Southern blot up shut

From the Cornell University site

In this extremely simplified cartoon, two different DNAs (single stranded following denaturation) 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 two DNAs 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 DNA to be perfectly complementary, i.E. A to T and C to G or for RNA; A to U and C to G.           

A wash step removes any probe which is not tightly bound to the DNA on the membrane. Only DNA that matches exactly will remain bound.

           

In this example, DNA "A" is the specific DNA homologous to the probe and therefor a band will develop where this DNA is bound to the membrane. No band will show where "B" is bound.