Unlimited increase in the resolution of DNA ladders

Fractionation of DNA ladders by gel electrophoresis is limited by the progr essive compressing of the long DNA end of a ladder. Improvement in the reso lution of this DNA is achieved by use of the following two-step electrophor esis. Initially, the DNA ladder is fractionated by conventional constant fi eld agarose gel electrophoresis. Subsequently, gel electrophoresis is perfo rmed in the reverse direction by pulsing the electrical field (PFGE). A new ly developed type of pulsing is used, which causes inversion of a double-st randed DNA ladder: the distance migrated increases as the length of the DNA molecule increases. Thus, the resolution of DNA bands continues to increas e during the PFGE. These two stages of electrophoresis are serially repeate d. Eventually, both the short and the long DNA ends of the ladder migrate o ut of the gel while a selected region of the ladder undergoes progressive i ncrease in resolution during back-and-forth migration. Improved resolution of DNA bands is achieved, without a known limit.