By analyzing the rate at which different sets of denatured DNA reannealed, the relative differences and similarities between species or between individuals within a species can be mapped and compared based on a DNA Difference Score.
There exist two kinds of nucleic acid hybridization. Both are methods in which radioactively labeled single DNA strands of known base sequences are used as a probe to detect the nucleotide sequence of another single stranded DNA or RNA molecule.
The first one is widely used for detection of specific genes in cellular DNA. It was developed by E.M. Southern and is called Southern Blotting. The DNA is digested with a restriction enzyme and the fragments are separated by electrophoresis. The gel is then overlaid with a nitrocellulose filter, to which the DNA fragments are transferred (blotting) to yield a replica of the gel. Following, the filter is incubated with a radiolabeled probe, which hybridizes to the complementary strand. This fragment is then visualized by exposure of the filter to X-ray film.
The second method is called northern blotting, it is used to identify the base pair sequence of RNA. RNA is extracted and fractionated according to size by electrophoresis. Then the procedure is the same as in Southern Blotting. Northern blotting is frequently used in studies of gene expression (e.g. whether specific mRNA is present in different types of cells).