The gel below illustrates the purification of the RecA protein from E. coli. This gel was loaded from the top. Which statement BEST describes how proteins "run" in an SDS gel?

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In SDS-PAGE, proteins are separated primarily by molecular weight. The SDS detergent denatures the proteins, and the negatively charged SDS molecules bind to the exposed hydrophobic regions, giving a uniform negative charge to the protein. Since the gel matrix acts like a physical sieve, the smaller proteins can move more easily through the matrix and will travel further down the gel, while the larger proteins will move more slowly and remain higher up in the gel. Therefore, the smallest MW proteins are on the bottom, and the largest are on the top.

The Gel Below Illustrates the Purification of the RecA Protein