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:
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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.
Step 1: Understanding the Concept:
Bacteria use Restriction-Modification (R-M) systems to protect themselves from foreign DNA. A methyltransferase enzyme adds a methyl group to specific bases within a recognition sequence in the bacterium's own DNA. This methylation blocks the corresponding restriction enzyme from cutting the host DNA. Step 2: Detailed Explanation:
The common sites for protective DNA methylation are:
On Adenine: The N6 position.
On Cytosine: The N4 position and, most commonly, the C5 position.
Analyzing the options for cytosine:
D. 5th Carbon: Methylation at the 5th carbon of the pyrimidine ring creates 5-methylcytosine. This is a major form of DNA modification for protection in R-M systems. This statement is correct.
A, B, C: The 4th, 7th, and 6th carbons are not the primary sites for this protective methylation. While methylation can occur on the amino group at C4, the most well-known and direct carbon methylation site is C5.
Step 3: Final Answer:
The correct statement is D (5th Carbon). Therefore, the correct option to choose is (A) D only.
