Bacterial defense — the native role of restriction endonucleases In their natural bacterial context, what is the primary physiological role of restriction endonucleases?

Introduction / Context:
Restriction endonucleases are sequence-specific DNA-cutting enzymes central to molecular cloning. However, their evolutionary origin lies in bacterial immunity. Understanding this protects against misconceptions and clarifies why bacteria also carry companion DNA methyltransferases that protect their own genomes.

Given Data / Assumptions:

• Bacteria compete with and defend against bacteriophages.
• Restriction enzymes recognize specific DNA sequences and cleave phosphodiester bonds.
• Host DNA is methylated at cognate sites to prevent self-digestion.

Concept / Approach:

Restriction–modification systems consist of a restriction endonuclease and a DNA methyltransferase. The enzyme cuts unmethylated foreign DNA, such as phage genomes, while host methylation marks self DNA to resist cleavage. This protective function is the native role of restriction enzymes in bacteria, distinct from their biotechnology applications.

Step-by-Step Solution:

Verification / Alternative check:

Genetic studies show that loss of methyltransferase function leads to self-restriction and lethality, underscoring the system’s defensive purpose.

Why Other Options Are Wrong:

Restriction enzymes do not shred chromosomes during replication (A) and do not synthesize RNA primers (C). mRNA splicing (E) is a eukaryotic process and not mediated by restriction enzymes.

Common Pitfalls:

Assuming bacteria use restriction enzymes for general DNA metabolism rather than targeted defense.

Final Answer:

To degrade invading bacteriophage DNA as part of a restriction–modification system