A technique for detecting one RNA within a mixture of RNAs (a Northern blot) or one type of DNA within 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 DNA is first transferred from the gel to a membrane by capillary action. Fluid wicks from the gel through the blotting membrane to several layers of absorbent paper, but the nucleic acids stick to the membrane. Baking the filter fixes the DNA or RNA to the filter.Specific bands are detected by hybridization. The filter membrane is incubated with radioactive probe, which hybridizes to some bands. After the filter is washed (to remove unused probe), an X-ray film exposed to the filter will show which bands have hybridized.
