The four nitrogenous bases in DNA are adenine (A), thymine (T), cytosine (C), and guanine (G). These are further divided into two types based on their structure: thymine and cytosine (called pyrimidines) have single-ring structures, while adenine and guanine (called purines) have double-ring structures. A double-ring base is always paired with a single-ring base on the opposite strand.