When a target DNA sequence and a complementary sequence are proximal enough and are properly oriented the two anneal together to what we call a hybrid. The complementary sequence is usually known as a probe. Several hybrids may exist such as DNA-DNA hybrids, DNA-RNA hybrids etc. The chemical reaction between the target and the probe is referred to as hybridization. If hybridization is carried out to analyze a sequence of interest that is present inside cells, tissue sections or isolated chromosomes it is known to be in-situ hybridization. However in contrast an in-vitro technique is carried out within suitable apparatus or typically in a test tube.
Tools for detection of hybridization sites
Hybridization itself is not sufficient on almost all instances. Detection is equally important or may be more important for further analysis. Several detection systems are present for detection. Usually the detectable utility known as the label is incorporated within the probe. Visualizing is either direct or indirect depending on the detection system applied. Fluorescence labeling and detection is one of the commonly used direct methods and radioactive labeling and detection or biotin streptavidin systems are usually employed indirect methods.
One of the well-known direct in-situ hybridization method is Fluorescence In-situ Hybridization (FISH). Different colors and combinations of colors are produced by different fluorescent reporter molecules. FISH is one of the widely used methods to map the locations of genes on chromosomes.
Applications of probes to any source of tissue or molecule
The most important application of in-situ hybridization lies in locating the molecules of interest (either DNA or RNA) within cells or tissues. It is also possible to visualize the specific positions of genes on a chromosome. However several other numerous applications of in-situ hybridization exists.