UPSC Zoology 2024 — Paper II

Framework: Zoology Paper 2: Define > Structure/Mechanism > Diagram > Example. (a) describe: define > structure or process in order > labelled diagram > significance | (b) compare: paired headings or point-wise contrast > key differences > significance > conclusion | (c) enumerate: list the items in order > one line each > no commentary | (d) compare: paired headings or point-wise contrast > key differences > significance > conclusion | (e) describe: define > structure or process in order > labelled diagram > significance Full marks: Precise definitions, correct examples, clear mechanisms, labelled diagrams where relevant.

• Formed by repeated endoreduplication without separation of chromatids; giant size
• Occurrence: larval salivary glands etc. of Diptera (Drosophila, Chironomus); discovered by Balbiani
• Structure: bands and interbands, somatic pairing of homologues, chromocentre
• Puffs and Balbiani rings as sites of active transcription induced by ecdysone; uses in mapping
• Definition of heterogamety
• Male heterogamety: XX–XY (mammals, Drosophila) and XX–XO (grasshopper)
• Female heterogamety: ZZ–ZW (birds, snakes, Lepidoptera) and ZZ–ZO
• Consequence: which parent determines the offspring's sex; 1:1 sex ratio
• Structural: deletion, duplication, inversion (para- and pericentric), translocation (reciprocal, Robertsonian)
• Numerical: aneuploidy (monosomy, trisomy, nullisomy) and euploidy (auto- and allopolyploidy)
• Examples: cri-du-chat, Bar eye, Down, Turner, Klinefelter
• Causes (breakage, non-disjunction) and meiotic consequences
• Geographic isolation: extrinsic physical barriers preventing gene flow; allopatric speciation
• Reproductive isolation: intrinsic prezygotic and postzygotic mechanisms
• Examples for both (e.g. Palghat Gap shortwings; mule sterility)
• Relationship: geographic isolation often precedes reproductive isolation; sympatric speciation without it
• Base substitutions: transitions and transversions
• Silent, missense (sickle-cell example) and nonsense mutations with effects
• Frameshift insertions and deletions; in-frame indels
• Phenotypic classes: loss-of-function, gain-of-function, dominant-negative, conditional; repeat expansions

(a) discuss: intro > 3-4 dimensions > example > balanced close | (b) compare: paired headings or point-wise contrast > key differences > significance > conclusion | (c) explain: definition/context > points in order > small example > short close Full marks: Comprehensive, accurate, with specific examples and clear mechanisms.

• RER: flattened interconnected cisternae with ribosomes attached via translocon, continuous with nuclear envelope
• SER: branched tubules without ribosomes; abundance in steroid-secreting cells, liver, muscle (sarcoplasmic reticulum)
• Secretory pathway: signal peptide, SRP, co-translational translocation, folding by chaperones, N-glycosylation, quality control/ERAD
• COPII vesicles to Golgi and exocytosis; SER roles in lipid/steroid synthesis, detoxification, Ca²⁺ storage
• Euchromatin: decondensed, early replicating, acetylated histones, transcriptionally active
• Heterochromatin: condensed in interphase, late replicating, H3K9 methylation, DNA methylation, largely inactive
• Constitutive heterochromatin (centromeres, telomeres, satellite DNA) and facultative heterochromatin (Barr body)
• Functional roles: gene regulation, chromosome stability, position-effect variegation
• Signal molecules and modes (endocrine, paracrine, autocrine, juxtacrine, synaptic)
• Receptor classes: GPCR, RTK, ion-channel, intracellular (nuclear) receptors
• Pathology from receptor, G-protein, kinase or second-messenger defects with examples (EGFR, Ras, cholera and pertussis toxins)
• Further examples: insulin resistance, androgen insensitivity, BCR–ABL in CML; therapeutic targeting

Framework: Zoology, Paper 2. (a) account for: state the phenomenon > the causes in order of weight > conclusion | (b) describe: define > structure or process in order > labelled diagram > significance | (c) discuss: intro > 3-4 dimensions > example > balanced close Full marks: Comprehensive, accurate, and well-structured answers with clear diagrams and examples.

