How can DNA segments, separated by gel electrophoresis, be visualized and isolated?

Explanation

DNA segments, that have been separated on an agarose gel by electrophoresis are transferred and permanently affixed to a nylon membrane, a process called southern blotting. The DNA bands are invisible to the eye so that they are flagged with some kind of tag that one can visualize. Therefore, a solution containing radioactive probe is spread over the membrane so that they bind to complementary DNA sequences and then can be visualized by autoradiography.