Similarly , the simple - ring structure of cytosine , uracil , and thymine is derived of pyrimidine , so those three bases are called the pyrimidine bases .
Adenine and guanine have a fused - ring skeletal structure derived of purine , hence they are called purine bases .
The simple - ring structure of cytosine , uracil , and thymine is derived of pyrimidine , so those three bases are called the pyrimidine bases .
Adenine and guanine have a fused - ring skeletal structure derived of purine , hence they are called purine bases .