Question

Common site for DNA methylation is

• A. CpG island
• B. ORF
• C. Terminator
• D. Initiator

Hint:

Hypermethylation of CpG islands in the promoter = Gene Turned OFF.
Hypomethylation = Gene Turned ON.
This is a key mechanism for cell differentiation and genomic imprinting.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question relates to epigenetics, specifically identifying where methyl groups are most commonly added to the DNA sequence to regulate gene expression.
Detailed Explanation:

• DNA Methylation:
This is a biochemical process where a methyl group ($-CH_3$) is added to the DNA molecule, typically at the number 5 carbon of the cytosine ring.

• CpG Sites:
In vertebrates and plants, methylation occurs almost exclusively at "CpG" sites, where a cytosine (C) is followed by a guanine (G) in the $5' \rightarrow 3'$ direction, linked by a phosphate (p) group.

• CpG Islands:
"CpG islands" are regions of DNA (often near the promoter of a gene) that have a high frequency of CpG sites.
Methylation of CpG islands in a gene's promoter generally leads to gene silencing (stable transcriptional repression).

• Analyzing other options:
ORF (Open Reading Frame): The part of a reading frame that contains no stop codons and is translated into protein.
Terminator: A sequence that signals the end of transcription.
Initiator: A core promoter element. While methylation happens near these, the specific site/motif is the CpG sequence.

Step 2: Final Answer:
CpG islands are the most common and significant sites for DNA methylation in the context of gene regulation.