Q.103 A DNA fragment shown below has restriction sites I and II, which create fragments X, Y, and Z.
Which ONE of the following agarose gel electrophoresis patterns represents the separation of these
fragments?
Introduction
Questions based on DNA restriction digestion and agarose gel electrophoresis are extremely important for NEET, CBSE Class 12 Biology, and other competitive exams. These problems test your understanding of:
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Restriction enzyme cutting sites
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DNA fragment sizes
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Migration pattern on agarose gel
In this article, we will analyze Question 18, interpret the given DNA diagram, and explain why only one electrophoresis pattern is correct, while the others are incorrect.
Understanding the Given DNA Fragment
The DNA fragment contains two restriction sites (I and II), which divide the DNA into three fragments:
Key Observation
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Z is the largest fragment
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X is of intermediate size
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Y is the smallest fragment
Key Concept: Agarose Gel Electrophoresis Rule
📌 DNA fragments migrate according to size:
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Smaller fragments move faster and travel farther
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Larger fragments move slower and stay closer to the wells
Migration Order (Top → Bottom of Gel):
Correct Answer
✅ Option (B)
Option B correctly shows:
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Z closest to the well (top)
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X in the middle
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Y farthest from the well (bottom)
This matches the expected migration pattern in agarose gel electrophoresis.
Explanation of All Options
Option (A): ❌ Incorrect
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Shows X above Y and Z
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Incorrect because Z is the largest fragment and should be closest to the well
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Violates size-based migration rule
Option (B): ✅ Correct
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Correct descending order: Z → X → Y
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Proper representation of DNA fragment separation
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Matches the known principles of agarose gel electrophoresis
Option (C): ❌ Incorrect
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Places X above Z
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Incorrect because Z is larger than X
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Larger fragments cannot migrate farther than smaller ones
Option (D): ❌ Incorrect
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Shows Z, Y, X from top to bottom
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Incorrect placement of Y above X
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Y is the smallest fragment and should migrate the farthest
Final Conclusion
✔ The correct agarose gel electrophoresis pattern is shown in Option (B).
This pattern accurately reflects the size-dependent migration of DNA fragments produced by restriction enzyme digestion.
