Southern Blotting

This technique, named after its inventor E.M. Southern, is a way of combining restriction enzyme and hybridization information. In Southern blotting, a DNA is digested with a restriction enzyme and the fragments are separated by gel electrophoresis. The DNA fragments are denatured by soaking the gel in base and then are transferred to a piece of nitrocellulose filter paper by capillary action. The result is like a “contact print” of the gel; each fragment is at a position on the paper that corresponds exactly to its position in the gel. Then the filter paper is mixed with a radioactively labeled single-stranded “probe” DNA or RNA. The probe DNA hybridizes to the complementary single-stranded DNA fragments on the filter paper. The blot is exposed to X-ray film and the resulting autoradiograph shows the positions of the complementary fragments of DNA. This information physically maps the regions of a large DNA that are complementary to a probe. See Figure 1 .