The specific structural difference between purine and pyrimidine nitrogenous bases lies in the arrangement of carbon and nitrogen atoms in their heterocyclic aromatic ring structures. Purines have a two-ring structure, consisting of a six-membered pyrimidine ring fused to a five-membered imidazole ring. The two purine bases found in nucleic acids are adenine A and guanine G . Pyrimidines, on the other hand, have a single six-membered ring structure containing four carbon atoms and two nitrogen atoms. The three pyrimidine bases found in nucleic acids are cytosine C , thymine T , and uracil U . Thymine is found only in DNA, while uracil is found only in RNA.This structural difference impacts their function in DNA and RNA in several ways:1. Base pairing: In both DNA and RNA, purines always pair with pyrimidines to maintain a consistent width of the double helix structure. In DNA, adenine A pairs with thymine T , and guanine G pairs with cytosine C . In RNA, adenine A pairs with uracil U , and guanine G pairs with cytosine C . This specific pairing, known as Watson-Crick base pairing, ensures accurate replication and transcription of genetic information.2. Stability: The difference in the number of hydrogen bonds between the base pairs also contributes to the stability of the DNA and RNA structures. A-T or A-U in RNA base pairs form two hydrogen bonds, while G-C base pairs form three hydrogen bonds. The higher number of hydrogen bonds in G-C pairs provides greater stability to the nucleic acid structure.3. Mutation rates: The difference in the structure and hydrogen bonding between purines and pyrimidines can also influence mutation rates. For example, the spontaneous deamination of cytosine can lead to the formation of uracil, which can result in a C-to-T or C-to-U in RNA mutation if not repaired. This type of mutation is more likely to occur in pyrimidines than in purines.In summary, the structural differences between purine and pyrimidine nitrogenous bases play a crucial role in maintaining the stability, specificity, and fidelity of genetic information storage and transmission in DNA and RNA.