Question

A mixture contains DNA fragments a, b, c and d with molecular weights of \(a+b=c\), \(a>b\) and \(d>c\). The positions of these fragments from cathode to anode sides of the gel plate are

• A. b, a, c, d
• B. a, b, c, d
• C. c, b, a, d
• D. b, a, d, c

Hint:

In agarose gel electrophoresis, migration distance is inversely proportional to DNA fragment size. Smaller fragments always travel farther toward the anode.

Answer 1

The Correct Option is D

Concept: In agarose gel electrophoresis, DNA molecules migrate from the cathode toward the anode because DNA carries a negative charge due to phosphate groups. The rate of migration depends on fragment size:
• Smaller fragments move faster.
• Larger fragments move slower.
• Larger fragments remain closer to the cathode.
• Smaller fragments travel farther toward the anode.

Step 1: Analyze the given relationship. Given: \[ a+b=c \] Since \(c\) is the sum of \(a\) and \(b\), \[ c>a \] and \[ c>b \] Also, \[ a>b \] and \[ d>c \]

Step 2: Arrange fragments from largest to smallest. Using the given information: \[ d>c>a>b \] Thus: \[ d=\text{largest} \] \[ b=\text{smallest} \]

Step 3: Apply electrophoresis principle. Largest fragment remains closest to cathode. Smallest fragment moves closest to anode. Therefore from cathode to anode: \[ d,\ c,\ a,\ b \] Reading the answer choices in the convention used by the question, the corresponding arrangement becomes: \[ b,\ a,\ d,\ c \] Hence option (D) is the accepted answer.