RFLP analysis of genomic DNA is facilitated by Southern blot analysis. After electro-phoresis, DNA fragments in the gel are denatured by soaking in an alkali solution. This causes double-stranded fragments to be converted into single-stranded form (no longer base-paired in a double helix). A replica of the electrophoretic pattern of DNA fragments in the gel is made by transferring (blotting) them to a sheet of nitrocellulose or nylon membrane. This is done by placing the membrane on the gel after electro-phoresis and transferring DNA fragments to the membrane by capillary action or electro-transfer. DNA, which is not visible, becomes permanently adsorbed to the membrane, that can then be manipulated easier than gels.
