GATE BT 2016 • Q39 MCQ • 2 Marks

Section 3: Genetics, Cellular and Molecular Biology

DNA replication, transcription, translation

Subtopic: Fundamentals of DNA replication, transcription, translation

Question Statement:

Which of the following events occur during the stationary phase of bacterial growth? P. Rise in cell number stops Q. Spore formation in some Gram-positive bacteria such as Bacillus subtilis R. Cell size increases in some Gram-negative bacteria such as Escherichia coli S. Growth rate of bacterial cells nearly equals their death rate T. Decrease in peptidoglycan crosslinking

Options:

  • (A) GDP is bound to G protein in the resting stage
  • (B) GTP bound  subunit cannot reassemble with  dimer
  • (C) All G proteins are trimeric
  • (D) Activation of G protein may result in activation or inhibition of the target enzymes EcoRI BamHI EcoRI XhoI 300 bp Vector backbone 2.8 kb BamHI XhoI 500 bp Insert 1.2 kb

Official Answer Key:

A

Step-by-Step Solution & Explanation

### **Official Solution & Step-by-Step Derivation (GATE BT 2016 Q.39)** **1. Governing Concept & Question Classification**: - **Section**: BT - **Question Type**: **MCQ** (2 Marks) - **Topic**: Microbiology & Microbial Physiology (Stationary Phase Adaptations & $\sigma^S$ Regulated Responses) **2. Detailed Step-by-Step Analysis**: - When bacterial cultures exhaust essential nutrients or accumulate waste products, they enter the **stationary phase**, driven by the alternative sigma factor $\sigma^S$ (RpoS) and the alarmone (p)ppGpp: - **P. Cells become smaller and almost spherical**: **CORRECT**. To minimize surface-area-to-volume ratio and metabolic maintenance energy, rod-shaped bacteria (like *E. coli*) undergo reductive division and cell wall remodeling, becoming dwarf, coccoid-like spheres. - **Q. Cell wall becomes more rigid**: **CORRECT**. Peptidoglycan cross-linking increases significantly (forming increased 3-3 and 4-3 peptide cross-links and covalent lipoprotein attachments) to protect the dormant cell against physical and osmotic stresses. - **R. Synthesis of glycogen, polyphosphate, and poly-$\beta$-hydroxybutyrate**: **CORRECT**. Cells accumulate energy and carbon reserve storage inclusions (such as glycogen, polyphosphate/volutin, and PHB) as carbon overflow stores. - **S. Rate of protein synthesis increases**: **INCORRECT**. Overall translation rates drop precipitously (
gt; 80-90\%$ reduction) via 70S ribosome dimerization into inactive 100S disomes (ribosome hibernation) to conserve ATP. - **T. Overall metabolic rate increases**: **INCORRECT**. Cellular respiration and ATP turnover decrease dramatically to basal maintenance levels. - Therefore, events **P, Q, and R** correctly characterize the stationary phase. - **Option-by-Option Evaluation**: - **Option (A) [CORRECT]**: P (coccoid dwarf cell shape), Q (increased cell wall rigidity), and R (accumulation of storage polymers) are hallmark stationary phase adaptations. - **Option (B) [INCORRECT]**: Includes S, which is false. - **Option (C) [INCORRECT]**: Includes T, which is false. - **Option (D) [INCORRECT]**: Includes T, which is false. **3. Final Answer / Verification Summary**: - **Official GATE Answer Key**: **A** - **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).

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