Gel Electrophoresis is a way to sort and measure the DNA strands. Scientists use the technique whenever they need to sort DNA strands according to lengths, this is because it helps them determine patterns, banding patterns create unique fingerprint for a certain individual. During the process the shorter strands move faster because they have less mass compared to the longer strand. Therefore, the short strands end up farther away from the starting point than the longer strands, which is how the different in lengths sort themselves out.
