Q.37 Two purified DNA samples A and B contain equal number of base pairs.
Each of these DNA samples has one site each for
EcoRI and BamHI restriction enzymes.
Complete digestion with both the enzymes yielded
3 DNA bands and 2 DNA bands respectively for A and B upon electrophoresis
of the digestion products.
Which one of the following explains the observation?

Options:

(A) A is circular DNA and B is linear

(B) B is circular DNA and A is linear

(C) A is circular DNA and B could be linear or circular

(D) B is circular DNA and A could be linear or circular

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Introduction

Restriction enzyme digestion followed by gel electrophoresis is a powerful technique to determine whether DNA is circular or linear. A common exam question involves analyzing the number of bands produced after digestion with enzymes such as EcoRI and BamHI. In this article, we explain how the number of restriction sites and DNA topology affect the digestion pattern.

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Key Concept: Restriction Digestion and DNA Topology

1. Linear DNA

• Number of fragments formed = number of cuts + 1

• With two restriction sites:

Fragments=2+1=3

2. Circular DNA

• Number of fragments formed = number of cuts

• With two restriction sites:

Fragments=2

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Applying the Concept to the Question

Sample A

• Produces 3 DNA bands

• Indicates linear DNA

Sample B

• Produces 2 DNA bands

• Indicates circular DNA

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✅ Correct Interpretation

• A is linear DNA

• B is circular DNA

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✅ Correct Answer

Option (B): B is circular DNA and A is linear

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Explanation of All Options

Option (A): A is circular DNA and B is linear

❌ Incorrect
Circular DNA with two cuts produces 2 fragments, not 3.

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Option (B): B is circular DNA and A is linear

✅ Correct

• Linear DNA → 3 fragments

• Circular DNA → 2 fragments

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Option (C): A is circular DNA and B could be linear or circular

❌ Incorrect
Band numbers clearly distinguish circular from linear DNA.

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Option (D): B is circular DNA and A could be linear or circular

❌ Incorrect
A giving 3 bands must be linear.

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Summary Table

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Final Conclusion

Restriction enzyme digestion patterns directly reveal DNA topology. Since:

• Sample A gives 3 bands → linear DNA

• Sample B gives 2 bands → circular DNA

Correct Answer: Option (B)