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Question

Which of the following statements about agarose gel electrophoresis is/are incorrect?

I. It is a common method used to separate the DNA fragments based on their size.
II. The DNA fragments can be detected by ethidium bromide staining followed by exposure to ultraviolet rays.
III. The movement of DNA is modulated by the strength of the electric field.
IV. The electric field allows the migration of positively charged DNA towards the negative terminal.

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

The correct option is D Only IV
Gel electrophoresis is a laboratory technique based on the principle of movement of charged particles towards oppositely charged electrodes under the influence of an electric field.
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, called agarose gel electrophoresis. DNA, being negatively charged, is forced to migrate towards the anode (positively charged terminal) in the electrophoresis unit.

Besides, the pore size of the gel is a determinant of the degree of migration. The pores create a sieving effect and help in the separation of DNA molecules based on their size. The greater the size of the DNA molecule, the greater is the resistance that it faces to pass through the pores of agarose gel. Hence, the smaller sized DNA can migrate over longer distances on the gel towards the anode.

Voltage also plays a significant role in the rate of separation of DNA fragments. Under a specific condition, the higher the voltage, the higher the electric field strength between the two electrodes. Hence, higher is the force acting on the charged molecule (here DNA) to migrate. Hence, the movement of DNA and speed of migration is affected by the strength of the electric field.

The DNA fragments can be visualized by staining with ethidium bromide (added in the gel during gel preparation), followed by exposure to ultraviolet rays.

Fig: Gel containing DNA bands on ethidium bromide staining and UV exposure

Hence, all other statements except statement IV, are correct. So, statement IV is the only incorrect statement.

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