The basic principle behind DNA recovery from agarose gel involves a sequence of bind, wash, and elute steps. Once the gel is in solubilizing buffer, it is applied onto a “spin column,” which, upon centrifugation, allows DNA molecules to selectively bind to a silica-filter while the impurities flow through into a collection tube.
DNA is able to bind to silica thanks to a high salt concentration in the gel solubilization buffer. This buffer is believed to disrupt the hydration structure around the filter and create a cation salt bridge between the strong negative charges on the filter and negative charges on the DNA. Residual impurities are removed by washing with ethanol.
Water or low salt buffer is added to the column and will “elute,” or free, the DNA from it, presumably by disrupting the cation bridge. The DNA is now purified from the gel.