UPSC Zoology 2026 — Paper II

(a) Explain what semiconservative replication of DNA means and prove it: state the Watson-Crick prediction, the Meselson-Stahl experiment that discriminated it from conservative and dispersive models, and outline the enzymatic machinery that executes it. A compact note, not a full essay on replication. (b) Differentiate pinocytosis from phagocytosis - two forms of endocytosis - on defined criteria: nature of material taken in, particle/vesicle size, cell types involved, specificity and receptors, molecular machinery, and fate of the vesicle. A point-by-point comparison is expected, not two separate paragraphs. (c) Two explicit parts: (i) the principle of AFLP (Amplified Fragment Length Polymorphism) - its step-by-step molecular logic - and (ii) its applications. Both halves must be present; an answer describing only the technique is incomplete. (d) Three explicit sub-parts must each be covered: the clinical symptoms of haemophilia, its causes (genetic basis and mode of inheritance, including the clotting factor deficiency), and its treatment. Equal weight to all three. (e) Explain why heritable variation is the raw material of evolution: classify the kinds of variation, identify their sources, and show through named examples how selection, drift and gene flow act on variation to produce evolutionary change. The emphasis must be on the role of variation, not on a general account of Darwinism.

• Meaning: each daughter duplex has one parental and one new strand
• Meselson–Stahl experiment with ¹⁵N and ¹⁴N and CsCl gradient, with the bands after each generation
• How it ruled out conservative and dispersive models
• Enzymes: helicase, primase, DNA polymerase III and I, ligase, topoisomerase
• Nature of material: fluid against solid particles
• Vesicle size and the cells involved
• Specificity and receptors, and the machinery, clathrin against actin
• Fate of the vesicle, phagolysosome and residual body
• Restriction digestion with two enzymes and ligation of adapters
• Pre-selective and selective PCR amplification
• Gel or capillary separation and scoring of bands
• Applications such as diversity, mapping, marker-assisted selection, fingerprinting
• Symptoms: prolonged bleeding, haemarthrosis, bruising
• Causes: X-linked recessive, F8 or F9 mutation, factor VIII or IX deficiency
• Carrier inheritance
• Treatment: factor replacement, desmopressin, emicizumab, gene therapy
• Variation as the raw material on which selection acts
• Kinds of variation: continuous, discontinuous, heritable, non-heritable
• Sources: mutation, recombination, gene flow
• Named examples of selection, drift and gene flow

(a) Two parts: define transcription (what it is, template polarity, product), then describe eukaryotic transcription as a process - the three RNA polymerases, promoter elements, assembly of the pre-initiation complex with general transcription factors, initiation, elongation with CTD phosphorylation, termination, and co-transcriptional processing. The word 'eukaryotes' is decisive: prokaryotic sigma-factor transcription may be used only for contrast. (b) Two linked parts: define primary sex determination (determination of gonadal sex, chromosomal/genetic in mammals) and secondary sex determination (the hormone-dependent development of ducts, external genitalia and secondary sexual characters); then explain in detail the hormonal regulation - AMH, testosterone, DHT - that produces the male or female phenotype in humans. (c) Two parts: define speciation (with the species concept and modes of speciation and isolating mechanisms) and then elaborate - i.e. develop with evidence and examples - how continental drift, by producing large-scale vicariance, has generated new species. Most marks lie in the second, 'elaborate' part.

