It is still a useful tool and you need to know about it so that you can interpret historical data. Named after its inventor, Prof. Ed. Southern, the blot is a fast way of analysing a small number of DNA fragments which may be present in a complex mixture. For instance, suppose that we wish to ask whether the sickle cell mutation is present in an individual and the only material which we have available is a DNA sample. DNA is first digested with a restriction endonuclease and then separated according to the size of DNA fragments by electrophoresis through an agarose gel.
Small fragments are able to migrate more quickly through the gel than are long fragments.
Most restriction enzymes will digest human DNA into about a million fragments. We will want to compare only those which contain the human beta globin gene. A probe is made. The single stranded probe is annealed to its target sequence which is then stringently washed to remove any probe which has bound other than by perfect fornation of a long run of complementary hydrogen bonds. As a consequence the size of the restriction fragment containing the 5 end of the gene is altered from 1.15kb to 1.35kb.