Methylation of CpG islands near gene promoters inhibits transcription primarily by facilitating heterochromatin formation. This is the correct answer (C) for the given multiple-choice question from exams like GATE BT 2023.

Option Analysis

Option A: Preventing RNA polymerase binding
Methylation does not directly block RNA polymerase II from binding to the promoter. Instead, it recruits proteins that indirectly hinder the transcription initiation complex assembly, making this mechanism secondary rather than primary.

Option B: Facilitating repressor binding
While methylated CpG sites attract methyl-CpG-binding domain (MBD) proteins like MeCP2, which can recruit repressors such as Sin3A and HDACs, this is part of a broader silencing process rather than the main mechanism for promoter CpG methylation.

Option C: Facilitating heterochromatin formation (Correct)
Promoter CpG methylation recruits MBD proteins and histone deacetylases, leading to histone deacetylation, H3K9/H3K27 methylation, and compaction into repressive heterochromatin that stably silences genes.

Option D: Inducing euchromatin formation
This is incorrect, as euchromatin is the open, transcriptionally active state marked by histone acetylation and H3K4 methylation; methylation promotes the opposite closed chromatin state.

Methylation of CpG islands near promoter regions serves as a key epigenetic mechanism to silence gene expression, commonly tested in competitive exams like IIT JAM and GATE Biotechnology. These unmethylated CpG-rich sequences typically mark active promoters, but adding methyl groups to cytosine bases triggers repression.

Core Mechanism

Methylation recruits MBD proteins that tether histone-modifying complexes, converting active chromatin to compact heterochromatin. This blocks transcription factor access and stabilizes silencing across cell divisions.

• H3K9me3 and H3K27me3 marks accumulate.

• DNA methyltransferases (DNMTs) maintain the pattern.

• Contrasts with intragenic CpG methylation, which tolerates transcription.

Exam Relevance

In GATE BT 2023 Q.40, option C (“facilitating heterochromatin formation”) is correct, as it captures the dominant pathway over direct polymerase or repressor effects. Euchromatin induction (D) reverses this process.

Biological Implications

Aberrant promoter hypermethylation drives cancer by silencing tumor suppressors, highlighting therapeutic potential of demethylases like 5-aza-cytidine. For students, mastering this links epigenetics to molecular biology sections in IIT JAM prep.