Locating specific alleles of genes

• Using DNA probes and DNA hybridisation, it is possible to locate a specific allele of a gene. Firstly, we must determine the sequence of nucleotide bases of the allele we are trying to locate. This can be achieved using DNA sequencing techniques or by referring to our extensive genetic libraries.
• After the sequence of nucleotide bases for the allele has been identified, a fragment of DNA is produced that has a sequence of bases that are complementary to the allele we are trying to locate. Multiple copies of our DNA probe are then formed using the PCR.
• A DNA probe is made by attaching a marker (e.g. a fluorescent dye) to the DNA fragment.
• After the DNA probe has been made, a DNA sample is taken from the person suspected to possess a certain allele, and the DNA is heated to separate the strands. The strands are then cooled in a mixture containing the DNA probes.
• If the DNA being tested contains the allele of interest, then one of the DNA probes is likely to bind to it because the probe has base sequences that are exactly complementary to those on the allele of interest.
• After the probes have had the chance to bind, the DNA is washed clean to remove any unbound probes. The remaining hybridised DNA is now fluorescently labelled with the dye on the probe. This dye is detectable by shining a light onto the DNA fragments, which will fluoresce under a special microscope.
• If complementary fragments are not present, the DNA probe will not fluoresce.