Where are methyl groups typically added in DNA?

Methyl groups are typically added to the nitrogenous bases in DNA, particularly to cytosine in a process known as DNA methylation. This modification plays a crucial role in regulating gene expression and is a key mechanism of epigenetics. When a methyl group is added to cytosine, especially when it is followed by guanine (in a CpG context), it can lead to the repression of nearby genes by inhibiting the binding of transcription factors or attracting proteins that modify the chromatin structure, thus affecting how tightly or loosely DNA is packaged and, consequently, its accessibility for transcription.

This process is vital for normal development, cellular differentiation, and maintenance of genomic integrity. It also has implications in various diseases, including cancer, where abnormal methylation patterns can lead to the silencing of tumor suppressor genes or activation of oncogenes. Therefore, the addition of methyl groups to the nitrogenous bases is a fundamental aspect of regulating biological processes at the molecular level.