Zoology 2024 Paper II 50 marks Discuss

Q2

(a) Give an account of ultrastructure of rough and smooth endoplasmic reticulum. Also discuss the role of endoplasmic reticulum in cell secretion. (20 marks) (b) Distinguish between heterochromatin and euchromatin in their structure and function with examples. (15 marks) (c) What are signal molecules? How do defects in signalling pathway cause pathological changes? (15 marks)

हिंदी में प्रश्न पढ़ें

(a) खुरदरी तथा चिकनी अंतर्द्रव्यी जालिका की सूक्ष्म संरचना का विवरण दीजिए। कोशिकीय स्रवण में अंतर्द्रव्यी जालिका की भूमिका की भी चर्चा कीजिए। (20 अंक) (b) विषमक्रोमैटिन (हेट्रोक्रोमैटिन) तथा युक्रोमैटिन की संरचना तथा कार्यों में उदाहरण सहित अंतर स्पष्ट कीजिए। (15 अंक) (c) संकेत अणु (सिग्नल मॉलिक्यूल) क्या हैं? संकेत मार्ग में दोष किस प्रकार रोगात्मक (पैथोलॉजिकल) परिवर्तन उत्पन्न करते हैं? (15 अंक)

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Directive word: Discuss

This question asks you to discuss. 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 'discuss' demands a comprehensive, analytical treatment with balanced coverage across all three sub-parts. Allocate approximately 40% of word budget to part (a) given its 20 marks, and roughly 30% each to parts (b) and (c). Structure as: brief introduction on cellular organization → detailed ultrastructure of RER/SER with secretion mechanisms → tabular/paragraph comparison of heterochromatin/euchromatin → signal molecule classification with disease examples → concluding synthesis on functional integration of these cellular components.

Key points expected

Part (a): Ultrastructure of RER (ribosomes on outer surface, flattened cisternae, luminal space) and SER (tubular network, absence of ribosomes); structural-functional correlation
Part (a): ER role in protein synthesis (RER), lipid synthesis/detoxification (SER), and secretion pathway via vesicular transport to Golgi
Part (b): Structural distinction—heterochromatin (condensed, late-replicating, transcriptionally inactive) vs euchromatin (dispersed, early-replicating, transcriptionally active)
Part (b): Functional examples—constitutive heterochromatin (centromeres, telomeres, Barr body in mammals) vs facultative heterochromatin (X-chromosome inactivation); euchromatin as house-keeping and inducible genes
Part (c): Signal molecules definition—hormones, growth factors, cytokines, neurotransmitters; autocrine/paracrine/endocrine modes
Part (c): Defect mechanisms—receptor mutations (e.g., EGFR in cancers), G-protein defects (cholera toxin, pertussis), kinase cascade dysregulation (Ras-MAPK pathway); specific Indian context—high incidence of oral cancers linked to signaling pathway mutations

Evaluation rubric

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	10	Precise terminology for all three parts: for (a) distinguishes cisernal vs tubular ER morphology and luminal continuity; for (b) correctly identifies constitutive vs facultative heterochromatin with accurate replication timing; for (c) classifies signal molecules by chemical nature and distinguishes first vs second messengers without conflating mechanisms	Generally accurate but with minor errors—e.g., vague on SER functions, conflates heterochromatin types, or lists signal molecules without functional classification; some structural details missing or imprecise	Major conceptual errors—claims ribosomes on SER, states heterochromatin is transcriptionally active, or confuses signal transduction with metabolic pathways; fundamental misunderstanding of ultrastructure-function relationships
Diagram / labelling	20%	10	For (a): detailed labeled diagram showing RER with ribosomes, translocon, lumen, and SER with associated enzymes; for (b): schematic chromosome showing banding pattern (G-bands/R-bands) or nucleosome packing levels; for (c): signaling cascade diagram with receptor, transducer, and effector components; all diagrams neat, proportional, with accurate labels	Basic diagrams present but incomplete—RER/SER distinction shown but lacking molecular detail; chromosome structure diagram generic without specific heterochromatin/euchromatin labeling; signaling pathway linear without feedback loops	Absent or seriously flawed diagrams; unlabeled sketches; diagrams contradict text (e.g., showing ribosomes on SER); messy presentation with no attempt at structural accuracy
Examples & nomenclature	20%	10	For (a): cites specific cell types (plasma cells for RER, hepatocytes for SER, steroidogenic cells); for (b): Barr body as classic example, mentions Indian muntjac chromosome studies; for (c): specific pathologies—Ras mutations in pancreatic/colorectal cancers, HER2/neu in breast cancer, cholera/pertussis as bacterial interference examples; uses standard nomenclature (H3K9me3, HP1 for heterochromatin)	Generic examples only—mentions 'protein-secreting cells' without specificity, 'cancer' without molecular detail; some nomenclature present but inconsistent or dated terminology	No specific examples; invented or incorrect nomenclature; irrelevant examples (e.g., mitochondrial diseases for ER function); failure to distinguish between parts (a), (b), (c) in example selection
Process explanation	20%	10	For (a): clear sequential explanation of co-translational import, signal recognition particle pathway, glycosylation in ER lumen, vesicle budding and fusion; for (b): explains molecular basis of condensation (histone modifications, chromatin remodeling complexes); for (c): stepwise signal transduction with amplification, specificity, and termination mechanisms; integrates all three parts thematically	Describes processes in static terms rather than dynamic sequence; mentions steps but not their regulation or interdependence; for (c) lists pathway components without explaining signal flow	No process explanation—only lists structures or states; confused temporal sequence; describes signaling as simple diffusion without receptor-mediated mechanisms; fails to explain 'how' for any of the three parts
Evolutionary / applied context	20%	10	For (a): mentions evolutionary origin of ER from invaginated plasma membrane in prokaryotes; for (b): discusses evolutionary significance of chromatin compaction (genome size vs nuclear volume, C-value paradox); for (c): therapeutic targeting of signaling pathways (imatinib for BCR-Abl, trastuzumab for HER2); connects to Indian public health—oral cancer screening, HPV-related signaling in cervical cancer; synthetic biology applications	Brief mention of disease relevance without mechanistic depth; generic statement about 'medical importance'; no evolutionary perspective or limited to one sub-part	No applied or evolutionary context; irrelevant digressions; fails to connect basic cell biology to human health or organismal biology; purely descriptive treatment without synthesis

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