Measurement of DNA length by gel electrophoresis II: Comparison of methods for relating mobility to fragment length

Relationships between DNA length and electrophoretic mobility in an agarose gel have been compared by estimating the lengths of known DNA polymer fragments, using other fragments in the series as standards. Global estimates were made using 10 fragments as standards; local estimates were made using the two closest fragments on either side of the unknown as standards. Most relationships were fitted by least squares. All the relationships gave more accurate local than global estimates. The most accurate results were obtained using the reciprocal relationship, where the maximum error in the local estimates was less than 0.1%. The semilog relationship gave the least accurate results, with a maximum error in the local estimates of almost 5%. The polymer fragments were also used as standards to estimate the lengths of λ DNA restriction fragments. Here the estimates were in error by up to 3%, indicating the influence of base composition and sequence on electrophoretic mobility.