Zoology 2021 Paper II 50 marks Describe

Q8

(a) What are morphogens ? Describe the cellular differentiation during morphogenesis. 20 (b) What are gap junction proteins ? Discuss the roles of connexins in cellular interaction. 15 (c) What is primordial germ cell ? With the help of suitable diagram, discuss the process of oogenesis. 15

हिंदी में प्रश्न पढ़ें

(a) आकारजन (मॉर्फोजेनेसिस) क्या है ? आकारजनन के दौरान कोशिकीय विभेदन का वर्णन कीजिए । 20 (b) अन्तराल संधि (गैप जंक्शन) प्रोटीन्स क्या हैं ? कोशिकीय पारस्परिक क्रिया में कनेक्सिन्स की भूमिकाओं की विवेचना कीजिए । 15 (c) प्राइमोर्डियल जनन कोशिका क्या है ? उपयुक्त आरेख की सहायता से अंडजनन प्रक्रिया का 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 processes and structures. Allocate approximately 40% of time/words to part (a) given its 20 marks, and 30% each to parts (b) and (c). Structure: brief introduction defining key terms, then sequential treatment of each sub-part with integrated diagrams for (a) and (c), concluding with synthesis of developmental principles across all three parts.

Key points expected

  • Part (a): Definition of morphogens as concentration-dependent signaling molecules (e.g., Bicoid, Sonic hedgehog) and their role in establishing embryonic axes
  • Part (a): Mechanism of cellular differentiation during morphogenesis—competence, determination, and differential gene expression via morphogen gradients
  • Part (b): Structure and function of gap junction proteins (connexins, pannexins) as intercellular channels allowing direct cytoplasmic communication
  • Part (b): Specific roles of connexins in embryonic patterning, metabolic coupling, electrical synchronization, and cell signaling regulation
  • Part (c): Definition and origin of primordial germ cells (PGCs) from epiblast via BMP signaling and their migration to gonadal ridges
  • Part (c): Complete oogenesis process with diagram—mitotic proliferation, meiotic arrest at prophase I (dictyate), follicular development, ovulation, and meiotic completion
  • Integration: Comparative developmental significance—morphogens pattern tissues, gap junctions coordinate cell behavior, PGCs ensure germline continuity

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10Precise definitions: morphogens as diffusible proteins creating concentration gradients (French flag model); connexins as hexameric hemichannels forming gap junctions; PGCs as bipotential precursors distinct from somatic cells. Correctly identifies meiotic arrest points and signaling pathways (BMP4, Wnt).Broadly accurate definitions but confuses morphogens with growth factors, or connexins with tight junction proteins; vague on PGC specification timing.Fundamental errors—describes morphogens as hormones, confuses gap junctions with desmosomes, or states PGCs arise in gonads rather than extraembryonic mesoderm.
Diagram / labelling20%10Two clear, well-proportioned diagrams: (a) morphogen gradient with threshold response zones (French flag), and (c) complete oogenesis with follicular stages (primordial, primary, secondary, Graafian), showing germinal vesicle, zona pellucida, and antrum formation. Proper labels and arrows indicating developmental progression.One adequate diagram (usually oogenesis) with basic labels; second diagram attempted but incomplete or poorly scaled; missing key structural details.No diagrams or unrecognizable sketches; missing essential structures (e.g., no polar bodies in oogenesis, no concentration axis in morphogen gradient).
Examples & nomenclature20%10Specific named examples: Bicoid/Dorsal in Drosophila, Sonic hedgehog (Shh) in vertebrate limb patterning; connexin 43 (Cx43) in cardiac development, Cx26 in hearing; PGC markers (Oct4, Blimp1, Stella); Indian species context where relevant (e.g., amphibian models for morphogen studies at Indian research stations).Generic references without specific gene/protein names; mentions 'some connexins' or 'certain morphogens' without exemplification; no species-specific context.No valid examples; invents non-existent nomenclature; confuses Drosophila bicoid with vertebrate homologs inappropriately.
Process explanation20%10Sequential, mechanistic clarity: for morphogenesis—threshold-dependent gene activation (e.g., Krüppel, Knirps); for connexins—gating by voltage/pH, permeability to cAMP/IP3; for oogenesis—hormonal regulation (FSH, LH), meiotic spindle repositioning, and cytoplasmic maturation. Shows cause-effect relationships.Lists stages without explaining mechanisms; describes what happens but not how or why; weak linkage between molecular events and cellular outcomes.Disordered or illogical sequence; fundamental misconceptions about meiosis timing; no explanation of how connexin channels actually function.
Evolutionary / applied context20%10Evolutionary conservation of morphogen gradients (Dpp/BMP across phyla), connexin gene family expansion in vertebrates; applied relevance—connexin mutations in human disease (Charcot-Marie-Tooth, deafness), PGC culture for conservation of endangered Indian species (e.g., cheetah, gharial), teratogenic risks of morphogen pathway disruption (thalidomide/Shh).Brief mention of evolutionary conservation without elaboration; generic statement about 'medical importance' without specific disease links.No evolutionary or applied context; or irrelevant digression into unrelated biotechnology.

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