decisions about family planning and lifestyle changes (e.g alterations in diet)

Personalised Medicine

The results from genetic screening can also be used in personalised medicine. Personalised medicine involves tailoring medical treatment to an individual based on their DNA.

This allows doctors to:

  • select the most effective drug and appropriate dosage for individuals
  • avoid drugs that are less effective due to genetic variation
  • reduce the risk of adverse drug reactions

Exam Question Practice

Q1

What is a DNA probe?

(2 marks)
Answer

Mark Scheme

  1. Short/small single-stranded piece/sequence of DNA (1 mark)
  2. With a (fluorescent/radioactive) label/tag (1 mark)
Q2

DNA probes are used to detect specific base sequences of DNA. The process is shown in Figure 6.

Figure 6

Describe how the DNA is broken down into smaller fragments.

(2 marks)
Answer

Mark Scheme

  1. Restriction endonuclease/enzyme (1 mark)
  2. (Cuts DNA at specific) base sequence OR (Breaks) phosphodiester bonds OR (Cuts DNA) at recognition/restriction site (1 mark)
Comments from mark scheme
  • Accept palindromic sequence.
  • Do NOT say “probe binds/attaches/anneals” - this is incorrect for this question.
Q3

The DNA on the nylon membrane is treated to form single strands. Explain why.

(1 marks)
Hint

Think about how DNA probes bind to their target sequence.

Answer

Mark Scheme

  1. So the (DNA) probe can bind/hybridise (to complementary bases) (1 mark)
Q4

A scientist used DNA probes and electrophoresis to screen four volunteers for five different viral DNA fragments.

Figure 7 shows the results the scientist obtained. The lanes numbered 2 to 5 represent the four volunteers.

Figure 7

Lane 1 of Figure 7 enabled the size of the different viral fragments to be determined. Suggest and explain how.

(2 marks)
Answer

Mark Scheme

  1. (Lane 1 has DNA fragments) of known sizes/lengths (1 mark)
  2. Compare (position of viral fragment/s) (1 mark)
Q5

The lengths of the viral DNA fragments were:

  • 600 base pairs
  • 250 base pairs
  • 535 base pairs
  • 300 base pairs
  • 500 base pairs

Which volunteers had at least one of the viral DNA fragments with 250 base pairs or 535 base pairs?

(1 marks)
Answer

Mark Scheme

  1. 3, 4, 5 (in any sequence) (1 mark)
Comments from mark scheme
  • All three numbers required.
  • Reject if more than three numbers given.
Q6

Suggest why a DNA probe for the mutated triplet was not considered a suitable method for detection of the E280A mutation.

(2 marks)
Hint

Think about whether the mutated triplet sequence is unique.

Answer

Mark Scheme

  1. (GCA/triplet) is common/found in other places (1 mark)
  2. Would not know if it was the mutation/allele/gene OR Produces ‘false positives’ (1 mark)
Comments from mark scheme
  • Accept: Probe will bind elsewhere.
Q7

The scientists used a radioactively labelled DNA probe to show that the cells of tobacco plant leaves contained the SUT1 gene. Describe how they would do this. Do not include PCR in your answer.

(4 marks)
Answer

Mark Scheme

  1. Extract DNA and cut it using restriction endonucleases (1 mark)
  2. Separate DNA fragments using gel electrophoresis (1 mark)
  3. Treat the DNA to form single strands / expose bases (1 mark)
  4. A radioactively labelled DNA probe binds (hybridises) to the complementary SUT1 gene sequence (1 mark)
  5. Use autoradiography to detect the bound probe (1 mark)
Comments from mark scheme
  • This was a tricky question, with only 1.2% of students scoring full marks.
  • Common mistakes:
  • Confusing a DNA probe with a marker gene (several answers incorrectly described in vivo gene cloning).
  • Describing a tracer experiment instead of DNA hybridisation.
  • Changing the radioactive probe into a fluorescent probe (the question specifically required autoradiography).
  • Failing to state that the DNA probe binds/hybridises to complementary bases.
  • Including PCR, despite being told not to.

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