DNA probes & hybridisation
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
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?
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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
- Radioactively labelled probes
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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
- Identify the DNA sequence of the mutant allele. The base sequence of the harmful mutation is known from DNA sequencing.
- 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.
- Obtain DNA from the person being tested. DNA is extracted from cells (e.g. saliva).
- Amplify the person’s DNA if necessary (PCR). PCR is used to produce enough copies of the target DNA region.
- Heat is used to separate the person’s double-stranded DNA into single strands.
- Mix the DNA probes with the person’s single-stranded DNA.
- DNA hybridisation occurs if the mutant allele is present. The probe binds to the complementary DNA sequence by base pairing.
- 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 allow 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
What you need to know (from the AQA specification)
What you need to know (from the AQA specification)
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
How this topic is tested
This analysis is based on past paper data from 2017 to 2025. It is intended for interest only and is not predictive of what will appear in future papers.
- Tested in 5 of 9 years (2017–2025): 10 question parts worth 22 marks.
- 37th most-examined topic overall by marks, 6th in Unit 8.
Marks by year
Most-tested spec points
- DNA Probes & Hybridisation: tested in 6 parts (14 marks)
- Screening, Counselling & Personalised Medicine: tested in 3 parts (8 marks)
Also links to: The Technique of Genetic Fingerprinting, Producing DNA Fragments.
Exam Question Practice
Doctors can use DNA probes to screen people for the allele that causes sickle cell disease.
Describe what a DNA probe is.
(2 marks)
Hint
How many strands does a probe have? How does it find the allele it’s looking for?
Mark Scheme
- A short, single-stranded piece of DNA (1 mark)
- Its bases are complementary to the allele being tested for (1 mark)
Also accepted / rejected
Tips
- Say it’s short and single-stranded; “a fragment of DNA” isn’t enough
- Say its bases are complementary to the target allele
- Define the probe; don’t describe how it’s labelled or used
Also accepted
Not accepted