Northern blot is a laboratory technique used to detect a specific RNA sequence in a blood or tissue sample. The sample RNA molecules are separated by size using gel electrophoresis. The RNA fragments are transferred out of the gel to the surface of a membrane. The membrane is exposed to a DNA probe labeled with a radioactive or chemical tag. If the probe binds to the membrane, then the complementary RNA sequence is present in the sample.
Northern blot is used to analyze molecules of RNA. So typically you would isolate a population of RNA from some cell sample or tissue sample, and then you would electrophoretically allow the RNA to migrate down a gel. So you're going to have the smaller fragments at the bottom and the larger fragments at the top. Once you've finished what we call running the gel, you apply a membrane to the gel and transfer, either by a salt gradient or by electrophoretic transfer, the molecules, the RNA molecules, in the gel to a membrane, typically nitrocellulose. Having that on nitrocellulose is going to look mostly like a white piece of paper. Having that, you're going to be able to then interrogate the differences in the RNA samples. We can ask if some RNA is gone or not by marking that RNA and then finding its match on the northern blot membrane, or you can ask, does it change in size? You might expect a change in size if you had an alteration in RNA splicing. The term northern blot actually is a play on words from how people originally would analyze DNA. And a man named Edward Southern actually developed the protocol in which he would do a similar analysis, but you would allow DNA molecules to migrate on a gel and then transfer that a membrane. And that protocol was named after him, [and] his name is Southern. And so it seemed kind of like a nice play on words that if you were going to analyze a similar RNA molecule in a similar way that you would then name it northern blot. The "blot" of northern blot refers to the protocol itself where you have a gel and then you lay it, it's like a sandwich, you put the membrane you want to transfer it to on top and then you add a stack of absorbent material--we use paper towels--and it's as if you're blotting the DNA up to the top of your membrane for further analysis.