A technique for detecting one RNA within a mixture of RNAs (a Northern blot) or one type of DNA within of a mixture of DNAs (a Southern blot). A blot can prove whether that one species of RNA or DNA is present, how much is there, and its approximate size. Basically, blotting involves gel electrophoresis, transfer to a blotting membrane (typically nitrocellulose or activated nylon), and incubating with a radioactive probe. Exposing the membrane to X-ray film produces darkening at a spot correlating with the position of the DNA or RNA of interest. The darker the spot, the more nucleic acid was present there.
The process of transferring nucleic acids or proteins from an unstable medium eg electrophoresis gel or agar plate onto nylon or nitrocellulose membrane. This allows the blotted material to be analysed by interaction with a specific, labelled probe to test for the presence of a specific molecular structure. For example a particular nucleic acid base sequence can be recognised by base pairing with a nucleic acid probe of complementary sequence.