DNA methylation refers to the addition of a methyl group (-CH3) to the cytosine base of DNA, specifically at cytosine-guanine dinucleotides (CpG sites). This methylation of CpG sites can have several effects on gene expression and chromatin structure:
- Gene Silencing: Methylation of CpG sites in the promoter region of a gene can inhibit the binding of transcription factors, preventing their interaction with the DNA and leading to gene silencing. This can result in reduced or suppressed gene expression.
- Chromatin Condensation: DNA methylation can recruit proteins called methyl-binding domain (MBD) proteins, which bind to methylated DNA. These MBD proteins can recruit additional proteins involved in chromatin compaction, leading to a more condensed chromatin structure. The condensed chromatin can further inhibit the accessibility of transcription factors and other regulatory proteins to the DNA, contributing to gene silencing.
- Epigenetic Inheritance: DNA methylation can be maintained through cell divisions, providing a stable epigenetic mark. During DNA replication, DNA methyltransferase enzymes copy the methylation pattern from the parent strand to the newly synthesized daughter strand, ensuring that the methylation pattern is faithfully transmitted to subsequent generations of cells. This allows DNA methylation to serve as a heritable epigenetic modification that can influence gene expression patterns over time.
It is important to note that while DNA methylation is generally associated with gene repression, the relationship between DNA methylation and gene expression can be context-dependent and complex. DNA methylation patterns can vary across different cell types, developmental stages, and disease conditions, and the effects of DNA methylation can be influenced by other epigenetic modifications and regulatory factors.