Q.35 Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: Dam methylase protects E. coli DNA from phage endonucleases
Reason [r]: E. coli Dam methylase methylates the adenosine residue in the sequence “GATC”
(A) Both [a] and [r] are true and [r) is the correct reason for [a]
(B) Both [a] and [r) are true but [r] is not the correct reason for [a]
(C) Both [a] and [r] are false
(D) [a] is false but [r] is true

Correct Answer: (A) Both [a] and [r] are true and [r] is the correct reason for [a]

Dam methylase (DNA adenine methylase) in E. coli methylates the adenine residue in GATC sequences, protecting the host DNA from cleavage by its own restriction endonucleases while allowing phage DNA (which lacks this methylation) to be degraded.​

Assertion Analysis

Assertion [a] states that Dam methylase protects E. coli DNA from phage endonucleases. This is true because Dam methylation distinguishes self from non-self DNA in restriction-modification systems. Unmethylated phage DNA gets cleaved by E. coli‘s restriction enzymes (like EcoKI), while methylated host DNA remains intact.​

Reason [r] states that E. coli Dam methylase methylates the adenosine in “GATC”. This is correct; Dam specifically adds a methyl group to the N6 position of adenine in the palindromic GATC sequence (5′-GATC-3’/3′-CTAG-5′).​

Option Breakdown

• (A) Both true, [r] explains [a]: Correct. The methylation at GATC directly enables protection by preventing host restriction enzymes from cutting E. coli DNA, while phage endonucleases (or host enzymes targeting phage) cleave unmethylated invaders.​

• (B) Both true, but [r] not reason: Incorrect. The specific GATC methylation is mechanistically why protection occurs.​

• (C) Both false: Incorrect. Both statements are factually accurate.​

• (D) [a] false, [r] true: Incorrect. [a] holds due to the restriction-modification role.​

Introduction to Dam Methylase in E. coli DNA Protection

Dam methylase plays a crucial role in E. coli defense against bacteriophages by methylating adenine in GATC sequences, preventing cleavage by restriction endonucleases. This mechanism is key for competitive exams like GATE Biotechnology and IIT JAM, where assertion-reason questions test molecular biology concepts.​

Mechanism of Dam Methylation and Phage Resistance

Dam (DNA adenine methyltransferase) recognizes 5′-GATC-3′ and methylates the adenine (N6 position), fully methylating both strands post-replication. In restriction-modification systems, methylated host DNA evades endonucleases, but unmethylated phage DNA is degraded, restricting viral growth. This GATC-specific action directly supports the assertion that Dam protects E. coli DNA from phage endonucleases.​

Assertion-Reason Question Breakdown for GATE Prep

In the given question, both [a] (protection role) and [r] (GATC methylation) are true, with [r] explaining [a] via the self/non-self discrimination. Students often confuse Dam with Dcm (CCWGG methylation), but Dam’s phage protection is GATC-dependent.​

• Primary function: Mismatch repair directionality and replication control.​

• Phage defense link: Complements type I/III restriction systems.​

• Exam tip: Option (A) fits when mechanism directly causes the effect.

Why This Matters for Biotech Entrance Exams

Mastering Dam methylase E. coli phage protection concepts aids in solving 2-3% of molecular biology questions in IIT JAM/GATE. Practice similar assertion-reason formats to score high in genetics and microbiology sections.​