In agarose gel electrophoresis, DNA fragments move towards the ____ and separate according to their ____.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → anode; size
Gel electrophoresis separates the DNA fragments made by restriction enzymes. DNA is negatively charged, so in an electric field it moves towards the positive electrode, the anode. The agarose gel acts like a sieve. Small fragments pass through it easily and move far; large fragments move slowly. So the fragments separate according to their size.
Option (1) cathode; charge — Incorrect. DNA is negatively charged, so it moves away from the cathode (the negative electrode). Also, all DNA fragments carry negative charge, so they are not separated by charge.
Option (2) anode; size — Correct. The fragments move towards the anode because DNA is negatively charged. The gel sieves them, so smaller fragments move farther and the fragments separate by size.
Option (3) anode; charge — Incorrect. The direction is right, but the basis of separation is wrong. Every DNA fragment has negative charge; it is the sieving effect of the gel on fragments of different sizes that separates them.
Option (4) cathode; size — Incorrect. The basis is right, but DNA does not move towards the cathode. Being negatively charged, it moves towards the anode.
NCERT Extract:
Separation and Isolation of DNA Fragments – “The cutting of DNA by restriction endonucleases results in the fragments of DNA. These fragments can be separated by a technique known as gel electrophoresis. Since DNA fragments are negatively charged molecules they can be separated by forcing them to move towards the anode under an electric field through a medium/matrix. Nowadays the most commonly used matrix is agarose which is a natural polymer extracted from sea weeds. The DNA fragments separate (resolve) according to their size through sieving effect provided by the agarose gel. Hence, the smaller the fragment size, the farther it moves.”
Figure 9.3 A typical agarose gel electrophoresis showing migration of undigested (lane 1) and digested set of DNA fragments (lane 2 to 4)