What is a DNA probe?
A DNA probe is a single strand of DNA made to recognise a chosen allele. The order of its bases is selected so that the probe can pair with a known region of the target allele.
Scientists must therefore know part of the target allele's base sequence before constructing the probe. A gene machine can then synthesise DNA with the required complementary sequence.
For a probe intended to locate a harmful allele, its bases must match the target allele by complementary base pairing without also matching the corresponding region of a normal allele.
| Feature | Why it is needed |
|---|---|
| Single-stranded DNA | Allows the probe bases to pair with complementary bases in the test DNA. |
| Known base sequence | Enables the probe to be designed for a particular target sequence. |
| Complementary sequence | Gives the probe specificity for the allele or DNA sequence being investigated. |
| Detectable label | Allows the location of a bound probe to be found after hybridisation. |
Labelling the probe
A probe can carry a radioactive or fluorescent label. The complementary base sequence determines where the probe can bind, while the label makes the position of a bound probe detectable.
Fluorescent label
The position of a bound fluorescent probe can be detected using suitable light, such as UV.
Radioactive label
The position of a radioactive probe can be revealed using autoradiography.
Exam Tip: Explain probe specificity using complementary base pairing. The label shows where the probe has bound; it does not make the probe specific.