The influence of agarose--DNA affinity on the electrophoretic separation of DNA fragments in agarose gels

The effects of DNA concentration, buffer composition, added "carrier" DNA, and chemical modification of agarose on the electrophoretic separation of DNA restriction fragments in agarose gels were tested. Electrophoretic zones of migrating DNA were found to broaden by trailing as sample load was decreased, and this effect was found to be more pronounced for species of higher molecular weight. As DNA sample load was increased, DNA fragments were found to move faster in the direction of electrophoresis (front forward). Sharp, well-resolved electrophoretic zones were obtained at very low DNA loads only when a high-salt, high-pH, high-EDTA buffer was employed or when "carrier DNA" having a broad and uniform molecular weight distribution was included in the sample. Moreover, DNA in high concentration was found to displace DNA in low concentration from a given gel region. Unmodified agaroses were found to differ only slightly in their effectiveness in retarding DNA fragments at a given agarose concentration. However, hydroxyethylated agarose was much more effective in retarding DNA, at a given gel concentration, than the unmodified agaroses tested. These results show that it is useful to consider the agarose gel matrix as possessing the properties of both a molecular sieve and a chromatographic adsorbent when designing electrophoretic separation techniques for DNA. A model for these separations which includes the effects of DNA-agarose interaction and molecular sieving is discussed.