Discussion
Gel electrophoresis separates DNA fragments on the basis of
Answer: B
Correct answer: size, with smaller fragments migrating further
Gel electrophoresis separates DNA fragments by size. The DNA is loaded into wells in an agarose gel and an electric field applied; because the phosphate backbone gives DNA a uniform negative charge per unit length, all fragments migrate towards the anode at a rate governed by how easily they thread through the pores of the gel. Smaller fragments travel further. The uniform charge is what makes the method work: since charge is proportional to length, charge-to-mass ratio is essentially the same for every fragment, and only sieving by the gel matrix distinguishes them. Fragments are visualised by staining with ethidium bromide or a safer alternative and viewing under ultraviolet light, and their sizes are read against a ladder of known standards run alongside. Agarose suits fragments from a few hundred bases to tens of kilobases; polyacrylamide resolves smaller ones more finely. Very large fragments need pulsed-field electrophoresis, which periodically changes the field direction.
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