Electrophoresis and DNA fragments?

Question

I know that smaller DNA fragments move quickly through electrophoresis gel because they're not encountering the same resistance and can travel more quickly than larger fragments, but WHY do the fragments seperate by size in the first place?

Answer 1

But that _is_ why they separate by size - the smaller ones move faster through the physical barrier of the gel in a given time period, leaving the larger ones behind. As the time passes in the run, the distance between the various sizes of fragment gets progressively greater. The larger the gel, the more you can separate the fragments before the smaller ones start running off the end.

Answer 2

As electrophoresis is taking place, the electric field moves the DNA fragments from the positive pole towards the negative pole since DNA fragments are positively charged. While they are traveling, the larger fragments get left behind because of their molecular weight, separating them from the smaller fragments that are still moving. The smaller fragments may even travel off of the gel that electrophoresis is being performed on. Hope my info is right and that it helped.