A voltage gradient‐induced arrest of circular DNA during agarose gel electrophoresis

During the gel electrophoresis of double‐stranded DNA, interaction with the gel fractionates the DNA by molecular weight and conformation. Here, it is shown that open circular bacteriophage λ DNA (M_r 32.1 × 10⁶) migrates into a 0.2% agarose gel during a first electrophoresis at 0.34 V/cm, but does not migrate when the voltage gradient is subsequently raised to 6.0 V/cm for a second, orthogonal electrophoresis. Linear DNA of the same molecular weight does migrate at 6.0 V/cm. The data suggest that the arrest observed at 6.0 V/cm is caused by threading of circular DNA by gel fibers. These observations have produced a procedure of two‐dimensional agarose gel electrophoresis capable of detecting comparatively small amounts of circular DNA. The concept of threading is potentially useful for understanding of DNA fractionations obtained with pulsed electrical fields.