According to epigenetic mechanisms described, which modification represses transcription?

• A. DNA Methylation
• B. Histone Acetylation
• C. DNA Demethylation
• D. mRNA Splicing

Answer: (A)

DNA methylation represses transcription by adding methyl groups to cytosines in CpG dinucleotides, especially in promoter regions. This mark can block transcription factor binding and recruit proteins that compact chromatin, such as methyl-CpG–binding proteins that bring in chromatin remodelers and histone deacetylases, leading to a closed, inaccessible chromatin state. Because the transcription machinery can’t access the DNA, gene expression is silenced.

In contrast, histone acetylation generally promotes transcription by loosening DNA-histone interactions and opening chromatin; DNA demethylation removes a repressive signal and can reactivate transcription; and mRNA splicing is a post-transcriptional process, not a chromatin modification that directly controls transcription initiation.