DNA probes and screening for genetic conditions
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires knowledge of DNA hybridisation and gene probes. You can test your knowledge on these below.
What is a DNA probe?
A short, single-stranded piece of DNA that is complementary to a target DNA sequence and is labelled (e.g., radioactively or fluorescently).
What does a DNA probe bind to?
A complementary DNA sequence in the sample being tested.
What is the purpose of using a DNA probe?
To locate specific genes or identify the presence of an allele linked to a genetic condition.
Topic Explainer Video
What is a DNA Probe?
A DNA probe is a short, labelled, single-stranded DNA sequence that is:
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Complementary to a part of a gene or allele being searched for.
- Labelled with either a fluorescent marker or a radioactive marker to allow detection.
- If a fluorescent probe is used, UV light is used in detection. If a radioactive probe is used, x-ray film is used in detection (a process known as autoradiography).
When added to a mixture of DNA, the probe will hybridise (bind) to the complementary sequence if it is present.
How Are DNA Probes Used?
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Sample Preparation
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DNA is extracted from cells and may be amplified using PCR.
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Denaturation
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DNA is heated to separate it into single strands.
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Hybridisation
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DNA probes are added and allowed to bind (hybridise) to complementary target sequences if present.
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Detection
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If the probe binds, the label (radioactive or fluorescent) allows scientists to detect the presence of the target gene or mutation.
Genetic Screening Using DNA Probes
Genetic screening involves testing an individual's DNA to:
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Detect mutations that cause genetic disorders (e.g. cystic fibrosis, Huntington's disease).
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Identify carriers of genetic conditions.
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Predict the likelihood of developing certain diseases (e.g. some cancers linked to BRCA1/2 mutations).
Genetic screening allows early diagnosis, informed reproductive choices, and targeted treatments.
How Does a Microarray Work? What Is a Microarray?
A DNA microarray is used to detect the presence of thousands of specific DNA sequences (or gene mutations) at the same time.
It uses many different DNA probes, each attached to a small fixed spot on a solid surface (like a glass slide or silicon chip).
Each spot on the array contains copies of a known DNA probe that is complementary to a different gene or mutation.
How Does a Microarray Work?
Step-by-Step:
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Extract DNA from the individual and label it with a fluorescent marker.
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Denature the DNA (separate into single strands).
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Wash the single-stranded DNA over the microarray.
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If the sample DNA has sequences that are complementary to the probes on the microarray, hybridisation occurs - the DNA binds to the spot.
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After washing away unbound DNA, the array is scanned for fluorescence.
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Fluorescent spots indicate where hybridisation has taken place, showing which genes or mutations are present.
Key Terms
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DNA Probe: A labelled, single-stranded piece of DNA used to detect specific DNA sequences.
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Genetic Screening: Testing DNA to identify the presence of genetic mutations.
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Hybridisation: The binding of a DNA probe to its complementary DNA sequence.
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Fluorescent Labelling: Attaching a fluorescent marker to a probe to allow visual detection.
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Radioactive Labelling: Attaching a radioactive isotope (e.g.,
) to a probe for detection by X-ray film.
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Exam Tips
When explaining DNA probe use:
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Always say the probe has a base sequence which is complementary to the target sequence.
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Clearly state that binding (hybridisation) occurs only if the sequence is present.
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Mention the detection method - fluorescence and UV light or radioactive and x-ray film - as a separate step.
No answer provided.
A husband and wife wanted to know whether they were carriers of the mutated form of a gene. This mutation is a deletion that causes a serious inherited genetic disorder in people who are homozygous.
A geneticist took samples of DNA from the husband and the wife. He used a DNA probe to look for the deletion mutation. The DNA probe was specific to a particular base sequence in the gene.
The geneticist compared the couple’s DNA with that of a person known not to carry this mutation.
The chart shows the geneticist’s results.
The geneticist told the couple they were both carriers of the mutated gene. Explain how he reached this conclusion. (3 marks)
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Carriers are heterozygous / have one normal copy and one mutant copy of gene.
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Both have DNA that binds to half / 50% amount of probe (compared to the person not carrying the mutation).
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Probe binds to dominant / healthy allele so they must have one copy of this allele for the probe to bind to.
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!