Gel electrophoresis separates fragments of nucleic acid that differ in size, charge or conformation. When charged molecules are placed in an electric field, they migrate toward either the positive or negative pole according to their charge. In contrast to proteins, which can have either a net positive or net negative charge, nucleic acids have a consistent negative charge imparted by their phosphate backbone, and migrate toward the anode during gel electrophoresis. The length of DNA/RNA generally determines its migration in the gel, with shorter DNA fragments traveling faster, while the longer fragments remain closer to the origin of the gel. This results in separation based on size. Oligonucleotide gels may be composed of either agarose or polyacrylamide. Agarose gels are more common for oligonucleotide separation, except for small fragments of DNA, which are usually purified using polyacrylamide. Alfa Aesar supplies reagents for oligonucleotide electrophoresis in acrylamide or agarose matrices.