Question

What are the positions of 'N' atoms present in purine ring of nucleic acids?

• A. 1, 3 and 5
• B. 1, 3 and 9
• C. 1, 5, 7 and 9
• D. 1, 3, 7 and 9

Hint:

Purine bases (Adenine, Guanine) attach to the sugar molecule specifically via the Nitrogen at position 9 (N-9 glycosidic linkage). Pyrimidines attach via the Nitrogen at position 1.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question tests biochemical structural knowledge, asking for the specific numbering positions of the nitrogen atoms within the skeletal framework of a purine molecule.

Step 2: Detailed Explanation:
Nucleic acids (DNA and RNA) contain two primary classes of nitrogenous bases: Pyrimidines and Purines.
1. Pyrimidines (Cytosine, Thymine, Uracil): These possess a simple six-membered heterocyclic aromatic ring. The nitrogen atoms in the pyrimidine ring are located at positions 1 and 3.
2. Purines (Adenine, Guanine): These possess a more complex double-ring structure consisting of a six-membered pyrimidine ring fused perfectly to a five-membered imidazole ring.
The IUPAC numbering convention for the purine double ring is highly specific and unique:
- Numbering begins at one of the nitrogens in the six-membered ring and proceeds counter-clockwise around the pyrimidine portion.
- Then it continues clockwise around the imidazole portion.
Following this standard numbering scheme, the four nitrogen atoms in the purine skeleton are located exactly at positions 1, 3, 7, and 9.

Step 3: Final Answer:
The positions are 1, 3, 7 and 9, matching option (d).