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GATE 2025 BT Q.21: solution and answer

GATE 2025 · BTQ.21 · 1 mark · MSQ

Which of the following features is/are used to distinguish Archaea from Bacteria?

  1. (A)

    Gram-staining

  2. (B)

    Peptidoglycan in the cell wall

  3. (C)

    Presence of N-acetylglucosamine

  4. (D)

    16S rRNA sequences

See the question in the paper
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Answer and solution

ANSWER

The features that distinguish Archaea from Bacteria are (B) peptidoglycan in the cell wall and (D) 16S rRNA sequences.

Official answer: B, DSource: Official answer key, GATE 2025 BT (IIT Roorkee)

  1. Archaea and Bacteria are both prokaryotes, so a distinguishing feature must separate the two domains and not merely describe prokaryotes in general.

  2. (A) Gram-staining: not correct. Gram staining reflects wall thickness and structure, and archaea can stain either Gram-positive or Gram-negative. The stain does not mark the domain boundary.

  3. (B) Peptidoglycan in the cell wall: correct. Bacterial walls contain peptidoglycan (murein), built from N-acetylglucosamine and N-acetylmuramic acid. Archaeal walls lack true peptidoglycan and have pseudomurein, S-layers or other polymers instead.

  4. (C) Presence of N-acetylglucosamine: not correct. This sugar occurs in both domains. Bacterial peptidoglycan contains it, and archaeal pseudomurein also contains it, paired with N-acetyltalosaminuronic acid. Its presence does not separate the domains.

  5. (D) 16S rRNA sequences: correct. The three-domain classification rests on 16S rRNA sequence signatures, and archaeal 16S rRNA sequences differ clearly from bacterial ones.

  6. Correct options: (B) and (D).

KEY IDEA

Archaea lack true peptidoglycan (they have pseudomurein, S-layers or other polymers), and their 16S rRNA sequences form a separate lineage from bacterial ones. These two features separate the domains.

WATCH OUT FOR

Treating Gram-staining or the presence of N-acetylglucosamine as domain markers. Gram reaction varies within both domains, and N-acetylglucosamine is also found in archaeal pseudomurein, so neither one separates Archaea from Bacteria.

From the GATE 2025 BT paper. Every question of this paper · Download the full paper · All 8 Microbiology questions, with notes

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