Final answer:
DNA is primarily methylated on the cytosine nucleotide within specific regions known as CpG islands, which play a role in gene regulation.
Step-by-step explanation:
DNA is methylated on the nucleotide cytosine at specific positions within the DNA sequence, typically within CpG islands.
Methylation of DNA is a crucial epigenetic modification that involves the addition of a methyl group to the 5-carbon of the cytosine ring. This process can change the activity of a DNA segment without changing the sequence. When cytosines are methylated, usually at CpG islands—which are regions with a high frequency of CG dinucleotides found in the promoter regions of genes—they can affect gene expression by altering how the DNA interacts with proteins, such as histone proteins and transcription factors. While cytosine is the most commonly methylated base in DNA, other bases like adenine can also be methylated, but this happens less frequently.
It is also worth mentioning that DNA contains other nitrogenous bases such as adenine, guanine, and thymine. Adenine pairs with thymine, and cytosine pairs with guanine in the DNA structure, maintaining the stability of the double helix. Chargaff's rules state that the amount of adenine is equal to thymine and the amount of guanine is equal to cytosine, illustrating base pairing specificity.