DNA methylation and gene expression: which statement is correct?

• A. DNA methylation (usually 5-methylcytosine at CpG sites) represses transcription by recruiting repressive proteins and compacting chromatin.
• B. DNA methylation enhances transcription by recruiting activator proteins.
• C. DNA methylation is the same as histone acetylation.
• D. DNA methylation only occurs in prokaryotes and has no effect on chromatin.

Answer: (A)

DNA methylation is a key way cells regulate gene activity. When a cytosine in a CpG site becomes methylated, it often leads to gene silencing, especially when this modification is in a gene’s promoter region. The added methyl group signals proteins that recognize methylated DNA (methyl-CpG-binding proteins), and these proteins recruit repressive complexes such as histone deacetylases. The combined effect is tighter packing of chromatin (a more closed, inactive state) and reduced access for transcription machinery, so transcription drops. That’s why the correct statement describes DNA methylation as repressing transcription by recruiting repressive proteins and compacting chromatin.

This mechanism contrasts with histone acetylation, which generally loosens chromatin and promotes transcription, and with the idea that methylation would enhance transcription. It also notes that methylation isn’t exclusive to prokaryotes (and isn’t simply the same thing as chromatin modification), since methylation in eukaryotes interacts with chromatin structure to influence gene expression.