Question

The DNA of the bacterial cell is protected from the cell's own restriction enzymes by the addition of methyl group (-CH3) to ___________ of cytosine.
A. 4th Carbon
B. 7th Carbon
C. 6th Carbon
D. 5th Carbon
Choose the correct answer from the options given below:

• A. D only
• B. B only
• C. B and D only
• D. B and C only

Hint:

Remember that methylation is like a "do not cut" signal for a bacterium's own restriction enzymes. The most common methylation sites are N6 of Adenine and C5 of Cytosine.

Answer 1

The Correct Option is A

Step 1: Concept Overview:
Bacteria utilize Restriction-Modification (R-M) systems to defend against foreign DNA. A methyltransferase enzyme adds a methyl group to specific bases within a recognition sequence of the bacterium's DNA, preventing the corresponding restriction enzyme from cleaving the host DNA.
Step 2: Detailed Explanation:
Common DNA methylation sites for protection include:


Adenine: N6 position.

Cytosine: N4 position, but most commonly the C5 position.

Analysis of cytosine methylation sites:


D. 5th Carbon: Methylation at the 5th carbon of the pyrimidine ring results in 5-methylcytosine, a significant DNA modification for R-M system protection. This is the correct statement.

A, B, C: The 4th, 7th, and 6th carbons are not primary sites for protective methylation. While C4 amino group methylation is possible, C5 is the most recognized direct carbon methylation site.

Step 3: Conclusion:
The correct statement is D (5th Carbon). Therefore, select option (A) D only.