Q.199 Zoology ⭐ 360° Option Elimination Active

NEET 2021 Zoology Question 199 Solution

Topic: Molecular Basis of Inheritance · Official NTA Booklet Code M1

Question

Statement I: The codon 'AUG' codes for methionine and phenylalanine. Statement II: 'AAA' and 'AAG' both codons code for the amino acid lysine. In the light of the above statements, choose the correct answer from the options given below.

(1)Both Statement I and Statement II are true
(2)Both Statement I and Statement II are false
(3)Statement I is correct but Statement II is false
(4)Statement is incorrect but Statement II is true

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Verified Against NTA Final Answer Key

Option (4) : Statement I is incorrect but Statement II is true

📚 Molecular Basis of Inheritance Medium

📋 Given Parameters & Problem Setup

Statement I The codon ‘AUG’ codes for methionine and phenylalanine

Statement II ‘AAA’ and ‘AAG’ both codons code for the amino acid lysine

Question Objective Evaluate the biological truth of Statement I and Statement II

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The 30-Second Exam Hall Shortcut: ⚡ "AUG = Methionine ONLY (Phenylalanine is UUU/UUC!); AAA & AAG = Lysine!"

⏱️ Save ~20 seconds

The codon AUG has dual functions: it acts as the initiation codon and codes strictly for Methionine (Met). It NEVER codes for phenylalanine (Phe is coded by UUU and UUC!). Thus, Statement I is completely FALSE. AAA and AAG both code for Lysine due to degeneracy, making Statement II TRUE. Statement I is incorrect but Statement II is true! Mark Option (4).

1 Check Statement I: Does AUG code for phenylalanine? No! AUG codes strictly for Methionine (Met). Phenylalanine is encoded by UUU and UUC. Statement I is FALSE.

2 Check Statement II: Do both AAA and AAG code for lysine? Yes! Due to degeneracy of the genetic code, AAA and AAG both specify Lysine (Lys). Statement II is TRUE.

3 Match with options: Statement I is incorrect but Statement II is true -> Option (4).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Molecular Basis of Inheritance (Genetic Code: The codon is triplet. 61 codons code for amino acids. AUG has dual functions: it codes for Methionine (met), and it also acts as initiator codon. UUU codes for phenylalanine. Some amino acids are coded by more than one codon, hence the code is degenerate – e.g. AAA and AAG both code for lysine).

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Critical Dissection of Statement I (Codon AUG)

The genetic code table maps 64 mRNA triplet codons to 20 standard proteinogenic amino acids and stop signals: - The codon AUG has dual functions in translation: 1. It serves as the canonical initiator codon where ribosomal translation initiation complexes assemble. 2. It specifies the amino acid Methionine (Met) (formylmethionine in prokaryotes). - The genetic code is unambiguous and specific: one codon codes for only one amino acid. Therefore, AUG can never simultaneously code for another amino acid. - Phenylalanine (Phe) is encoded specifically by UUU and UUC, as historically deciphered by Marshall Nirenberg and Heinrich Matthaei using poly-U RNA templates. - Hence, Statement I is unequivocally incorrect/false.

Step 2

Critical Dissection of Statement II (Codons AAA and AAG)

- The genetic code is degenerate: most amino acids are specified by more than one synonymous triplet codon (with the exceptions of Methionine [AUG] and Tryptophan [UGG]). - The basic amino acid Lysine (Lys) is encoded by two synonymous codons that differ only in their wobble third base: AAA and AAG. - Hence, Statement II is completely true.

\begin{cases} \textbf{AUG} &\implies \mathbf{Methionine\,(Met)} \quad (\text{Initiator Codon; NEVER Phenylalanine}) \\[4pt] \textbf{UUU, UUC} &\implies \mathbf{Phenylalanine\,(Phe)} \\[4pt] \textbf{AAA, AAG} &\implies \mathbf{Lysine\,(Lys)} \quad (\text{Degenerate Codons}) \end{cases}

⭐

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:

Incorrect because Statement I is false (AUG does not code for phenylalanine).

⚠️

Student Fallacy (Why candidates pick this trap): Assuming AUG has multiple amino acid assignments because it has "dual functions".

2 2

✗ Incorrect

Verdict & Scientific Reason:

Incorrect because Statement II is true (AAA and AAG both code for lysine).

⚠️

Student Fallacy (Why candidates pick this trap): Denying the degeneracy of the genetic code for lysine.

3 3

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Statement I is false and Statement II is true.

⚠️

Student Fallacy (Why candidates pick this trap): Completely inverting the veracity of both statements.

4 4

✓ 100% Correct

Verdict & Scientific Reason:

Correct. Accurately identifies that Statement I is false and Statement II is true.

💡

Why this is airtight: None. Candidate remembers the exact codon-amino acid mappings.

⚠️

NTA Trap Alert: Dual Function vs Dual Amino Acid Trap

NCERT states that "AUG has dual functions". Students mistakenly interpret "dual functions" as "codes for two different amino acids (methionine and phenylalanine)". In reality, its dual function is: (1) coding for Methionine, and (2) acting as the Initiator codon.

🛡️ How to avoid the -1 mark: Remember: AUG dual function = Methionine + Start codon. It NEVER codes for phenylalanine.

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Core NEET Syllabus Standard

NEET / NCERT Biology · Molecular Basis of Inheritance Standard Benchmark

Core Scientific Principle Tested:

"AUG codes for methionine and initiates translation; phenylalanine is coded by UUU/UUC; AAA and AAG both code for lysine."

Syllabus Focus: The Genetic Code

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Zoology Question 199 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "AUG = Methionine ONLY (Phenylalanine is UUU/UUC!); AAA & AAG = Lysine!"

Step-by-Step Rigorous Workflow & Mathematical Proof

Critical Dissection of Statement I (Codon AUG)

Critical Dissection of Statement II (Codons AAA and AAG)

360° Option Elimination Engine Key Feature

NTA Trap Alert: Dual Function vs Dual Amino Acid Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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