Gene regulation — DNA methylation: In general, how does cytosine methylation across gene promoters affect gene activity?

Introduction / Context:
DNA methylation is a key epigenetic modification with profound effects on chromatin structure and transcription. Promoter methylation is widely used in development, imprinting, and cancer to repress gene expression.

Given Data / Assumptions:

• Methylation typically refers to 5-methylcytosine at CpG dinucleotides.
• Focus is on promoter or regulatory region methylation, not gene bodies.
• Question asks the general effect on expression.

Concept / Approach:
Methylated promoters recruit methyl-CpG binding proteins and corepressors, compacting chromatin and excluding transcription factors. The net effect is transcriptional silencing. Demethylation or hypomethylation correlates with activation for many genes.

Step-by-Step Solution:

Verification / Alternative check:
Experiments using demethylating agents (e.g., 5-azacytidine) reactivate silenced genes, confirming the repressive role of methylation.

Why Other Options Are Wrong:

• “Active” contradicts the typical promoter methylation effect.
• “Dynamic switching” is overly broad and not a standard outcome of methylation per se.
• “Either (a) or (b)” lacks specificity and misleads learners.

Common Pitfalls:
Confusing promoter methylation (repressive) with gene-body methylation (which can correlate with active transcription in some contexts). Always consider genomic context.

Final Answer:
Genes are silent