Nucleosides are fundamental building blocks of nucleic acids like DNA and RNA, consisting solely of a nitrogenous base linked to a pentose sugar. The correct statement about a nucleoside from the given options is option 1: Nitrogenous base attached to OH of C-1′ of pentose sugar.

Correct Answer

Option 1 is true.
In a nucleoside, the nitrogenous base (purine or pyrimidine) forms a β-N-glycosidic bond with the hydroxyl (OH) group at the C-1′ anomeric carbon of the pentose sugar (ribose in RNA or deoxyribose in DNA), replacing the OH with the base via dehydration.

Option Breakdown

Option 1: True

The nitrogenous base attaches specifically to the C-1′ position of the pentose sugar through a glycosidic linkage, distinguishing nucleosides from other biomolecules.

Option 2: False

No phosphoryl group exists in a nucleoside; this describes a nucleotide, where phosphate attaches to the C-5′ OH of the sugar.

Option 3: False

Phosphoryl groups in nucleotides link to C-5′, not C-3′; C-3′ phosphorylation is rare and not standard for nucleosides or nucleotides.​

Option 4: False

Phosphate never attaches to the 2′ carbon in standard nucleosides or nucleotides; the 2′ position varies (OH in ribose, H in deoxyribose) but lacks phosphate.​