Correct option: (A) Modification of DNA

Introduction

Bacteria produce enzyme methylase which mainly plays role in protecting their own DNA through specific chemical modification. This enzyme is a key component of the bacterial restriction-modification (R-M) system, working alongside restriction enzymes to distinguish self from foreign DNA. This concept is fundamental for understanding bacterial defense mechanisms and frequently tested in competitive life science exams.

Primary Function: DNA Modification (Correct Answer)

Methylase (DNA methyltransferase) adds methyl groups (-CH₃) to specific adenine (A) or cytosine (C) bases in DNA at recognition sequences.

Key mechanism:

Protection workflow:

1. Methylase recognizes specific DNA sequences (e.g., GAATC)

2. Adds methyl group to A or C base within sequence

3. Bacterial DNA becomes “self-marked”

4. Restriction enzymes ignore methylated sites

5. Foreign phage/bacteriophage DNA (unmethylated) gets cleaved

Example: EcoRI methylase methylates GAATTC → protects bacterial DNA while EcoRI restriction enzyme cuts unmethylated GAATTC from invaders.

Why Other Options Are Incorrect

(B) Cutting of DNA – Incorrect

Cutting performed by: Restriction endonucleases (not methylase)
Methylase role: Protects DNA from cutting by methylating it
Opposite function: Methylase prevents restriction enzymes from cutting bacterial DNA
Analogy: Methylase = “lock”, Restriction enzyme = “key that only cuts unlocked DNA”​

(C) Repair of DNA – Incorrect (Secondary Role)

DNA repair role: Methylation helps mismatch repair by marking template strand (methylated) vs new strand (unmethylated)
Primary role: Defense against foreign DNA, not repair
Dam methylase (E. coli) aids repair, but R-M methylases primarily protect genome
Exam focus: Question emphasizes “mainly plays role” = defense modification

(D) Rearrangement of DNA – Incorrect

Rearrangement involves: Site-specific recombination, transposons, integrases
Methylase function: Chemical modification only, no cutting/pasting/rearranging
No structural change: Methyl groups don’t alter DNA sequence or organization
Completely unrelated mechanism​

Restriction-Modification System Summary Table

Exam Strategy: Why (A) is Always Correct

Standard MCQ pattern: Bacterial methylase questions test R-M system knowledge
Key memory point: “Methylase modifies → Restriction cuts unmethylated”
Elimination logic:

• Cutting = Restriction enzymes ✅

• Repair = Secondary role ❌

• Rearrangement = Wrong enzyme ❌

• Modification = Primary role ✅

Clinical/Research Relevance

• Biotechnology: Methylases used in cloning to protect vector DNA

• Gene therapy: Understanding R-M systems prevents bacterial degradation of therapeutic DNA

• Antibiotic resistance: Some bacteria use methylation patterns for phase variation

For MCQ “Bacteria produce enzyme methylase which mainly plays role”: Answer = (A) Modification of DNA – protects self DNA in R-M defense system.