Final answer:
The nitrogenous bases with 2 fused heterocyclic rings are adenine (A) and guanine (G), which are known as purines. They are one of the two types of nitrogenous bases found in nucleic acids, alongside pyrimidines, which have a single-ring structure.
Step-by-step explanation:
The nitrogenous bases with 2 fused heterocyclic rings are known as purines. These bases, adenine (A) and guanine (G), are essential components of nucleic acids such as DNA and RNA. Purines are distinguished by their double-ring structure, composed of a six-member ring fused to a five-member ring with several nitrogen atoms. In contrast, pyrimidines, which include cytosine (C), thymine (T), and uracil (U), have a single-ring structure.