A limitation to the Southern blotting procedure for the teaching laboratory is that it requires multiple laboratory sessions. The procedures have been streamlined for this experiment so that they can be carried out in a single 3-hour laboratory period. In the exercise, students electrophorese three unknown DNA samples on an agarose gel. One of the samples (instructor keyed) contains biotinylated DNA from the bacterial virus lambda. Following electrophoresis, the DNA in the gel is transferred to a nylon membrane during a 15-minute blotting step and the viral DNA on the blot is then detected by using an enzyme-color-producing reaction.