Question

Which one of the following is a nucleoside?

• A. Purine + Pyrimidine
• B. Pyrimidine + Phosphate group
• C. Purine + Phosphate group
• D. Pyrimidine + Pentose sugar

Hint:

Remember the simple structural hierarchy:
- Base + Sugar = Nucleoside
- Base + Sugar + Phosphate = Nucleotide
(Think of the "t" in nucleotide as representing the "three" components).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Nucleic acids (DNA and RNA) are polymers made of nucleotide monomers.
Understanding the structural components of these monomers requires distinguishing between nitrogenous bases, nucleosides, and nucleotides.

Step 2: Detailed Explanation:
Let us review the structural components:
- Nitrogenous base: Planar, nitrogen-containing aromatic rings, classified into purines (double-ringed, e.g., adenine, guanine) and pyrimidines (single-ringed, e.g., cytosine, thymine, uracil).
- Nucleoside: A compound formed when a nitrogenous base (purine or pyrimidine) is covalently linked to a five-carbon pentose sugar (ribose or deoxyribose) via a \( \beta \)-N-glycosidic bond.
Therefore:
\[ \text{Nucleoside} = \text{Nitrogenous Base (Purine or Pyrimidine)} + \text{Pentose Sugar} \]
This matches the combination "Pyrimidine + Pentose sugar".
- Nucleotide: A compound formed when a phosphate group is esterified to the sugar hydroxyl group of a nucleoside:
\[ \text{Nucleotide} = \text{Nucleoside} + \text{Phosphate Group} \]
Therefore, the combination "Pyrimidine + Pentose sugar" is a nucleoside.

Step 3: Final Answer:
A nucleoside is composed of a pyrimidine (or purine) base and a pentose sugar, corresponding to option (D).