Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Carl Woese used the gene sequence of which one of the following for phylogenetic
taxonomy of prokaryotes?
Options:
(A) A ribosomal RNA of large ribosomal subunit
(B) A ribosomal RNA of small ribosomal subunit
(C) A ribosomal protein of large ribosomal subunit
(D) A ribosomal protein of small ribosomal subunit
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2020 Q.43)**
**1. Governing Concept**:
- **Topic**: Microbial Phylogeny & Three-Domain System of Life
- In 1977, **Carl Woese** and George Fox revolutionized biological taxonomy by utilizing the nucleotide sequence of the **16S ribosomal RNA (16S rRNA)** gene (the small subunit rRNA in prokaryotes, homologous to 18S rRNA in eukaryotes) as an evolutionary chronometer.
- Because 16S rRNA is universally present, functionally invariant, and contains alternating conserved and variable regions, its comparative sequence analysis revealed that Archaea are a distinct domain of life, establishing the Three-Domain Tree of Life (**Bacteria, Archaea, Eukarya**).
**2. Option Analysis**:
- **Option (A) [5S rRNA] is INCORRECT**: Too short ($\sim 120\text{ nt}$) to provide sufficient phylogenetic resolution.
- **Option (B) [16S rRNA] is CORRECT**: The universal molecular marker used by Carl Woese to define the three domains of life.
- **Option (C) [23S rRNA] is INCORRECT**: Large subunit rRNA, less commonly used due to size and sequencing constraints in the 1970s.
- **Option (D) [cytochrome c] is INCORRECT**: Used by Margoliash for early protein trees, not Carl Woese for the three-domain tree.
**3. Final Answer**:
- **Correct Choice**: **B**
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