Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Which of the following features is/are used to distinguish Archaea from Bacteria?
Options:
(A) Gram-staining
(B) Peptidoglycan in the cell wall
(C) Presence of N-acetylglucosamine
(D) 16S rRNA sequences
Official Answer Key:
B;D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2025 Q.21)**
**1. Governing Concept**:
- **Topic**: Microbiology & Three-Domain System of Life (Archaea vs. Bacteria)
- While both Archaea and Bacteria are prokaryotes lacking membrane-bound nuclei, Carl Woese's three-domain system established fundamental molecular differences:
1. **Cell Wall Composition**: Bacteria possess **peptidoglycan (murein)** containing $N$-acetylmuramic acid (NAM) and $D$-amino acids. Archaea completely lack peptidoglycan; their cell walls contain pseudopeptidoglycan (pseudomurein with $N$-acetyltalosaminuronic acid), methanochondroitin, or proteinaceous S-layers.
2. **Ribosomal RNA (16S rRNA)**: Distinct signature nucleotide sequences and secondary structures in 16S rRNA phylogenetically separate Archaea from Bacteria.
3. **Lipid Chemistry**: Archaea possess ether-linked, branched phytanyl isoprenoid chains (often glycerol diether or tetraether monolayers), whereas Bacteria have ester-linked, unbranched fatty acids in lipid bilayers.
**2. Option Analysis**:
- **Option (A) [Gram-staining] is INCORRECT**: Both Archaea and Bacteria can stain either Gram-positive or Gram-negative depending on their wall thickness; Gram reaction does not phylogenetically distinguish domains.
- **Option (B) [Peptidoglycan in the cell wall] is CORRECT**: Found uniquely in Bacteria and absent in Archaea.
- **Option (C) [Presence of N-acetylglucosamine] is INCORRECT**: $N$-acetylglucosamine (NAG) is present in both bacterial peptidoglycan and archaeal pseudomurein.
- **Option (D) [16S rRNA sequences] is CORRECT**: The molecular chronometer that definitively distinguishes Archaea from Bacteria.
**3. Final Answer**:
- **Correct Options**: **B, D**
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