Question

When the phage DNA enters the host bacterium, the host cell protects itself from the viral DNA attacks with the help of \rule{2cm{0.15mm} enzyme.}

• A. exonuclease
• B. restriction endonuclease
• C. DNA ligase
• D. helicase

Hint:

Restriction enzymes = Molecular scissors They are widely used in genetic engineering to cut DNA at specific sites.

Answer 1

The Correct Option is B

Concept: Bacteria have a defense mechanism against invading viral DNA (such as bacteriophage DNA) known as the restriction-modification system.
• Restriction endonucleases are enzymes that cut DNA at specific recognition sequences.
• They identify foreign DNA and cleave it, thereby protecting the host cell.
• The host DNA is protected by methylation, preventing self-digestion.

Step 1: Understand the problem.
When phage DNA enters a bacterial cell, it is recognized as foreign genetic material.

Step 2: Identify the defensive enzyme.
Restriction endonucleases cut the foreign DNA into fragments, thereby inactivating it.

Step 3: Analyze options.
• (A) Exonuclease: Removes nucleotides from ends, not specific defense
• (B) Restriction endonuclease: Correct, cuts foreign DNA internally
• (C) DNA ligase: Joins DNA fragments
• (D) Helicase: Unwinds DNA

Step 4: Conclusion.
Thus, the host bacterium uses restriction endonuclease enzymes to defend against phage DNA. Final Answer: Option (B)