Southern blotting involves the binding of a radioactive probe to cellular DNA which has been digested by restriction enzymes. This form of hybridization is particularly useful in the detection of restriction fragment length polymorphisms (RFLP), caused by structural variations in DNA which results in restriction fragments of various lengths.
Northern blotting involves the binding of a radioactive probe to RNA. This technique is particularly useful for monitoring the synthesis of proteins. Adding a denaturant such as formaldehyde to the agarose gel prevents the hybridization of mRNA molecules with one another during electrophoresis.