Where do DNA methyltransferases mainly function?

DNA methyltransferases primarily function at the C5 position of cytosine residues within CpG dinucleotides. This process involves adding a methyl group to the cytosine base, which plays a crucial role in the regulation of gene expression and the maintenance of genomic stability. The methylation of CpG sites often leads to gene silencing, as it can prevent the binding of transcription factors and recruit other proteins that are involved in chromatin remodeling.

This mechanism is particularly significant in mammals, where many regulatory regions of genes are rich in CpG islands. Methylation patterns can be heritable and thus influence gene expression profiles across different cell types and developmental stages. This aspect of DNA methylation is critical in processes such as embryonic development, X-chromosome inactivation in females, and the silencing of retrotransposons.

The other choices do not represent the primary function of DNA methyltransferases. They do not appropriately define the specific site of action or the biological context in which methyltransferases operate.