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Question

The following figure shows the direction of migration of DNA under the applied electric field. Which of the following statement(s) is/are correct about A and B?


I. B is anode and A is cathode
II. B is cathode and A is anode
III. A is positive terminal and B is negative terminal
IV. A is negative terminal and B is positive terminal

A
I and III
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B
I and IV
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C
II and III
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D
II and IV
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Solution

The correct option is C II and III
Gel electrophoresis is a laboratory technique based on the principle of movement of charged particles towards oppositely charged electrodes under the influence of electric fields. The technique is widely used for the separation of a mixture of DNA (a negatively charged molecule) fragments of varying sizes on agarose gel (a natural biopolymer) matrix under the influence of an electric field. DNA, being negatively charged, migrates towards the anode (positively charged terminal) in the electrophoresis unit.

The rate of migration is determined by the size of the DNA molecule. The greater the size of a DNA molecule, the greater the resistance that it faces to percolate through the pores present in the agarose gel. Hence, a smaller sized DNA with lower molecular weight travels more distance than a heavier one. Hence, the former is located near the anode. The heavier ones are located closer to the cathode, which is the negative terminal.
Since, in this figure, A is the terminal near which low molecular weight analytes are located, hence, A is the anode/positive terminal. B is the terminal near which a higher molecular weight fraction is located, hence, B is the cathode/negative terminal.


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