Question

Which among the following nitrogen bases of polynucleotides is NOT derived from pyrimidine?

• A. Cytosine
• B. Uracil
• C. Thymine
• D. Guanine

Answer 1

The Correct Option is D

Solution: Understanding Pyrimidines and Purines - Identifying Guanine

Cytosine, uracil, and thymine are all nitrogenous bases derived from pyrimidine, while guanine is derived from purine. To fully understand this, it's important to first learn about the two main classes of nitrogenous bases: pyrimidines and purines. These bases play crucial roles in the structure of DNA and RNA, which are fundamental to life. Let’s break down the details:

Pyrimidines: Pyrimidines are a class of nitrogenous bases that include cytosine (C), uracil (U), and thymine (T). These bases are characterized by having a single six-membered ring structure. Pyrimidines are typically found in RNA and DNA. Uracil is found in RNA, while thymine is found in DNA, and cytosine is found in both.

Purines: Purines, on the other hand, have a two-ring structure that consists of a six-membered ring fused with a five-membered ring. Guanine (G) and adenine (A) are the two purines found in both RNA and DNA. Purines are larger in size compared to pyrimidines due to their two-ring structure.

Guanine: Guanine (G) is a purine, meaning it has a distinct structure compared to pyrimidines. Guanine contains two rings in its structure and plays an essential role in the genetic material of both DNA and RNA. It pairs with cytosine (C) in DNA through three hydrogen bonds, which contributes to the stability of the DNA double helix.

Comparison Between Pyrimidines and Purines:

• Pyrimidines: Cytosine (C), Uracil (U), Thymine (T)
• Purines: Guanine (G), Adenine (A)

Conclusion: Based on the structural and chemical differences between pyrimidines and purines, we conclude that guanine belongs to the purine family due to its two-ring structure. Therefore, the correct answer is (D) Guanine.