Zoology 2023 Paper II 50 marks Describe

Q3

(a) Describe the structure of mitochondrion. Why is it known as powerhouse of the cell? (20 marks) (b) Explain the mechanism of sex determination in human beings. (15 marks) (c) Describe various means which result in change in gene frequencies in a population. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) सूत्रकणिका (माइटोकॉन्ड्रियन) की संरचना का वर्णन कीजिए। यह कोशिका का ऊर्जा-घर क्यों जाना जाता है? (20 अंक) (b) मनुष्यों में लिंग-निर्धारण की क्रियाविधि की व्याख्या कीजिए। (15 अंक) (c) विभिन्न तरीकों का वर्णन कीजिए जिनके परिणामस्वरूप किसी जनसंख्या में जीन आवृत्तियों में परिवर्तन होता है। (15 अंक)

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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 structure and function. Allocate approximately 40% of time/words to part (a) given its 20 marks, with 30% each to parts (b) and (c). Structure: brief introduction linking the three sub-parts under cell biology and population genetics; body addressing each part sequentially with clear sub-headings; conclusion synthesizing how mitochondrial inheritance, sex determination, and gene frequency changes collectively illustrate fundamental genetic principles.

Key points expected

  • Part (a): Double membrane organization (outer smooth, inner folded into cristae); matrix composition (enzymes, mtDNA, ribosomes); elementary particles/F1 particles on cristae; ATP synthesis via chemiosmotic mechanism justifying 'powerhouse' designation
  • Part (b): XX-XY system with SRY gene on Y chromosome; gonadal differentiation pathway (indifferent gonad → testis/ovary); hormonal cascade (testosterone, AMH, estrogen); brief mention of sex chromosome aneuploidies (Klinefelter, Turner) as supporting evidence
  • Part (c): Five evolutionary forces—mutation (ultimate source), genetic drift (founder/bottleneck effects), gene flow/migration, natural selection (directional/stabilizing/disruptive), non-random mating; Hardy-Weinberg equilibrium as baseline reference
  • Part (a): Specific enzymes and carriers—NADH dehydrogenase, cytochrome oxidase, ATP synthase; proton gradient establishment across inner membrane
  • Part (b): Dosage compensation via X-inactivation (Barr body); recent discoveries of sex-determining genes beyond SRY (SOX9, DAX1)
  • Part (c): Indian examples where relevant—sickle cell trait and malaria resistance (natural selection); genetic drift in isolated populations (Andaman tribes)

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Accurately describes mitochondrial ultrastructure with correct membrane terminology; precisely states SRY-mediated sex determination mechanism; correctly identifies all five forces changing gene frequencies with proper definitionsBasic structure described with minor errors (e.g., confusing matrix with cytoplasm); sex determination described as 'XY system' without gene-level explanation; lists 3-4 evolutionary forces with some confusion between themFundamental misconceptions—single membrane mitochondria, environmental sex determination in humans, conflates gene flow with genetic drift, or omits mutation as evolutionary force
Diagram / labelling15%7.5Clear, proportionate diagram of mitochondrion showing double membrane, cristae, matrix with 8+ accurate labels; may include schematic of ATP synthase or sex chromosome karyotype for (b)Acceptable diagram with 5-7 labels, minor structural inaccuracies; or describes structure adequately without visual representationNo diagram where essential, or seriously flawed diagram with <4 correct labels; confusing cristae with other organelles
Examples & nomenclature20%10Uses precise terminology—cristae, F1-F0 ATPase, SRY, gonadal ridge, Mullerian inhibiting substance; cites specific conditions (Leber's hereditary optic neuropathy for mitochondrial inheritance; 47,XXY/45,X0 syndromes)Generally correct terminology with occasional imprecision; mentions some genetic disorders without clear linkage to mechanismVague or incorrect nomenclature—'mitochondrial DNA' without context, 'male chromosomes,' generic 'birth defects' without specification; no named examples
Process explanation25%12.5For (a): clear chemiosmotic coupling—electron transport chain, proton gradient, ATP synthesis; for (b): sequential developmental cascade from genetic to phenotypic sex; for (c): explains how each force alters allele frequencies mathematically or mechanisticallyDescribes processes in correct sequence but lacks mechanistic depth—states 'energy produced' without proton gradient, 'testes form' without SRY trigger, lists forces without explaining frequency change mechanismNo process explanation—only static descriptions; or fundamentally incorrect sequences (ATP made in matrix without membrane involvement, hormones determine genetic sex)
Evolutionary / applied context15%7.5Connects mitochondrial endosymbiotic origin briefly; notes maternal inheritance pattern; links sex determination conservation to mammalian evolution; applies gene frequency changes to conservation genetics, human evolution, or disease resistance (e.g., Indian population studies)Mentions evolutionary significance superficially or for only one part; no application to real-world contextsNo evolutionary or applied context; treats all topics as isolated facts without synthesis

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