Why does DNA flow toward the positive electrode of the gel chamber?

DNA molecules have negative charges, and so when placed in an electric field they migrate toward the positive pole.

Explanation:

Electrophoresis is a technique that uses differences in electrical charge to separate the molecules in a mixture. DNA molecules have negative charges, and so when placed in an electric field they migrate toward the positive pole. The rate of migration of a molecule depends on two factors, its shape, and its charge-to-mass ratio.

A gel, which is usually made of agarose, polyacrylamide, or a mixture of the two comprises a complex network of pores, through which the DNA molecules must travel to reach the positive electrode. The smaller the DNA molecule, the faster it can migrate through the gel. Gel electrophoresis, therefore, separates DNA molecules according to their size.