Bacterial defenses against phages: Which mechanism refers to host restriction of foreign DNA and is a classic bacterial defense against bacteriophage infection?

Introduction / Context:
Bacteria possess several anti-phage defenses. A foundational system is restriction–modification, in which host-encoded restriction endonucleases cleave invading unmodified DNA, while host DNA is protected by methylation. Recognizing the terminology is essential for microbial genetics and biotechnology.

Given Data / Assumptions:

• Restriction is enzymatic cleavage of specific DNA sequences.
• Modification (methylation) protects self DNA.
• Only one option names the defense mechanism itself.

Concept / Approach:
Restriction endonucleases survey DNA for defined sites. Foreign phage DNA lacking host methylation patterns is cut, preventing replication. This concept underpins many molecular cloning tools (type II restriction enzymes) and explains host-range barriers in phage infection.

Step-by-Step Solution:
Identify the defense mechanism: restriction–modification. Choose the term “restriction” as the defensive act. Exclude unrelated processes (concatamerization, polymerization). Note that lysogeny is a phage life cycle, not a host defense.

Verification / Alternative check:
Genetic studies show methyltransferase-deficient cells become hypersensitive to their own restriction enzymes, validating the specificity and protective role of modification.

Why Other Options Are Wrong:
Concatamerization describes DNA joining in phage packaging; polymerization is generic; lysogeny benefits temperate phages rather than host defense; autotransduction is unrelated.

Common Pitfalls:
Assuming CRISPR-Cas is the only defense; restriction–modification long predates CRISPR in textbooks and exams.

Final Answer:
Restriction.