Gel electrophoresis is normally set up with the negative electrode at the top of the gel and the positive electrode at the bottom of the gel. The DNA products are loaded at the top of the gel, and then a current is applied to separate them. However, when preparing to run a gel, you accidentally switched the locations of the negative and positive electrodes such that the positive electrode is at the top and the negative electrode is at the bottom. You still loaded the DNA products at the top of the gel as normal. What result are you most likely to observe if you apply an electric current to this gel setup?

A) Shorter DNA molecules will move up the gel, and longer DNA molecules will move down the gel.
B) All DNA molecules will migrate up the gel toward the positive electrode.
C) Longer DNA molecules will move up the gel, and shorter DNA molecules will move down the gel.
D) All DNA molecules will migrate down the gel toward the negative electrode.

Because the DNA has a negative charge - regardless the size-, all DNA molecules will migrate towards the positive electrode, at the top of the gel. This is due to the Coulomb's physic law "force of the interaction between the charges is attractive if the charges have opposite signs".

Instead of migrate down the gel , the DNA will do migrate up the gel (opposite direction).