Genetics: Analysis and Principles 4th Edition by Robert Brooker

Exercise 9

A segment of DNA that contains a loop is more compact than the same DNA segment without a loop. Therefore, when these two alternative structures (looped versus unlooped) are electrophoresed through a gel, the looped structure migrates more quickly to the bottom of the gel because it can more easily penetrate the gel matrix (see solved problem S6). Let's suppose that a mutant E. coli strain has been identified in which the AraC protein represses the ara operon, even in the presence of arabinose. In the experiment shown here, mutant or normal AraC protein was mixed with a segment of DNA containing the ara operon in the absence or presence of arabinose and then run on a gel.