DNA Probes

• A DNA probe is a short, single-stranded length of DNA that has some sort of label attached that makes it easily identifiable. The two main types of probe are radioactively labelled probes and fluorescently labelled probes.
• Radioactively labelled probes are made up of nucleotides with the isotope phosphorous-32 attached. The probe is identified using X-ray film that is exposed by radioactivity.
• Fluorescently labelled probes emit light (fluoresce) under certain conditions, for instance when the probe has bound to the target DNA sequence.
• DNA probes are used to identify particular alleles of genes. Firstly, a DNA probe is made that has base sequences that are complementary to part of the base sequence of DNA that makes up the allele of the gene that we want to find.
• After creating the DNA probe, the double-stranded DNA that is being tested is treated to separate its two strands. These strands are mixed with the probe, which binds to the complementary base sequence on one of the strands. This is known as DNA hybridisation.
• The site where a DNA probe binds can be identified by the radioactivity or fluorescence that the probe emits.