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Chemistry Question 62 – NEET-UG 2023

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Given below are two statements : Statement I : A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside Statement II : When nucleoside is linked to phosphorous acid at 5′-position of sugar moiety, we get nucleotide. In the light of the above statements, choose the correct answer from the options given below :

Recall the fundamental definitions and structures of nucleosides and nucleotides.

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Ninja Strategy•Precise Chemical Terminology

A quick check of the exact chemical name 'phosphorous acid' versus 'phosphoric acid' reveals the error in Statement II, allowing for rapid elimination.

Step 1: Evaluate Statement I: Definition of Nucleoside✦ Active

A nucleoside is indeed formed by the attachment of a nitrogenous base (purine or pyrimidine) to the 1′ carbon of a pentose sugar (ribose or deoxyribose) via an N-glycosidic bond. This definition is accurate.

Therefore, Statement I is true.

Step 2: Evaluate Statement II: Definition of Nucleotide○ Expand

A nucleotide is formed when a phosphate group is attached to a nucleoside. Specifically, the phosphate group is typically linked to the 5′ hydroxyl group of the sugar moiety in a nucleoside through a phosphodiester bond.

However, the phosphate group in nucleotides is derived from phosphoric acid (H3PO4), not phosphorous acid (H3PO3). The statement incorrectly mentions 'phosphorous acid'.

Therefore, Statement II is false.

💡 Teacher's Secret Hint

Distinguish carefully between 'phosphoric acid' (H3PO4) and 'phosphorous acid' (H3PO3). This is a common point of confusion.

Step 3: Determine the Correct Option○ Expand

Based on the evaluation, Statement I is true, and Statement II is false. This combination corresponds to option (3).

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