Trapping in PFGE has been reduced, and sharper bands obtained by interrupting the long electric-field pulses with short high-voltage spikes in the reverse direction ( 7). E crit appears to be higher for PFGE than for steady-field electrophoresis. Lowering the field below 2 V/cm prevents trapping ( 6) but at the expense of very long electrophoretic runs ( 4). The value of E crit falls as the DNA size is increased. Trapping occurs in both steady-field and pulsed-field experiments if the electric field is higher than some critical value, E crit ( 7). Unfortunately, molecules longer than 1–2 Mbp can become permanently immobilized or trapped after traveling various distances through the gel, leading to band smearing ( 6). In PFGE, the electric field is periodically alternated in two directions and DNA separation depends on the way the molecules reorient through the gel in response to the changing electric field ( 4, 5). ![]() ![]() Steady-field electrophoresis is commonly used to separate molecules from a few bp to about 20 kbp, while pulsed-field gel electrophoresis (PFGE) methods ( 1– 3) are required to separate molecules beyond this size range, up to 10 megabase pairs (Mbp). Gel electrophoresis is the method of choice for the size fractionation of DNA in analytical biochemistry.
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