A blotting membrane is a thin support onto which separated biological molecules are transferred so they can be retained and probed. In a Southern blot, DNA fragments separated by gel electrophoresis keep their relative spatial pattern during transfer. A complementary labelled probe then detects fragments carrying a matching sequence. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 8, Analyzing DNA by Southern Blotting: transfer and nucleic-acid hybridization
The membrane and the probe do different jobs: the first immobilizes the sample pattern, while the second supplies sequence-selective detection. A signal at the same migration position establishes neither complete sequence identity nor equal copy number without appropriate controls. Transfer efficiency, sample loading and probe binding can all affect intensity. The support therefore turns a transient separation into a reusable surface for molecular recognition.
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