What refers to the addition of methyl groups to DNA, often silencing gene expression?

The process that refers to the addition of methyl groups to DNA, leading to the silencing of gene expression, is known as DNA methylation. This modification occurs at specific sites on the DNA molecule, particularly at cytosine bases in a cytosine-guanine (CpG) dinucleotide context. When these methyl groups are added, they can inhibit the binding of transcription factors and other proteins that are necessary for gene expression, effectively turning off or silencing the gene.

DNA methylation plays a crucial role in various biological processes, including development, cellular differentiation, and the regulation of gene expression patterns. It is also involved in maintaining genomic stability and is a key factor in processes such as X-chromosome inactivation and imprinting.

In contrast, the other options refer to different processes. DMSO modification does not pertain to the addition of methyl groups in the context of DNA but rather relates to solvent use in biochemical applications. RNA interference involves small RNA molecules that inhibit gene expression by targeting messenger RNA (mRNA) for degradation, which differs from the epigenetic modification that occurs in DNA methylation. Gene silencing is a broader term that can encompass various mechanisms of turning off genes, including but not limited to DNA methylation