Electric field gradients and band sharpening in DNA gel electrophoresis

A mathematical study of the effect of non-uniform electric fields on the width of DNA electrophoretic bands is presented. Using a simple model, we show that field gradients sharpen these bands during an experiment if the corresponding gradient of electrophoretic velocity is large enough. This is in agreement with experimental results indicating that narrower bands form when pulsed field electrophoresis is carried out in the presence of field gradients. Moreover, it is shown that there is in fact an optimal experimental duration that maximizes separation. Finally, gradients are also predicted to reduce the relative mobilities of the DNA fragments, which is a serious drawback of this technique.