8.4.7 DNA Probes & DNA Hybridisation

DNA Probes

• A DNA probe is a short, single-stranded DNA molecule designed to be complementary to a specific sequence of DNA that you want to detect.
• DNA probes are used to locate specific alleles of genes or mutations in a DNA sample.
• Probes are labelled in two main ways:
  1. Radioactive isotopes, such as 32P, which can be detected using X-ray film.
  2. Fluorescent dyes, which emit light when exposed to certain wavelengths.

How DNA Probes Are Used

1. Target DNA is extracted from the sample and is made single-stranded (denatured).
2. The DNA probe is added to the sample.
3. The probe binds to its complementary sequence through base pairing if the target sequence is present.
4. The sample is then washed to remove any unbound probes.
5. Detection occurs using methods appropriate for the probe's label (e.g., X-ray film for radioactive probes or UV light for fluorescent probes).

Applications of DNA Probes:

• Identifying heritable genetic conditions such as Huntington's disease.
• Screening for mutations or detecting carriers of genetic diseases.
• Identifying genes associated with diseases, e.g., cancer susceptibility genes.
• DNA fingerprinting in forensic science.

DNA Hybridisation

• DNA hybridisation is the process by which a single-stranded DNA fragment binds with a complementary strand of DNA or RNA through hydrogen bonding.

Steps in DNA Hybridisation:

1. Target DNA is extracted and denatured to form single strands.
2. Probes are added, allowing complementary base pairing to occur.
3. Hybridisation occurs, where the probe binds to the complementary DNA sequence in the sample.
4. Unbound probes are washed away, leaving only hybridised fragments.
5. Detection is carried out based on the probe's label.