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?
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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Update: June 2012