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Name: Sara
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Question:
Is DNA an anion or a cation? I thought since it was negatively charged it was an anion but mt teacher in class today said it was a cation because negatively charged molecules logically migrate to the positively charged plate of the cathode, ie molecules that migrate towards a cathode are cations. Where is the error in my logic or there error in my logic?



Replies:
DNA is negatively charged due to the phosphate ions present in the ribose-phosphate backbone. It moves towards the positive pole during electrophoresis.

The definition kation/anion is confusing because:
1. a cation moves to the cathode
2. the cathode is negative, thus
3. a cation is positive

DNA is an anion.

The confusion is that a cathode is negative, but a cation is positively charged. For that reason these terms are not generally used in this context.

Dr. T. Wassenaar


The cathode is called that because it attracts positive ions (or cations) therefore it is negatively charged. I hate to contradict your teacher but it is a common misconception. If you go to a college level chemistry book this will be confirmed. So therefore you are correct, because of the phosphate groups in the backbone of the DNA molecule it has an overall negative charge and will migrate toward the positive electrode which is the anode.

K. Van Hoeck


negative charge = anion
positive charge = cation

anode = electrode taking in electrons
cathode = electrode giving off electrons

So your teacher had the right derivation of the names anion and cation, but the wrong final answer. Molecules that migrate toward the cathode are cations, but the cathode has a negative potential. "Cathode rays," the particles given off by a cathode, are electrons.

Richard E. Barrans Jr., Ph. D.



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