• Definition: RNA synthesis from a DNA template by RNA polymerase, template and polarity
• Three RNA polymerases with their products, and the promoter with TATA box and enhancers
• Initiation with general transcription factors and PIC, elongation with CTD phosphorylation, termination
• Co-transcriptional capping, splicing and polyadenylation, and contrast with prokaryotes
• Primary as gonadal sex by SRY, and secondary as hormone-driven development of ducts and genitalia
• AMH from Sertoli cells regressing Müllerian ducts
• Testosterone maintaining Wolffian ducts, and DHT forming male external genitalia
• Female pathway without AMH and androgens, and puberty hormones
• Definition of speciation, species concept, modes and isolating mechanisms
• Vicariance from the break-up of Pangaea and Gondwana leading to allopatric speciation
• Named examples: lungfishes, marsupials, ratites, cichlids, lemurs
• Evidence and a balanced note on the role of dispersal

(a) Two parts: define recombinant DNA technology and outline its essential steps and tool enzymes; then describe - with capacities, hosts and uses - the different classes of cloning vector. The second part carries the greater weight and requires several named vectors, not just 'plasmids'. (b) Describe how cladistic methodology is applied in modern taxonomy. The directive is 'applications', so the answer must move quickly past definitions to concrete uses - monophyly-based classification, phylogeny reconstruction, species delimitation, divergence dating, conservation and comparative biology - each with a named example. (c) Differentiate mitosis from meiosis on clearly stated criteria - site and cell type, number of divisions and daughter cells, behaviour of homologues in prophase I, chiasmata and crossing over, centromere behaviour in anaphase I versus anaphase II, ploidy outcome, genetic consequence and biological significance. A criterion-wise comparison, not two separate descriptions.

• Definition and steps: cutting with restriction enzymes, joining with ligase, introduction into host, selection
• Plasmids such as pBR322 and pUC with capacity and selection
• Phage λ vectors, cosmids and phagemids with capacities
• BAC, YAC and PAC, shuttle and expression vectors, and viral or Ti vectors, with hosts and uses
• Cladistics defined by synapomorphy and monophyly, with the cladogram
• Phylogeny reconstruction by parsimony, likelihood and Bayesian methods
• Monophyly-based classification, such as birds among dinosaurs and whales among artiodactyls
• Species delimitation, dating, conservation (EDGE), biogeography and epidemiology
• Site and cell type, number of divisions and daughter cells, and chromosome number outcome
• Prophase I with pairing, chiasmata and crossing over, absent in mitosis
• Anaphase I against anaphase II centromere and chromosome behaviour
• Genetic consequence and biological significance

(a) Two parts: state what Oparin's concept of the origin of life is, and then 'comment on' the chemical (abiogenic/chemosynthetic) theory - which requires not only setting out its stages and experimental support but also weighing its strengths, gaps and modern alternatives. (b) Explain the cis-acting functional elements that allow a eukaryotic chromosome to replicate, segregate and remain stable - origins of replication (ARS), centromere and telomere as the essential triad, plus the organisational elements (nucleosome/chromatin, NOR, MARs, insulators, heterochromatin). The YAC experiment that demonstrated the triad should anchor the answer. (c) A yes/no question that must be answered explicitly and then justified: mitochondria do contribute to inheritance through their own genome (extranuclear/cytoplasmic inheritance). Explain mtDNA and maternal transmission, heteroplasmy and the threshold effect, and illustrate with at least one worked example - the instruction 'with suitable example' makes the example compulsory.

• Oparin–Haldane concept: reducing atmosphere, primordial soup, coacervates, heterotrophs first
• Stages of chemical evolution from simple organics to polymers, protobionts and the first cell
• Experimental support: Miller–Urey, Oró, Fox, RNA world
• Critical comment: strengths, gaps and alternatives such as hydrothermal vents and panspermia
• Three essential elements: origins of replication (ARS), centromere, telomere
• The YAC experiment of Murray and Szostak
• Roles of each: replication, segregation, protection of ends and telomerase
• Organisational elements: nucleosomes, MARs, NOR, insulators, heterochromatin
• Clear answer yes, with mtDNA as a separate genome
• Human mtDNA features: 16,569 bp, 37 genes
• Maternal inheritance and non-Mendelian pattern
• Heteroplasmy and threshold effect, with a named example such as LHON, MERRF or MELAS

