Blotting: Southern, Northern, Western

Blotting finds one particular molecule among thousands. The molecules are first separated (usually on a gel), then transferred and fixed onto a membrane (nitrocellulose, nylon or PVDF), and then the wanted one is found, either by a probe that binds to it (hybridisation) or by an antibody. The signal may be a colour, fluorescence, light, or radioactivity.

There are three main blots, one for each kind of molecule in the flow of genetic information:

• Southern blot, for DNA, invented by Edwin Southern (the original blotting method). DNA is cut with restriction enzymes and separated on a gel. It is then drawn up out of the gel onto the membrane by capillary action, through a stack of paper, and fixed by baking. A labelled, single-stranded probe finds the matching sequence, and the result is seen on X-ray film (autoradiography).
• Northern blot, for RNA (Alwine, Kemp and Stark, 1977). It uses a positively charged nylon membrane. Formamide is added to lower the temperature at which the probe binds, so the fragile RNA is not damaged. The RNA is fixed by UV light or heat.
• Western blot, for proteins (named by W. Neal Burnette, 1981). Proteins are separated by SDS-PAGE, transferred to nitrocellulose, and found with an antibody specific to the protein.

Together they can follow one gene from DNA (Southern) to its RNA (Northern) to its protein (Western). The names are a joke on Southern's surname: once his was 'Southern', the others were named after other directions. There is even an Eastern blot, which detects changes made to proteins after they are made (post-translational modifications).