Q.143 Zoology ⭐ 360° Option Elimination Active

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NEET 2025 Zoology Question 143 Solution

Topic: Human Health and Disease · Official NTA Booklet Code 45

Question

Identify the statement that is NOT correct.

(1)Each antibody has two light and two heavy chains.
(2)The heavy and light chains are held together by disulfide bonds.
(3)Antigen binding site is located at C-terminal region of antibody molecules.
(4)Constant region of heavy and light chains are located at C-terminus of antibody molecules.

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Verified Against NTA Final Answer Key

Option (3) : Antigen binding site is located at C-terminal region of antibody molecules.

📚 Human Health and Disease Easy

📋 Given Parameters & Problem Setup

Molecule Antibody (Immunoglobulin / Ig)

Target Statement Statement that is NOT correct (False statement)

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The 30-Second Exam Hall Shortcut: ⚡ Antibody Molecular Structure Fingerprint

⏱️ Save ~20 seconds

Antigen binding site (paratope) is formed by the Variable (V) regions at the N-TERMINUS of heavy and light chains! The C-terminus contains the CONSTANT (Fc) region! Statement (3) claiming antigen-binding site is at C-terminus is WRONG!

1 Recall immunoglobulin structure from NCERT Section 8.2.2:

2 - Formula: H₂L₂ (two light chains, two heavy chains) -> Statement (1) is CORRECT.

3 - Chains are held together by inter-chain disulfide bonds -> Statement (2) is CORRECT.

4 - Constant (C) regions of both heavy and light chains are located towards the C-terminus -> Statement (4) is CORRECT.

5 - Antigen-binding site (Fab fragment) is located at the N-terminal (amino-terminal) variable region of each arm! Claiming it is at the C-terminus is FALSE.

6 Statement (3) is NOT correct!

7 Select Option (3)!

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Why You Can 100% Trust This Shortcut: Life Cycle of Plasmodium (NCERT Biology · Human Health and Disease: Female Anopheles primary host, human secondary host).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Quaternary Structure of an Antibody Molecule (H₂L₂)

An antibody (immunoglobulin) is a Y-shaped glycoprotein composed of four polypeptide chains:

• Two identical light (L) chains of lower molecular weight (~25 kDa each).
• Two identical heavy (H) chains of higher molecular weight (~50 kDa each).
• The four chains are held together in a Y-configuration by covalent interchain disulfide (-S-S-) bonds and non-covalent interactions. Hence, statements (1) and (2) are correct.

Step 2

Domain Architecture: N-terminus vs C-terminus

Functional differentiation between the variable tips and constant stalk:

• Variable regions ( $V_L$ and $V_H$ ): Located at the N-terminal (amino-terminal) ends of the Y-shaped arms. The hypervariable segments (complementarity-determining regions / CDRs) form the unique antigen-binding site (paratope) that binds specifically to an epitope.
• Constant regions ( $C_L$ and $C_H$ ): Located towards the C-terminal (carboxyl-terminal) end. They mediate effector functions such as complement fixation and binding to Fc receptors on phagocytes. Hence, statement (4) is correct.

Step 3

Identification of the Incorrect Statement

Statement (3) claims that the "Antigen binding site is located at C-terminal region of antibody molecules". This is biologically false; the antigen-binding site is located at the N-terminal region.

\text{Antigen-Binding Site} = \text{N-terminus} \neq \text{C-terminus}

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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 A

✗ Incorrect

Verdict & Scientific Reason:

Incorrect choice (true statement). Every antibody has an H₂L₂ structural formula.

⚠️

Student Fallacy (Why candidates pick this trap): Failing to realize that option 1 is a correct fact.

2 B

✗ Incorrect

Verdict & Scientific Reason:

Incorrect choice (true statement). Disulfide bridges stabilize the quaternary immunoglobulin assembly.

⚠️

Student Fallacy (Why candidates pick this trap): Thinking chains are held only by hydrogen bonds.

3 C

✓ 100% Correct

Verdict & Scientific Reason:

100% correct choice (as the incorrect statement). The antigen-binding site is formed by the N-terminal variable domains, not the C-terminal domains.

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Why this is airtight: None; pristine knowledge of antibody biochemistry.

4 D

✗ Incorrect

Verdict & Scientific Reason:

Incorrect choice (true statement). The constant effector domains reside at the C-terminus.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing N-terminus with C-terminus.

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NTA Trap Alert: N-terminus vs C-terminus of Polypeptides in Antibodies

Polypeptides have an N-terminus (amino end) and a C-terminus (carboxyl end). The N-terminal tips are VARIABLE and BIND ANTIGEN (arms of the Y). The C-terminal base is CONSTANT (stalk of the Y).

🛡️ How to avoid the -1 mark: N = "New/Novel/Binding (Antigen site)"; C = "Constant/C-terminus (Stalk)".

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Core NEET Syllabus Standard

NEET Biology Core Syllabus · Human Health and Disease Standard Benchmark

Core Scientific Principle Tested:

"The antigen-binding site is located at the N-terminal variable region of the heavy and light chains."

Syllabus Focus: Innate and Acquired Immunity · Structure of Antibody Molecule

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

N-terminus binds New antigens; C-terminus is Constant!

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NEET 2025 Zoology Question 143 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ Antibody Molecular Structure Fingerprint

Step-by-Step Rigorous Workflow & Mathematical Proof

Quaternary Structure of an Antibody Molecule (H₂L₂)

Domain Architecture: N-terminus vs C-terminus

Identification of the Incorrect Statement

360° Option Elimination Engine Key Feature

NTA Trap Alert: N-terminus vs C-terminus of Polypeptides in Antibodies

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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