What is the primary role of DNA methyltransferases?

The primary role of DNA methyltransferases is to add methyl groups to nucleotides, specifically to the cytosine base of DNA. Methylation is a crucial process that can regulate gene expression without altering the DNA sequence itself. This modification can impact how genes are turned on or off, playing a significant role in processes such as development, cellular differentiation, and the maintenance of genomic stability.

Methylation can also serve as a signal for the binding of proteins that regulate transcription, thus influencing various cellular functions and the response of the organism to environmental factors. This function is vital in epigenetics, where changes that do not involve alterations to the underlying DNA sequence can lead to heritable changes in gene expression.

In contrast to this, other options do not relate to the primary activity of DNA methyltransferases. For instance, digesting nucleic acids involves enzymes such as nucleases, joining DNA fragments pertains to the role of ligases, and unwinding DNA double helices is primarily the function of helicases. Therefore, the option emphasizing the addition of methyl groups accurately reflects the essential function of DNA methyltransferases.