Nucleic Acids

Nucleic acids are polymers whose repeating units are built in two stages, and distinguishing the intermediates clarifies the polymer's architecture. A nucleoside is formed when a nitrogenous base (a purine or pyrimidine) attaches through an N-glycosidic bond to the C1 carbon of a pentose sugar, either ribose or deoxyribose. A nucleotide is produced when a phosphate group is then esterified, usually at the C5 hydroxyl of that same sugar. Therefore the single component that a nucleotide possesses beyond a nucleoside is the phosphate unit, making the first option correct. The claim that a nucleotide lacks the base is wrong, since both retain the base. The option tying nucleotide exclusively to ribose and nucleoside to deoxyribose is false because either sugar can appear in both. The suggestion of two bases on one sugar describes no standard unit. This matches the NCERT description of nucleosides and nucleotides in the Biomolecules chapter. As a consistency check, linking nucleotides through phosphodiester bonds between the C3 and C5 positions of adjacent sugars builds the nucleic acid backbone, which requires exactly the phosphate group that defines a nucleotide.