How Restriction Endonucleases Protect Bacteria from Lambda Phage Invasion

Bacteria are not defenseless against viral invaders. When under attack by bacteriophages like lambda (λ) phage, bacteria deploy specialized enzymes to destroy foreign genetic material. One of the most critical tools in this defense system is the restriction endonuclease.

In this article, we explore how restriction endonucleases work, their role in bacterial defense, and why they are the correct answer to the question about the degradation of lambda phage DNA.

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What Are Restriction Endonucleases?

Restriction endonucleases, or restriction enzymes, are molecular scissors used by bacteria to:

• Recognize and bind to specific DNA sequences (often palindromic).

• Cut the DNA within or near these sequences.

• Distinguish between foreign (phage) and native DNA.

Native bacterial DNA is typically protected from cleavage by methylation, while foreign DNA—such as from a lambda phage—is unmodified and thus vulnerable.

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Role in Defending Against Lambda Phage

When a lambda phage infects a bacterium, it injects its viral DNA into the host cell. The bacterium:

1. Detects unmethylated viral DNA.

2. Uses restriction endonucleases to cleave it at specific sites.

3. Destroys the phage genome before it can hijack the cell’s replication machinery.

This rapid and targeted response is a first line of defense in prokaryotic immune systems.

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Why Restriction Endonucleases Are the Correct Answer

Let’s evaluate the options:

• (a) Exonuclease:
  Removes nucleotides from the ends of DNA strands but does not offer the sequence specificity needed for defending against lambda phage.

• (b) Phagodegradase:
  This term is not a recognized biological enzyme and seems fictitious.

• (c) Restriction endonuclease:
  ✅ Correct Answer
  Specifically cleaves foreign DNA like that of lambda phage, making it the main defense tool.

• (d) Topoisomerase:
  Involved in relieving DNA supercoiling during replication; not related to DNA degradation or bacterial immunity.

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Conclusion

The bacterial enzyme responsible for degrading the genome of the invading lambda phage is the:

✅ Restriction endonuclease

These enzymes serve as a powerful defense mechanism, preventing phages from taking over the host cell. Their ability to selectively destroy non-self DNA has also made them indispensable tools in molecular biology, genetic engineering, and biotechnology.