• Definition of sex determination; Bridges' genic balance theory: sex set by X:A ratio, Y not sex-determining in Drosophila
• Ratios: 1.0 female, 0.5 male, between 0.5 and 1.0 intersex, >1.0 metafemale, <0.5 metamale
• Abnormal karyotypes: XXY female, XO sterile male, 3X:2A metafemale, 2X:3A intersex, X:3A metamale, 3X:3A triploid female
• Molecular basis: X-signal elements → Sxl → tra → dsx; dosage compensation
• Definition; semiconservative nature (Meselson–Stahl)
• Initiation at oriC: DnaA, DnaB helicase, SSB, gyrase, primase
• Elongation by Pol III holoenzyme; leading and lagging strands, Okazaki fragments; Pol I primer removal; ligase
• Termination at Ter/Tus and decatenation; proofreading fidelity; bidirectional theta mode
• Operon concept (Jacob–Monod): regulator, promoter, operator, structural genes
• lac operon: negative inducible control by LacI and allolactose; positive control by cAMP–CAP (catabolite repression)
• trp operon: negative repressible control by TrpR–tryptophan and attenuation via the leader peptide
• Other mechanisms: AraC dual control, sigma factors, small RNAs/riboswitches

Framework: null. (a) describe: define > structure or process in order > labelled diagram > significance | (b) explain: definition/context > points in order > small example > short close | (c) compare: paired headings or point-wise contrast > key differences > significance > conclusion Full marks: Comprehensive, chronologically accurate, with precise terminology and clear differentiation of concepts.

• Early hominins: Sahelanthropus (~7 Mya), Orrorin (~6 Mya), Ardipithecus (5.8–4.4 Mya); Ramapithecus now Sivapithecus (not ancestral)
• Australopithecines (A. anamensis, afarensis, africanus; Paranthropus) with dates and features
• Homo habilis, H. erectus, H. heidelbergensis, H. neanderthalensis, H. sapiens with dates, cranial capacity and tool cultures
• Trends: bipedalism before brain expansion, dental reduction, tool culture; out-of-Africa and Indian evidence (Narmada hominin)
• Statement and assumptions; p² + 2pq + q² = 1; equilibrium after one generation
• Mutation: forward/backward rates, Δp = vq − up, equilibrium q̂ = u/(u+v); slow but source of variation
• Genetic drift: random sampling in small populations, variance pq/2N, fixation/loss, loss of heterozygosity
• Bottleneck and founder effects with examples
• Systematics: study of diversity and relationships (Simpson); taxonomy: theory and practice of classification and nomenclature
• Classical taxonomy: morphology, anatomy, embryology, cytotaxonomy; tools such as specimens, keys, numerical taxonomy
• Molecular taxonomy: DNA sequencing, barcoding (COI), rRNA, microsatellites/SNPs, RFLP, protein electrophoresis; BLAST, ClustalW, MEGA phylogenies
• Comparison of strengths and limitations; integrative taxonomy

Framework: Zoology Paper 2: Define > Structure/Mechanism > Diagram > Example. (a) write short notes: define > 3-4 key features > one example > one-line significance | (b) write short notes: define > 3-4 key features > one example > one-line significance | (c) write short notes: define > 3-4 key features > one example > one-line significance | (d(i)) write short notes: define > 3-4 key features > one example > one-line significance | (d(ii)) write short notes: define > 3-4 key features > one example > one-line significance | (e(i)) write short notes: define > 3-4 key features > one example > one-line significance | (e(ii)) write short notes: define > 3-4 key features > one example > one-line significance Full marks: Precise definitions, correct mechanisms, named taxa, and clear diagrams where applicable.

