Zoology 2024 Paper II 50 marks Describe

Q3

(a) What is sex-determination? Give an account of genic balance theory of sex-determination in Drosophila with examples from abnormal karyotypes. (20 marks) (b) What is DNA replication? Describe the process of DNA replication in prokaryotes. (15 marks) (c) Discuss gene regulation mechanisms in E. coli. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) लिंग-निर्धारण क्या है? ड्रोसोफिला में लिंग-निर्धारण के जीनी संतुलन (जीनिक बैलेंस) सिद्धांत का विवरण असामान्य गुणसूत्रप्ररूपों (कैरियोटाइप) के उदाहरणों सहित दीजिए। (20 अंक) (b) डी० एन० ए० प्रतिकृतियन क्या है? प्राक्केंद्रकी में डी० एन० ए० प्रतिकृतियन की प्रक्रिया का वर्णन कीजिए। (15 अंक) (c) ई० कोलाई में जीन नियमन (जीन रेगुलेशन) तंत्र की चर्चा कीजिए। (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

This question demands descriptive exposition across three molecular genetics topics. Allocate approximately 40% of time/words to part (a) given its 20 marks, with 30% each to parts (b) and (c). Structure with brief definitions for each sub-part, followed by detailed mechanistic descriptions, supporting diagrams, and concluding with significance. For (a), emphasize the genic balance equation and karyotype examples; for (b), detail enzymatic machinery; for (c), compare lac and trp operons.

Key points expected

  • Definition of sex-determination and distinction from sex differentiation; genic balance theory with X:A ratio calculation (1.0 = female, 0.5 = male, 0.5-1.0 = intersex)
  • Abnormal Drosophila karyotypes: 2X:3A (metafemale/sterile), 3X:2A (superfemale), 2X:2A with Y (normal female), XY with 2A (normal male), XO (sterile male), XXY (female)
  • DNA replication definition; semiconservative mechanism; initiation at oriC, DnaA binding, helicase unwinding, primase synthesis, continuous vs discontinuous synthesis on leading/lagging strands
  • Prokaryotic replisome components: DNA pol III (main polymerase), DNA pol I (primer removal), ligase, SSB proteins, topoisomerases; Okazaki fragment processing
  • E. coli gene regulation: lac operon (negative inducible control with catabolite repression/CRP-cAMP), trp operon (negative repressible control, attenuation mechanism)
  • Regulatory elements: operator, promoter, regulatory gene, effector molecules (allolactose, tryptophan), feedback inhibition and transcriptional attenuation in trp operon

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness22%11Precise definitions with accurate mechanistic details: correct X:A ratio thresholds for (a), semiconservative replication fidelity for (b), distinction between negative/positive regulation and inducible/repressible systems for (c); no conceptual errors in genetic terminologyBroadly correct concepts with minor inaccuracies in ratios or regulatory logic; some confusion between DNA pol I and III functions, or between operon typesFundamental misconceptions: confusing genic balance with chromosomal sex determination, describing conservative replication, or equating lac and trp operon regulation mechanisms
Diagram / labelling18%9Three clear, well-labelled diagrams: (a) Drosophila karyotype comparisons showing X and autosome numbers; (b) replication fork with leading/lagging strands, primers, Okazaki fragments; (c) lac operon structure with promoter, operator, structural genes and regulatory circuitTwo adequate diagrams with minor labelling omissions; or all three with cluttered presentation; missing key features like directionality arrows or enzyme labelsSingle or no diagrams; poorly drawn with confused structures; missing critical labels (e.g., no oriC indicated, operon genes unlabelled, no 5'-3' direction shown)
Examples & nomenclature20%10Complete nomenclature: Bridges' genic balance theory with exact karyotype notations (2X:3A, etc.), all replication proteins named correctly (DnaA, DnaB helicase, primase DnaG), regulatory molecules specified (allolactose vs IPTG distinction, corepressor tryptophan)Most terms correct but inconsistent notation (e.g., X/A vs X:A), missing some protein names (referring to 'helicase' generically), or conflating inducer with substrateIncorrect or invented terminology; confusing genotype with phenotype descriptions; missing all specific examples of abnormal karyotypes or regulatory mutants
Process explanation25%12.5Stepwise chronological descriptions with causal linkages: for (a) how X:A ratio triggers Sex-lethal (Sxl) expression cascade; for (b) ordered assembly of replisome and coordination of leading/lagging strand synthesis; for (c) molecular events from inducer binding to RNA polymerase accessCorrect sequence but lacking mechanistic detail; descriptive without explaining 'how' processes occur; missing temporal coordination between replication eventsDisordered or fragmented description; listing components without explaining their interaction; no sense of process flow or regulatory logic
Evolutionary / applied context15%7.5Brief contextualization: evolutionary significance of genic balance vs chromosomal systems in insect diversification; replication fidelity and error rates in genome stability; operon organization as prokaryotic adaptation for metabolic efficiency; mention of biotechnological applications (plasmid replication, recombinant protein expression using lac promoter)Generic concluding statements without specific connections; or context limited to one sub-part onlyNo evolutionary or applied perspective; purely descriptive answer ending abruptly without synthesis or significance

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