Zoology 2022 Paper II 50 marks Describe

Q2

(a) What is rough endoplasmic reticulum (RER)? Describe in detail the various modifications of secretory proteins occurring in the lumen of RER. 20 (b) Define endomitosis. Describe in detail the structure of a polytene chromosome highlighting the importance of puffs. 15 (c) What is facilitated diffusion? Describe the mechanism of active transport across the plasma membrane with suitable example. 15

हिंदी में प्रश्न पढ़ें

(a) खुरदरी अंतर्द्रव्यी जालिका (आर० ई० आर०) क्या है? आर० ई० आर० की अवकाशिका में होने वाले स्रावी प्रोटीनों के विभिन्न परिवर्तनों का विस्तृत वर्णन कीजिए। 20 (b) अंतःसूत्रण की परिभाषा लिखिए। आफुट (पफ) के महत्व को दर्शाते हुए बहुपट्टीय गुणसूत्र की संरचना का विस्तृत वर्णन कीजिए। 15 (c) सुकृत विसरण क्या है? प्रद्रव्य झिल्ली के आर-पार सक्रिय अभिगमन की प्रक्रिया का उपयुक्त उदाहरण के साथ वर्णन कीजिए। 15

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Directive word: Describe

This question asks you to describe. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

The directive 'describe' demands detailed, systematic exposition of structures and processes across all three parts. Allocate approximately 40% of time/words to part (a) given its 20 marks, with ~30% each to parts (b) and (c). Structure: brief comparative introduction distinguishing secretory vs. membrane vs. transport processes; then three dedicated sections each with definition → structural details → functional mechanisms → significance; conclude with integrative statement on how these cellular processes coordinate for organismal function.

Key points expected

  • Part (a): RER structure with ribosome-studded cisternae; co-translational import via SRP-SRP receptor; N-linked glycosylation (dolichol pathway), disulfide bond formation by PDI, and protein folding with chaperones (BiP, calnexin/calreticulin cycle)
  • Part (b): Endomitosis as chromosome replication without nuclear/cytoplasmic division; polytene chromosome structure with chromomeres, bands (1024 chromatids in Drosophila), interbands; puffs as sites of active transcription (Balbiani rings), gene amplification in Dipteran salivary glands
  • Part (c): Facilitated diffusion as carrier/channel-mediated passive transport down concentration gradient; active transport requiring energy (ATP/ion gradients), primary vs. secondary active transport; Na⁺-K⁺ pump as prime example with 3:2 stoichiometry and ouabain sensitivity
  • Comparative distinction: RER modification (covalent) vs. polytene amplification (structural) vs. membrane transport (energetic); all represent solutions to cellular logistics at different scales
  • Applied significance: RER in protein secretion diseases (cystic fibrosis ΔF508, ER stress/UPR); polytene chromosomes in developmental gene mapping; active transport in nerve conduction and renal function

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%11Precise definitions for all three parts: RER as ribosome-bound ER with correct membrane topology; endomitosis distinguished from endoreduplication and amitosis; facilitated diffusion correctly contrasted with simple diffusion and active transport; no conflation of co-translational vs. post-translational importGenerally correct definitions with minor errors (e.g., confusing RER/SER functions, vague endomitosis definition, or treating facilitated diffusion as active transport); some membrane orientation errorsFundamental misconceptions: RER described as DNA-containing, endomitosis equated with mitosis, or active transport described as not requiring energy; confusion between lumenal and cytosolic compartments
Diagram / labelling18%9Three quality diagrams: (a) RER with ribosomes, translocon complex, vesicle budding; (b) polytene chromosome with band-interband pattern and puff structure showing decondensation; (c) Na⁺-K⁺-ATPase with E1-E2 conformational states; all with accurate labels and directional arrowsTwo adequate diagrams with some labels missing or structural inaccuracies (e.g., ribosomes on wrong side, polytene without chromatid number indication, generic pump without conformational detail)Single diagram or poorly executed sketches; missing critical labels (SRP, signal peptide, band vs. puff distinction, phosphorylated intermediate); diagrams not referenced in text
Examples & nomenclature20%10Specific nomenclature: SRP (signal recognition particle), Sec61 translocon, PDI, oligosaccharyltransferase, BiP/GRP78, calnexin; Balbiani rings in Chironomus; Na⁺-K⁺-ATPase (sodium-potassium pump), SGLT1 as secondary active transport example; disease examples from Indian context (malaria-related transport studies)Generic terms like 'chaperones' without naming, 'pump' without specifying; mentions Drosophila for polytene but no species detail; active transport example without molecular specificityIncorrect or no examples; invented terminology; confusing RER with Golgi enzymes; no mention of specific organisms for polytene chromosomes; missing Na⁺-K⁺ pump entirely
Process explanation25%12.5Stepwise mechanistic detail: (a) signal hypothesis with SRP cycle, translocation, glycosylation mechanism with dolichol phosphate cycle, quality control (ERAD mention); (b) endomitosis mechanism with endocycle phases, puff formation as local chromatin decondensation with RNA polymerase II recruitment; (c) conformational coupling model for Na⁺-K⁺-ATPase with specific steps and stoichiometrySequential but superficial descriptions; missing key intermediates (e.g., no mention of signal peptide cleavage, vague 'DNA unwinds' for puffs, 'pump works against gradient' without mechanism)Descriptive without mechanism; lists structures without explaining how they function; no temporal or causal sequencing; confused order of events in protein modification
Evolutionary / applied context15%7.5Evolutionary significance: RER in eukaryogenesis (endosymbiosis connection to protein targeting); polytene chromosomes as evolutionary solution to high transcriptional demand in secretory tissues; active transport origin in LUCA ion pumps; applied: ER stress in diabetes/Alzheimer's, polytene in Drosophila developmental genetics (Indian contributions: JNCASR work), Na⁺-K⁺ pump as drug target (cardiac glycosides, malaria research at CDRI)Brief mention of disease relevance without elaboration; generic statement about 'important for research'; no evolutionary perspectiveNo applied or evolutionary context; purely descriptive answer; missing significance entirely or providing irrelevant applications

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