What is the general effect of DNA methylation on gene expression?

• A. Methylation of DNA generally represses transcription, contributing to gene silencing.
• B. It always activates transcription.
• C. It does not influence transcription.
• D. It changes the amino acid sequence.

Answer: (A)

DNA methylation generally reduces transcription by adding methyl groups to cytosines, especially at CpG sites in promoter regions. This mark is recognized by proteins that recruit chromatin-remodeling complexes and histone deacetylases, leading to a more compact chromatin state and blocking access of transcription factors and RNA polymerase to the DNA. Because of this, methylation commonly silences gene expression. While there are nuanced effects in different genomic contexts, the usual outcome in promoters is transcriptional repression. It does not change the amino acid sequence, since that would require changes to the DNA code itself, not just its methylation.