MP Board · Class 12 · Chemistry · BiomoleculesExplain the structure of a nucleotide and a nucleoside with a neat comparative description. How do they link together to form a polynucleotide chain?

Nucleic acids are polymers of nucleotides. To understand their structure, we must distinguish between nucleosides and nucleotides:

1. Nucleoside: A nucleoside is formed by the attachment of a nitrogenous base (purine or pyrimidine) to C-1' of a pentose sugar ($\beta$-D-ribose or 2'-deoxyribose) through a $\beta$-N-glycosidic linkage. $$\text{Nucleoside} = \text{Pentose Sugar} + \text{Nitrogenous Base}$$

2. Nucleotide: A nucleotide is formed when a nucleoside is esterified with phosphoric acid at the C-5' hydroxyl group of the pentose sugar. Thus, it contains three components: a pentose sugar, a nitrogenous base, and a phosphate group. $$\text{Nucleotide} = \text{Nucleoside} + \text{Phosphoric Acid}$$ \nPolymerization into Polynucleotide Chain:\nNucleotides are linked together through phosphodiester linkages. The phosphate group of one nucleotide forms an ester bond with the 3'-carbon hydroxyl group of the pentose sugar of the next nucleotide, creating a repeating sugar-phosphate backbone (5'$\to$3' direction), with nitrogenous bases projecting outward.