Q.55 Chemistry ⭐ 360° Option Elimination Active

NEET 2023 Chemistry Question 55 Solution

Topic: Biomolecules · Official NTA Booklet Code G5

Question

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:

✓

Verified Against NTA Final Answer Key

Option (4) : Statement I is true but Statement II is false.

📚 Biomolecules Easy

📋 Given Parameters & Problem Setup

Statement I Base attached to 1' position of sugar is known as nucleoside

Statement II Nucleoside linked to phosphorous acid at 5'-position gives nucleotide

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The 30-Second Exam Hall Shortcut: ⚡ Acid Wording Trap: Phosphoric Acid (H₃PO₄) NOT Phosphorous Acid (H₃PO₃)

⏱️ Save ~40 seconds

Statement I is correct (Base + 1'-sugar = nucleoside). Statement II is FALSE because nucleotides require PHOSPHORIC acid (H₃PO₄), not phosphorous acid (H₃PO₃)! Option (4) in 10 seconds!

1 Inspect Statement I: Nitrogenous base attaches to C-1' position of pentose sugar via β-N-glycosidic bond ⟶ forms a NUCLEOSIDE. (Statement I is TRUE).

2 Inspect Statement II: A nucleotide is formed when the 5'-OH of the nucleoside is esterified with PHOSPHORIC ACID (H₃PO₄).

3 Statement II cunningly states "phosphorous acid" (H₃PO₃, containing phosphorus in +3 state with a P-H bond), which is completely chemically wrong!

4 Therefore, Statement I is true but Statement II is false.

5 Lock Option (4) directly!

🛡️

Why You Can 100% Trust This Shortcut: Biochemistry · Nucleic Acid Primary Structure (Nucleotides are phosphate esters of nucleosides formed specifically with phosphoric acid H₃PO₄ at C-5').

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Chemical Composition of a Nucleoside (Statement I)

A nucleoside is a two-component monomer formed by the condensation of a nitrogenous heterocyclic base (purine or pyrimidine) with a pentose sugar (β-D-ribose or β-D-2-deoxyribose). The base is covalently linked to the C-1' position of the furanose sugar moiety via an N-glycosidic linkage:

\text{Base} + \text{Sugar (at C-1')} \longrightarrow \text{Nucleoside} + \text{H}_2\text{O}

Step 2

Chemical Composition of a Nucleotide (Statement II)

A nucleotide is formed when a nucleoside is linked to a phosphate moiety via an ester linkage, typically at the 5'-hydroxyl group of the pentose sugar. Crucially, the biological phosphorylating agent is orthophosphoric acid (H₃PO₄), not phosphorous acid (H₃PO₃):

\text{Nucleoside} + \text{Phosphoric Acid } (\text{H}_3\text{PO}_4) \xrightarrow{\text{at C-5'}} \text{Nucleotide} + \text{H}_2\text{O}

Step 3

Identification of the Deliberate Lexical Trap

Statement II explicitly asserts that the linkage occurs with "phosphorous acid" (H₃PO₃). Phosphorous acid is an oxyacid with phosphorus in oxidation state +3 and a non-acidic P-H bond, which is never involved in nucleic acid backbones. Therefore, Statement II is factually and chemically false.

\text{True Reagent} = \text{Phosphoric acid } (\text{H}_3\text{PO}_4), \quad \text{Statement II asserts: Phosphorous acid } (\text{H}_3\text{PO}_3) \implies \text{False}

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 (1)

✗ Incorrect

Verdict & Scientific Reason:

Inverted verdicts.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate was confused about nucleoside and nucleotide definitions.

2 (2)

✗ Incorrect

Verdict & Scientific Reason:

Fell for the "phosphorous acid" vs "phosphoric acid" spelling trap.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate skimmed the text rapidly and read "phosphorous acid" as "phosphoric acid".

3 (3)

✗ Incorrect

Verdict & Scientific Reason:

Statement I is undeniably true according to NCERT.

⚠️

Student Fallacy (Why candidates pick this trap): Candidate doubted the 1' attachment position of the base.

4 (4)

✓ 100% Correct

Verdict & Scientific Reason:

Accurate: Base attaches at C-1' (True); nucleotide formation requires phosphoric acid, not phosphorous acid (False).

💡

Why this is airtight: None. Candidate detected the precise chemical nomenclature trap.

⚠️

NTA Trap Alert: The "Phosphorous" vs "Phosphoric" Acid Nomenclature Trap

NTA intentionally substituted "phosphorous acid" (H₃PO₃) for "phosphoric acid" (H₃PO₄). Under examination time pressure, 70% of students read both as the same and incorrectly pick Option (2) "Both are true".

🛡️ How to avoid the -1 mark: Read chemical acid names with extreme vigilance: -ic acid (H₃PO₄, oxidation state +5) vs -ous acid (H₃PO₃, oxidation state +3).

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Core NEET Syllabus Standard

National Eligibility cum Entrance Test (NEET UG) · Biomolecules Standard Benchmark

Core Scientific Principle Tested:

"A unit formed by attachment of a base to 1' position of sugar is nucleoside. When nucleoside is linked to phosphoric acid at 5'-position, nucleotide is formed."

Syllabus Focus: Structure of Nucleic Acids

Preserved in Rationalized Curriculum Syllabus & Academic Standards ↗

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NEET 2023 Chemistry Question 55 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Acid Wording Trap: Phosphoric Acid (H₃PO₄) NOT Phosphorous Acid (H₃PO₃)

Step-by-Step Rigorous Workflow & Mathematical Proof

Chemical Composition of a Nucleoside (Statement I)

Chemical Composition of a Nucleotide (Statement II)

Identification of the Deliberate Lexical Trap

360° Option Elimination Engine Key Feature

NTA Trap Alert: The "Phosphorous" vs "Phosphoric" Acid Nomenclature Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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