(a) Explain calcium as the trigger and switch of muscle contraction: its release from the sarcoplasmic reticulum during excitation-contraction coupling, its binding to troponin C and the resulting steric unblocking of actin, and its removal by SERCA during relaxation. Smooth/cardiac variations and clinical illustrations add depth. (b) Contrast excretion (removal of nitrogenous metabolic waste and foreign solutes) with osmoregulation (regulation of body-fluid water and solute concentration and volume) in mammals - their aims, the substances handled, the nephron segments and mechanisms involved, and the hormonal controls - while noting that the kidney performs both and that urea links them. (c) Trace the biosynthetic route of a peptide hormone from gene to secreted product: transcription, translation of a preprohormone on the rough ER with signal-peptide targeting, cleavage to prohormone, Golgi processing, packaging into secretory granules with prohormone convertases, and regulated exocytosis - illustrated with a named hormone such as insulin or POMC. (d) Explain how the placenta sustains human pregnancy - chiefly its endocrine functions (hCG rescuing the corpus luteum, the luteo-placental shift, progesterone's myometrial quiescence, oestrogens via the fetoplacental unit, hPL and CRH), together with its transport, immunological-tolerance and barrier roles. The focus must be on maintenance of pregnancy, not placental anatomy for its own sake. (e) Define sperm capacitation, state where and how long it takes, describe the membrane and signalling changes that constitute it (cholesterol efflux, bicarbonate and calcium entry, cAMP-PKA-driven tyrosine phosphorylation, hyperactivated motility), and state its significance - competence for the acrosome reaction and fertilisation, and its exploitation in assisted reproduction.

• Release of Ca²⁺ from the sarcoplasmic reticulum after the action potential via T tubules and the ryanodine receptor
• Ca²⁺ binding troponin C and tropomyosin moving off actin sites
• Removal of Ca²⁺ by SERCA for relaxation
• Variations in smooth and cardiac muscle
• Excretion as removal of nitrogenous wastes and foreign solutes, and osmoregulation as regulating water and solute concentration
• Substances handled by each
• Nephron mechanisms: filtration and secretion against loop of Henle, gradient and collecting duct
• Hormones ADH, aldosterone, ANP, and the urea link
• Transcription and translation on rough ER with a signal peptide
• Preprohormone to prohormone by signal peptidase
• Golgi processing and packaging into secretory granules
• Cleavage by prohormone convertases and regulated exocytosis, with a named example such as insulin or POMC
• hCG rescuing the corpus luteum
• Luteo-placental shift with progesterone and oestrogens
• Roles of progesterone, oestrogens, hPL and CRH
• Transport and immune tolerance
• Definition and the discoverers, with site and duration
• Membrane changes: cholesterol efflux, bicarbonate, calcium, cAMP and PKA, tyrosine phosphorylation
• Hyperactivated motility
• Significance for the acrosome reaction and for IVF

(a) Describe all four levels of protein organisation - primary, secondary, tertiary and quaternary - giving for each the defining feature, the bonds that stabilise it and at least one named protein example, as the phrase 'with suitable examples' requires. Supersecondary structure/domains and denaturation may be added. (b) Three things: define the nerve impulse, explain how the action potential is generated (resting potential, threshold, voltage-gated Na+ and K+ channel behaviour, all-or-none, refractory periods), and explain how it is propagated along the axon (local circuits, continuous versus saltatory conduction, factors affecting velocity). (c) Explain how the embryonic body axes - anteroposterior, dorsoventral and left-right - are established, i.e. the concept of axis specification by localised maternal determinants, morphogen gradients and organiser tissue; and illustrate it in detail with at least one named model system (Drosophila or Xenopus), as 'with suitable example' requires.

