Southern blotting A chromatographic technique for isolating and identifying specific fragments of DNA, such as the fragments formed as a result of DNA cleavage by restriction enzymes. The mixture of fragments is subjected to electrophoresis through an agarose gel, followed by denaturation to form single-stranded fragments. These are transferred, or `blotted', onto a nitrocellulose filter where they are immobilized in their relative positions. Specific gene probes labelled with a radioisotope are then added. These hybridize with any complementary fragments on the filter, which are subsequently revealed by autoradiography. The technique was devised by US biologist E. M. Southern (1938– ). A similar technique for detecting RNA fragments is called Northern blotting, by analogy. See also Western blotting.

Southern blotting A technique for transferring single-stranded (denatured) DNA fragments from a gel used for electrophoresis to a nitrocellulose filter. A gel containing the denatured DNA fragments is placed between blotting paper and a cellulose nitrate filter is saturated with a buffer. DNA is elued from the gel on to the cellulose nitrate, to which it binds permanently after baking at 80°C for two hours. The technique is used in DNA finger-printing and genetic engineering, for testing DNA hybridization.