Botany 2025 Paper II 50 marks Elaborate

Q2

(a) What are the different methods of DNA sequencing? Elaborate the shotgun sequencing method. (20 marks) (b) Describe the phenomenon of linkage by giving suitable examples. Why is the linkage an exception to Mendel's second law? (15 marks) (c) Explain the structure and behaviour of B chromosomes in plants. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) डी० एन० ए० अनुक्रमण के विभिन्न तरीके क्या हैं? शॉटगन अनुक्रमण तरीके को विस्तृत कीजिए। (20 अंक) (b) उपयुक्त उदाहरणों की सहायता से सहलग्नता की क्रिया का वर्णन कीजिए। सहलग्नता मेंडल के दूसरे नियम का एक अपवाद क्यों है? (15 अंक) (c) पौधों में B गुणसूत्रों की संरचना तथा गतिविधि को समझाइए। (15 अंक)

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Directive word: Elaborate

This question asks you to elaborate. 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 'elaborate' in part (a) demands comprehensive expansion with technical depth, while parts (b) and (c) require 'describe' and 'explain' respectively. Allocate approximately 40% of time/words to part (a) given its 20 marks, covering Sanger, NGS, and third-generation methods with detailed shotgun sequencing workflow; 30% each to (b) and (c). Structure: brief introduction on genomic advances → systematic treatment of each sub-part with clear sub-headings → concluding synthesis on how these genetic mechanisms advance crop improvement and evolutionary studies.

Key points expected

  • Part (a): Classification of DNA sequencing methods (Sanger dideoxy, Next-Generation Sequencing—Illumina, Ion Torrent; third-generation—PacBio, Oxford Nanopore); detailed shotgun sequencing workflow including library preparation, random fragmentation, cloning, sequencing, and computational assembly with overlap detection
  • Part (a): Comparison of shotgun vs. hierarchical/clone-by-clone sequencing; mention of Indian contributions (e.g., NIPGR, IARI work on crop genomes) and applications in de novo genome assembly
  • Part (b): Definition of linkage (complete and incomplete), Bateson and Punnett's sweet pea (Lathyrus odoratus) experiments showing deviation from 9:3:3:1 ratio; Morgan's Drosophila work establishing linkage groups
  • Part (b): Explanation of why linkage violates Mendel's second law (Independent Assortment)—genes on same chromosome tend to inherit together unless separated by crossing over; recombination frequency and map units
  • Part (c): Structure of B chromosomes (heterochromatic, smaller than A chromosomes, lack functional genes, possess specific DNA sequences like B-specific repeats); occurrence in maize (Zea mays), rye (Secale cereale), and Indian plants like Coix
  • Part (c): Behaviour—non-Mendelian inheritance, accumulation mechanisms (drive systems), effects on A chromosome pairing, phenotypic consequences; evolutionary significance as selfish genetic elements

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Demonstrates precise understanding across all three parts: for (a) correctly distinguishes sequencing generations and explains shotgun assembly algorithm; for (b) accurately defines linkage phases and recombination mechanics; for (c) correctly describes B chromosome molecular structure and drive mechanisms without confusing with A chromosome behaviourShows generally correct concepts with minor errors—may confuse shotgun with pairwise end-sequencing, oversimplify linkage as merely 'genes close together', or describe B chromosomes only as 'extra chromosomes' without structural detailFundamental misconceptions present—treats all sequencing methods identically, believes linkage disproves Mendel's first law, or describes B chromosomes as 'bacterial chromosomes'
Diagram / labelling15%7.5Includes well-drawn, properly labelled diagrams for at least two parts: shotgun sequencing workflow showing fragmentation, cloning, sequencing, and assembly stages; linkage map with recombination frequencies; or B chromosome structure showing centromere position and heterochromatin distribution relative to A chromosomesProvides one relevant diagram with basic labelling, or describes diagrams textually without clear visual representation; diagrams may lack critical labels (e.g., missing 'contig assembly' in shotgun)No diagrams attempted, or diagrams are irrelevant/misleading (e.g., showing DNA replication instead of sequencing methods)
Examples & nomenclature20%10Uses specific, accurate examples throughout: for (a) cites Human Genome Project, rice genome (Oryza sativa, IRGSP); for (b) cites Bateson-Punnett sweet pea, Morgan's Drosophila melanogaster sex-linked genes, Indian examples like linkage in Cicer; for (c) names maize B chromosome variants, rye Bs, and mentions Indian research at BHU or IARIProvides generic examples without specificity—mentions 'pea plants' without Bateson-Punnett, 'fruit flies' without Morgan, or 'some grasses' for B chromosomes; nomenclature mostly correct but inconsistentExamples absent or wrong—uses Mendel's original peas for linkage, confuses B chromosomes with bacterial artificial chromosomes (BACs), or invents non-existent examples
Process explanation25%12.5Explains dynamic processes with mechanistic clarity: for (a) stepwise shotgun sequencing from physical/chemical fragmentation through bioinformatic assembly; for (b) how crossing-over creates recombination and breaks linkage; for (c) B chromosome accumulation through directed nondisjunction and their effects on nuclear organizationDescribes processes in static terms—lists steps without causal connections, states that recombination 'happens' without explaining crossing-over mechanism, or notes B chromosome presence without explaining how they accumulateProcess descriptions absent or incoherent—presents shotgun as 'shooting DNA', describes linkage as 'genes sticking together' without chromosomal basis, or cannot explain B chromosome behaviour beyond 'they divide'
Application / ecology15%7.5Connects concepts to practical significance: shotgun sequencing for crop genome projects (e.g., Indian wheat, chickpea genomes); linkage analysis for marker-assisted selection in Indian agriculture; B chromosome research for understanding genome evolution and potential applications in chromosome engineering for crop improvementMentions applications superficially—notes 'used in genomics' or 'important for breeding' without specific Indian or contemporary relevance; or focuses only on one part while ignoring othersNo application context provided, or applications are irrelevant (e.g., suggesting B chromosomes are used for gene therapy); fails to demonstrate why these genetic concepts matter beyond academic interest

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