• Basic structure: two heavy and two light chains, Fab and Fc regions
• Classes and functions: IgG (opsonisation, neutralisation, complement, placental transfer), IgM (primary response, agglutination, complement)
• IgA (mucosal secretory immunity, colostrum), IgE (mast-cell binding, allergy, helminths), IgD (B-cell receptor)
• Effector mechanisms: neutralisation, opsonisation, ADCC, complement activation
• Antigens A, B and H on RBCs and natural anti-A/anti-B antibodies; transfusion compatibility
• Genetic basis: multiple alleles Iᴬ, Iᴮ, i on chromosome 9; codominance; glycosyltransferases; Bombay phenotype
• Erythroblastosis foetalis: Rh-negative mother sensitised, IgG anti-D crossing placenta in later pregnancy, fetal haemolysis
• Prevention with anti-D immunoglobulin; ABO-HDN as milder form
• Olfactory epithelium: bipolar receptor neurons, supporting and basal cells, Bowman's glands
• Transduction: odorant GPCRs, Gαolf, adenylyl cyclase, cAMP, CNG channels, Ca²⁺-activated Cl⁻ channels
• Pathway: olfactory nerve through cribriform plate → bulb glomeruli → mitral/tufted cells → lateral olfactory tract → piriform cortex, amygdala, entorhinal cortex/hippocampus
• Adaptation and odour coding (one receptor per neuron, glomerular map); accessory olfactory system
• Begins at grey crescent with bottle cells forming dorsal lip of blastopore
• Involution of mesoderm and endoderm, archenteron formation, epiboly of ectoderm
• Yolk plug and closure of blastopore; convergent extension
• Spemann–Mangold organizer and germ-layer fates
• Haeckel's statement: ontogeny recapitulates phylogeny
• Evidence cited: pharyngeal arches, tail, two-chambered embryonic heart
• Criticism: embryos resemble ancestral embryos, not adults; heterochrony, caenogenesis, paedomorphosis
• Von Baer's laws as the accepted modern view
• Definition: self-renewal and differentiation; niche
• Potency: totipotent, pluripotent, multipotent, unipotent
• Types/sources: embryonic, adult (haematopoietic, mesenchymal, neural), iPS cells (Yamanaka factors)
• Applications: regenerative medicine, disease modelling, bone-marrow transplant; ethical issues
• Histological (Grosser) classification: epitheliochorial, syndesmochorial/synepitheliochorial, endotheliochorial, haemochorial with examples
• Morphological: diffuse, cotyledonary, zonary, discoidal with examples
• Deciduate vs non-deciduate
• Functional significance of barrier thickness

Framework: UPSC Zoology Paper 2. (a(i)) explain: definition/context > points in order > small example > short close | (a(ii)) explain: definition/context > points in order > small example > short close | (b) describe: define > structure or process in order > labelled diagram > significance | (c) explain: definition/context > points in order > small example > short close Full marks: Precise mechanisms, labelled diagrams, named enzymes/agents, clear distinction between types.

• Parts: renal corpuscle (glomerulus, Bowman's capsule, podocytes), PCT, loop of Henle, DCT, collecting duct; cortical vs juxtamedullary nephrons
• Glomerular filtration: filtration barrier, net filtration pressure, GFR ~125 mL/min
• Tubular reabsorption and secretion in PCT, loop, DCT and collecting duct
• Counter-current multiplier and vasa recta exchange producing concentrated urine
• ADH: aquaporin-2 insertion in collecting duct, water reabsorption
• Aldosterone: Na⁺ reabsorption and K⁺/H⁺ secretion in DCT/collecting duct
• Renin–angiotensin system from the juxtaglomerular apparatus
• ANP (natriuresis) and PTH (Ca²⁺ reabsorption)
• Classification by chemical nature (peptide/protein, steroid, amino-acid derived) and by mechanism/source
• Cholesterol import by StAR and side-chain cleavage by CYP11A1 to pregnenolone (rate-limiting)
• Pathways to progesterone, mineralocorticoids, glucocorticoids, androgens and oestrogens with key enzymes (3β-HSD, CYP21, CYP11B1/B2, CYP17, 17β-HSD, aromatase)
• Zonation of adrenal cortex and tissue specificity; regulation by ACTH/LH; enzyme-defect disorders (CAH)
• Definition: production of structural/functional birth defects during development
• Critical period: organogenesis (weeks 3–8); all-or-none period earlier
• Wilson's principles: genotype, stage, dose, mechanism
• Outcomes: death, malformation, growth retardation, functional deficit
• Genetic: chromosomal aneuploidies (trisomies 21, 18, 13; Turner) and single-gene defects
• Induced by drugs (thalidomide, isotretinoin, valproate), alcohol, radiation
• Infections (TORCH, Zika) and maternal conditions (diabetes, iodine/thyroid deficiency); endocrine disruptors (DES)
• Mechanisms and prevention (folic acid, vaccination, avoidance, counselling)

Framework: Zoology Paper 2: Define > Structure/Mechanism > Diagram > Example. (a) describe: define > structure or process in order > labelled diagram > significance | (b) justify: claim > 3-4 reasons > evidence > conclusion | (c) explain: definition/context > points in order > small example > short close Full marks: Precise terminology, labelled diagrams, and clear logical flow for all parts.