• Primary structure: amino acid sequence and peptide bonds, with an example such as insulin
• Secondary structure: α-helix and β-sheet with hydrogen bonds, and examples
• Tertiary structure: side-chain interactions and disulphide bonds, with myoglobin
• Quaternary structure: subunits, with haemoglobin
• Nerve impulse defined, with the resting potential and the Na⁺–K⁺ pump
• Generation: threshold, voltage-gated Na⁺ opening, depolarisation, K⁺ repolarisation, all-or-none
• Refractory periods and their role
• Propagation by local circuits, continuous and saltatory conduction, and factors affecting velocity
• Concept: anteroposterior, dorsoventral and left–right axes set by maternal determinants, morphogen gradients and organisers
• Drosophila: bicoid and nanos gradients, and Dorsal gradient with Toll
• Xenopus: cortical rotation, β-catenin, Nieuwkoop centre and the Spemann–Mangold organiser
• Left–right axis and Hox genes

(a) Two distinct tasks: (i) name the proteins of skeletal muscle contraction - contractile, regulatory and structural - and state the function of each; (ii) explain physiologically why cardiac muscle, unlike skeletal muscle, is fatigue-resistant. Both must be answered; the second is a reasoning question, not a list. (b) Three named stages must each be covered: generation of T-cell precursors in the bone marrow and their entry into the thymus; differentiation within the thymus (TCR gene rearrangement, double-negative to double-positive to single-positive, positive and negative selection); and activation of a mature naive T cell in the periphery (the three signals and the resulting effector subsets). (c) Two parts: define teratogenesis precisely, then 'illustrate' - i.e. explain with named agents and specific defects - the mechanisms by which teratogens disturb early embryonic development, including the critical-period principle and dose dependence.

• Contractile proteins actin and myosin with functions
• Regulatory proteins tropomyosin and troponin (TnT, TnC, TnI)
• Structural proteins such as titin, nebulin, α-actinin and dystrophin
• Fatigue-resistance of cardiac muscle: aerobic metabolism, mitochondria and myoglobin, rich blood supply, long refractory period
• Origin in bone marrow and entry into the thymus
• Thymic differentiation: TCR gene rearrangement, DN to DP to SP
• Positive and negative selection with central tolerance
• Peripheral activation by three signals and effector subsets
• Definition of teratogenesis
• Named teratogens and specific defects
• Mechanisms: DNA damage, cell death, disturbed signalling
• Critical period and dose dependence

(a) Four explicit tasks: define apoptosis; give its characteristics (morphological and biochemical, contrasted with necrosis); set out its molecular pathways (intrinsic/mitochondrial and extrinsic/death-receptor, with caspases); and state its significance in development, homeostasis, immunity and disease. All four must appear. (b) Describe how the endocrine pancreas regulates blood glucose: the islet cell types and their hormones, the glucose-sensing mechanism of the beta cell, the metabolic actions of insulin and glucagon on liver, muscle and adipose tissue, the paracrine role of somatostatin, and the consequences of failure (diabetes mellitus). (c) Two parts: define vitamins (with their classification), then differentiate them from coenzymes on the two criteria the question names - structure and function - while acknowledging the overlap, since most B-complex vitamins are precursors of coenzymes but not all vitamins are coenzymes and not all coenzymes are vitamin-derived.

• Definition: programmed, energy-dependent, non-inflammatory cell death
• Characteristics: shrinkage, chromatin condensation, blebbing, apoptotic bodies, DNA laddering, contrast with necrosis
• Intrinsic and extrinsic pathways with Bcl-2 family, cytochrome c, apoptosome, FasL/TNF, FADD and caspases
• Significance in development, homeostasis, immunity and disease
• Islet cell types with hormones: α glucagon, β insulin, δ somatostatin
• Beta-cell glucose sensing and insulin release
• Actions of insulin and glucagon on liver, muscle and adipose tissue
• Failure of regulation: diabetes mellitus and hypoglycaemia
• Definition of vitamins as essential dietary organic micronutrients
• Classification into fat-soluble and water-soluble
• Structure difference: vitamin as precursor, coenzyme as active derivative with examples
• Functional difference: coenzyme in group or electron transfer, vitamins with wider roles