How are the DNA fragments detected in Southern blotting?

In a Southern blot, the DNA fragments are first separated by size using gel electrophoresis. After the DNA fragments are separated based on size in the gel, the double-stranded pieces of DNA are denatured into single strands. Then, these fragments are moved from the gel to a solid membrane. This membrane is later exposed to a DNA probe that has been labeled with a tag, such as fluorescent, radioactive, or chemical markers. This tag helps to identify any DNA fragments on the membrane that have sequences complementary to the probe. In specific, when the probe hybridizes to its complementary DNA fragment on the membrane, it allows for the detection of that specific fragment.