We have our base, the nitrogenous base, attached to our center sugar, which is also attached to our phosphate group. Now, there are four different types of nitrogenous bases. We have adenine, guanine, cytosine and thymine. Now, adenine and guanine, or simply A and g are two types of nitrogenous bases that consist of two rings. And these types of nitrogenous bases that consist of two rings are known as purines. And notice thymine and cytosine or TNC consist of only a single ring structure.