From the Cornell University site:

http://drylab1.vet.cornell.edu/Block4/Reference/Diagnostics/northupclose.html


Northern - up close

In this extremely simplified cartoon, two different mRNAs are bound to a membrane.
The membrane is incubated with a solution containing a probe (another single stranded DNA or RNA, or oligonucleotide) which is homologous to one of the 2 mRNAs on the membrane. The probe has an indicator attached to it so that it may be detected later on. The binding is very specific and requires the base pairs of the probe and the bound mRNA to be perfectly complementary, i.e. A to U and C to G.
A wash step removes any probe which is not tightly bound to the mRNA on the membrane. Only probe that matches exactly will remain bound.
In this example, mRNA "A" is the specific mRNA homologous to the probe and therefor a band will develop where this mRNA is bound to the membrane. No band will show where "B" is bound.

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