Pulse field gel electrophoresis:

Problem:

What is the primary principle behind pulse field gel electrophoresis? 

Answer:

While there is large number of DNA samples, it is extremely difficult to separate them. Pulsed area electrophoresis is a technique in that the direction of current flow in the electrophoresis chamber is periodically altered. This permits fractionation of pieces of DNA ranging from 50,000 to 5 millon bp that is much larger and cannot be resolved on standard gels.