Explanation - Feedback
DNA is negatively charged due to the phosphate groups in the sugar-phosphate backbone. So, DNA will migrate in an electric field towards the positive electrode, which is the anode in electrophoresis (types of electrolytic cells). (A) is correct.
▾ Explanation - Expert Approach
The question asks about the principle underlying the separation of DNA fragments in gel electrophoresis—a technique used to separate DNA fragments based on their size and charge. DNA molecules are negatively charged due to the phosphate groups in their backbone. In an electric field, the fragments will migrate towards the positive electrode, which, in an electrolytic cell, is the anode. Therefore, the correct answer is (A).
Simply knowing that DNA is negatively charged allowed me to immediately eliminate (C) and (D). I think that most people who miss this question, though, will probably get that far. They’ll stumble because they forgot whether the anode or cathode was the positive electrode. It’s confusing, because in galvanic cells, the polarity is reversed. This is why I try to develop strong conceptual understandings of things like this rather than simply memorizing them. If I have that, I don’t have to rely on perfect recall, but can instead “reconstruct” my knowledge. In this case, I know that by convention, oxidation always happens at the anode and reduction at the cathode (Anode Oxidation Reduction Cathode). I also know that the polarity of the electrodes in electrophoresis is created by an external power source such as a battery. The battery is pushing electrons down onto one electrode (negative) and vacuuming them up from the other electrode (positive). Therefore, molecules approaching the positive electrode will be likely to have their electrons stolen (be oxidized, making that electrode the anode), while those approaching the negative electrode will be more likely to gain new electrons (be reduced, making that electrode the cathode). Trying to understand things this way may seem sort of elaborate compared to rote memorization, but it’s a lot more robust and durable. Once I’ve mastered something conceptually like this, it’s very hard to forget. And that’s hugely useful for the MCAT, where there’s just so much content to learn.
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