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The 8 pages that link to Agarose gel electrophoresis, each with the reason it gives.
ElectrophoresisBroader topic: Its gel matrix makes migration depend strongly on nucleic-acid length.
Southern blotRelated: It sorts restriction fragments by length before they are blotted.
Ethidium bromideRelated: Ethidium bromide reveals nucleic-acid bands after electrophoresis.
Gel electrophoresisBroader topic: Agarose pores sort DNA fragments chiefly by length.