Final answer:
The nitrogenous base in DNA that is methylated in mammals and other vertebrates is cytosine.
Step-by-step explanation:
In mammals and other vertebrates, the nitrogenous base in DNA that is methylated is cytosine. Cytosine is one of the four nitrogenous bases found in DNA, the others being adenine (A), thymine (T), and guanine (G).
Methylation typically occurs at the 5' position of the cytosine pyrimidine ring and is important for gene regulation and protecting DNA from the insertion of viral genomes. Adenine and guanine are purines, which have a two-ringed structure, while cytosine and thymine are pyrimidines, having a single-ring structure.