GATE BT 2017 • Q8 MCQ • 1 Mark

Section 2: General Biotechnology

Biomolecules, enzyme kinetics

Subtopic: Fundamentals of Biomolecules, enzyme kinetics

Question Statement:

If the nucleotide composition (%) of a viral genome is A = 10, U = 20, C = 40, and G = 30, which one of the following is this genome?

Options:

  • (A) Double stranded RNA
  • (B) Single stranded RNA
  • (C) Single stranded DNA
  • (D) Double stranded DNA

Official Answer Key:

B

Step-by-Step Solution & Explanation

### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.08)** **1. Governing Concept & Question Classification**: - **Section**: BT - **Question Type**: **MCQ** (1 Mark) - **Topic**: Molecular Biology & Virology (Nucleic Acid Base Composition & Chargaff's Rules) **2. Detailed Step-by-Step Analysis**: - The viral genome composition is given as: $A = 10\%, \quad U = 20\%, \quad C = 40\%, \quad G = 30\%$ - First, the presence of **Uracil (U)** and the absence of Thymine (T) unequivocally proves that the genetic material is **RNA**, completely ruling out single-stranded or double-stranded DNA. - Second, in any canonical double-stranded nucleic acid duplex (including dsRNA), complementary base pairing dictates **Chargaff's rules**: $\%A = \%U \quad \text{and} \quad \%G = \%C$ Here: $\%A (10\%) \neq \%U (20\%) \quad \text{and} \quad \%G (30\%) \neq \%C (40\%)$ - Because Chargaff's parity rule is fundamentally violated, the bases are not paired into a complementary duplex. - Hence, the viral genome is definitively **Single-stranded RNA (ssRNA)**. - **Option-by-Option Evaluation**: - **Option (A) [INCORRECT]**: dsRNA requires $\%A = \%U$ and $\%G = \%C$. - **Option (B) [CORRECT]**: The presence of Uracil with unequal $\%A \neq \%U$ and $\%G \neq \%C$ conclusively defines single-stranded RNA. - **Option (C) [INCORRECT]**: DNA contains Thymine, not Uracil. - **Option (D) [INCORRECT]**: DNA contains Thymine, not Uracil. **3. Final Answer / Verification Summary**: - **Official GATE Answer Key**: **B** - **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).

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