Q.66 Bacteria produce an enzyme methylase, which mainly play a role in –

1. modification of DNA
2. cutting of DNA
3. repair of DNA
4. rearrangement of DNA

Bacterial methylase enzymes are crucial in protecting bacterial DNA from their own restriction enzymes through epigenetic modification. This MCQ tests understanding of restriction-modification (R-M) systems, a key microbiology concept for exam success.

Correct Answer: Option 1 – Modification of DNA

Bacterial DNA methylases (methyltransferases) add methyl groups to specific DNA bases (usually adenine or cytosine), modifying the host DNA. This protects it from cleavage by the bacteria’s own restriction endonucleases, which cut unmethylated foreign DNA (like phage). Classic R-M system function.

Detailed Explanation of All Options

Option 1: Modification of DNA (CORRECT)

Methylases recognize specific sequences and methylate bases (e.g., Dam methylase at GATC). This “tags” bacterial DNA as self, preventing restriction enzyme attack. Primary role in protection and regulation.

Option 2: Cutting of DNA (INCORRECT)

Cutting is the role of restriction endonucleases (e.g., EcoRI), not methylases. Methylases modify to prevent cutting of host DNA.

Option 3: Repair of DNA (INCORRECT)

While methylation aids mismatch repair (e.g., MutHLS system uses hemimethylated DNA to distinguish strands), this is secondary. Primary/main role is modification/protection in R-M systems.

Option 4: Rearrangement of DNA (INCORRECT)

DNA rearrangement involves recombinases/site-specific enzymes (e.g., Cre-lox). Methylases don’t rearrange sequences—just add chemical tags.

Restriction-Modification System Quick Reference

Why This Matters for Exams

• NEET/CSIR Link: Biotechnology principles, microbial genetics chapters.

• Common Trap: Confusing methylase (modification) with restriction enzyme (cutting).

• Mnemonic: “Methylase = Modify self; Restriction = Rip foreign.”

Mastering bacterial defense mechanisms boosts microbiology scores significantly!