The Southern blot is a molecular biology technique for determining whether a specific DNA sequence is present within a complex mixture of DNA, and if so, in what size fragment. Its logic combines three independent principles: enzymatic cleavage reduces intractably long DNA into a reproducible population of fragments; gel electrophoresis resolves those fragments spatially by size and charge; and a labeled single-stranded **probe** complementary to the target sequence anneals by Watson–Crick base pairing only where that sequence resides, converting an invisible sequence-identity question into a visible positional signal. An intermediate transfer step immobilizes the resolved fragments onto a durable filter membrane, and detection of the label — historically radioactive, read out on X-ray film — reports both presence and fragment size. The technique belongs to molecular biology's analytical methods and is the archetype for the wider blotting family that applies the same separate-immobilize-probe logic to other macromolecules.
The Southern blot is a molecular biology technique for determining whether a specific DNA sequence is present within a complex mixture of DNA, and if so, in what size fragment. Its logic combines thr…