Fluorescence in situ hybridization (FISH).

The two complementary deoxyribonucleic acid (DNA) strands are bound by hydrogen bonds. Heat and chemicals break the hydrogen bonds but they re-form when the conditions are favorable; this is the basis of nucleic acid hybridization. The probe is either tagged with biotin or digoxigenin, and they are detected by fluorophore conjugated streptavidin or antidigoxigenin antibody, respectively. Fluorophores are tagged directly to the probe, thus enabling rapid visualization of the target DNA. Fluorescent-labeled probes are safe, simple to use, and provide low background and high resolution. There are mainly three types of probes: The locus specific probe is used to locate the position of a particular gene on the chromosome, the centromeric repeat probe binds to the repetitive sequences found in the centromere of the chromosome, and the whole chromosome probe maps different regions along the length of any given chromosome. Thousands of bacterial artificial chromosome (BAC) clones obtained from the Human Genome Project are used as probes to map chromosomes. Probes are also available commercially.