• Site: seminiferous tubules; roles of Sertoli (support, blood–testis barrier) and Leydig cells (testosterone)
• Phases: multiplication (spermatogonia A and B), growth (primary spermatocyte), maturation (meiosis I and II) with chromosome numbers
• Spermiogenesis: nuclear condensation, acrosome, flagellum from distal centriole, mitochondrial sheath, loss of cytoplasm; spermiation
• Hormonal control by FSH, LH/testosterone and inhibin; duration ~64–74 days
• Golgi produces proacrosomal granules/vesicles
• Fusion into single acrosomal vesicle with granule attaching to the nucleus
• Spreading as acrosomal cap; Golgi remnant discarded
• Phases (Golgi, cap, acrosome, maturation) and enzymes (hyaluronidase, acrosin) for fertilisation
• Concept of apoenzyme + coenzyme = holoenzyme; vitamins as precursors of coenzymes needed in catalytic amounts
• B-complex examples: B1→TPP, B2→FMN/FAD, B3→NAD/NADP, B5→CoA, B6→PLP, B7→biotin, B9→THF, B12→cobamides with reactions
• Fat-soluble vitamins mostly not coenzymes (except K as carboxylase cofactor); vitamin C as hydroxylase cofactor
• Deficiency diseases showing enzyme failure (beriberi, pellagra, megaloblastic anaemia)
• Neuromuscular transmission: ACh, nicotinic receptor, end-plate potential, muscle action potential
• Excitation–contraction coupling: T-tubules, DHP receptors, ryanodine receptors, Ca²⁺ release
• Ca²⁺–troponin C, tropomyosin shift, cross-bridge cycle with ATP (attachment, power stroke, detachment)
• Sliding filament evidence (A band constant, I band and H zone shorten); relaxation by SERCA
• Strength–duration curve
• Rheobase: minimum current strength that excites when applied for long duration
• Chronaxie: minimum duration needed at twice the rheobase
• Use as an index of excitability (shorter chronaxie = more excitable)

Framework: Zoology Paper 2: Define > Structure/Mechanism > Diagram > Example. (a(i)) explain: definition/context > points in order > small example > short close | (a(ii)) explain: definition/context > points in order > small example > short close | (b) discuss: intro > 3-4 dimensions > example > balanced close | (c) describe: define > structure or process in order > labelled diagram > significance Full marks: Precise mechanisms, labelled diagrams, specific enzyme/hormone names, clear developmental stages.

• Bile composition and bile salts: emulsification, micelle formation, fat and fat-soluble vitamin absorption; enterohepatic circulation
• Other hepatic roles: bilirubin excretion, processing of absorbed nutrients (glycogen, deamination/urea, plasma proteins, storage)
• Pancreatic juice: bicarbonate, proteases as zymogens (trypsinogen activated by enteropeptidase), amylase, lipase, nucleases
• Endocrine pancreas: insulin, glucagon, somatostatin in nutrient handling
• Neural: vagal cholinergic stimulation in cephalic/gastric phases; sympathetic inhibition
• Secretin from S cells (acid) → bicarbonate-rich pancreatic and biliary secretion
• CCK from I cells (fat, amino acids) → enzyme secretion, gall bladder contraction, relaxation of sphincter of Oddi
• Other modulators: GIP/GLP-1, gastrin, somatostatin; potentiation between secretin and CCK
• Electron transport chain complexes I–IV, ubiquinone, cytochrome c, O₂ as final acceptor
• Mitchell's chemiosmotic theory: proton gradient and ATP synthase rotary mechanism; P/O ratios
• Inhibitors and uncouplers (rotenone, antimycin, cyanide, oligomycin, DNP); ATP yield per glucose
• Tissue roles: heart/brain dependence, muscle fibre types, shuttles, brown fat thermogenesis via UCP-1
• Optic vesicle from diencephalon, contact with surface ectoderm, induction of lens placode
• Optic cup formation; lens vesicle and lens fibres
• Neural retina and RPE from optic cup layers; iris and ciliary body from cup rim; optic nerve via optic stalk; choroid, sclera and cornea from mesenchyme/neural crest and ectoderm
• Molecular control (Pax6, Rx, Six3), Spemann's induction experiments, and defects (coloboma)