What is the criterion for the movement of DNA fragments on agarose gel during gel electrophoresis?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The smaller the fragment size, the farther it moves.
DNA fragments carry a negative charge, so in an electric field they all move towards the anode (positive electrode). The agarose gel acts like a sieve. Small fragments slip through its pores easily and travel far, while large fragments are held back. So the fragments separate according to their size.
Option (1) Negatively charged fragments do not move. — Incorrect. Every DNA fragment is negatively charged, and it is this charge that makes them move towards the anode.
Option (2) The larger the fragment size, the farther it moves. — Incorrect. This is the reverse of what happens. Large fragments meet more resistance from the gel and stay nearer the wells.
Option (3) The smaller the fragment size, the farther it moves. — Correct. The sieving effect of the agarose gel lets the smaller fragments move faster, so the smaller the fragment, the farther it moves.
Option (4) Positively charged fragments move to the farther end. — Incorrect. DNA fragments are not positively charged. They are negatively charged molecules and move towards the anode.
NCERT Extract:
Restriction Enzymes – “… 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)