UPSC Civil Services (Main) Examination 2026 — Zoology Optional Paper II: Questions with Model Answers | Plutus IAS

08 Sep UPSC Civil Services (Main) Examination 2026 — Zoology Optional Paper II: Questions with Model Answers | Plutus IAS

The questions below are from Zoology Optional Paper II of UPSC Civil Services (Main) Examination 2026 (held 2026-08-30) — the actual paper, which is public. Each carries a model answer written by Aanya in Plutus IAS teaching style, to the marks and word limit, following our faculty’s discussion of the paper (video below).

Official source: official (upsc.gov.in).

Q1. Write on the following in about 150 words each : 10×5=50 (a) Semiconservative replication of DNA (b) Difference between pinocytosis and phagocytosis (c) Principle and applications of AFLP (d) Symptoms, causes and treatment of haemophilia (e) Role of variations in evolutionary change (15 marks)

How to approach this question

The directive word “Write on” requires a concise, structured explanation with key definitions, mechanisms, and applications. Examiners test understanding of core concepts, their biological significance, and real-world relevance. A top answer must: (1) define the term precisely, (2) explain the mechanism step-wise, and (3) connect it to broader biological or medical implications. The common mistake is overloading with unnecessary details or missing the evolutionary/medical linkage that elevates the answer.

Model answer

Semiconservative replication of DNA is the process by which each strand of a parental DNA double helix serves as a template for the synthesis of a complementary daughter strand, resulting in two daughter DNA molecules each containing one original (parental) strand and one newly synthesized strand. Discovered by Meselson and Stahl (1958) using ¹⁵N-labeled E. coli grown in ¹⁴N medium, the experiment demonstrated that after one generation all DNA was hybrid (15N–14N), and after two generations half was hybrid and half was light (14N–14N), confirming the semiconservative model. Mechanistically, the process begins with helicase-mediated unwinding at origins of replication, forming replication forks; single-strand binding proteins stabilize exposed strands; primase synthesizes RNA primers; DNA polymerase III elongates the new strand in the 5′→3′ direction using dNTPs; leading strand synthesis is continuous, while lagging strand synthesis proceeds discontinuously via Okazaki fragments; finally, DNA polymerase I replaces RNA primers with DNA and DNA ligase seals nicks. This mechanism ensures genetic fidelity and provides a substrate for variation: replication errors introduce mutations that fuel evolution, while proofreading and mismatch repair systems maintain genomic integrity. Clinically, understanding semiconservative replication underpins CRISPR-Cas9 gene editing, PCR-based diagnostics, and cancer genomics, where replication stress and fidelity defects drive tumorigenesis.

Q2. (a) What is DNA transcription? Explain the process of DNA transcription in eukaryotes. (b) What is primary and secondary sex determination? Explain hormonal regulation of sex determination in humans. (c) What is speciation? Elaborate the role of continental drift in the formation of new species. (15 marks)

How to approach this question

The directive word “explain” requires you to define, delineate the steps, and give concrete evidence. Examiners test (a) conceptual clarity of transcription, (b) hormonal cascades in sex determination, and (c) geological-biological linkage in speciation. A top answer has three parts: definition → stepwise mechanism with named molecules and checkpoints → real-world or geological example. The common mistake is to stop at the definition or to miss the regulatory hormones (FSH, testosterone) and geological time scales.

Model answer

(a) DNA transcription is the enzymatic synthesis of a complementary RNA strand from a DNA template, serving as the first step in gene expression. In eukaryotes it occurs in the nucleus and proceeds in three phases:

• Initiation: The pre-initiation complex (TFIID/TBP) binds the TATA box in the promoter; RNA polymerase II is recruited and the transcription bubble forms.
• Elongation: RNA polymerase II moves 3′→5′ on the template strand, synthesising pre-mRNA (5′→3′) with ribonucleotides; introns remain embedded.
• Termination & processing: A polyadenylation signal (AAUAAA) triggers cleavage and poly(A) tail addition; 5′ capping and splicing remove introns (via spliceosomes) to yield mature mRNA.

(b) Sex determination is hierarchical: Primary is genetic (SRY on Y chromosome upregulates SOX9 → testes development → testosterone/MIS); Secondary is hormonal, where fetal testosterone masculinises Wolffian ducts and external genitalia, while Müllerian-inhibiting substance regresses female structures. Post-natally, FSH and LH regulate gametogenesis and steroidogenesis.

(c) Speciation is the evolutionary process by which populations diverge into distinct species. Continental drift (plate tectonics) fragments habitats; allopatric speciation ensues. Classic evidence: the breakup of Gondwana (~160–80 Ma) isolated ratite birds on separate landmasses, leading to ostriches (Africa), rheas (South America), and emus/cassowaries (Australia).

Thus, transcription converts genetic information into functional transcripts, hormonal axes translate genotype into phenotype, and geological forces provide the spatial template for biodiversity.

Q3. (a) What is recombinant DNA technology? Describe the different vectors used in DNA cloning. (b) Describe the applications of cladistics in modern taxonomy. (c) Differentiate between mitosis and meiosis division. (15 marks)

How to approach this question

The directive word “Describe” in (a) and (b) demands definition plus detailed enumeration, while “Differentiate” in (c) requires a point-wise comparison. The examiner is testing conceptual clarity on recombinant DNA technology, vector classification, cladistics applications, and precise cell-division mechanics. A top answer must be structured in three parts: (1) concise definition, (2) enumerated points with real examples or data, and (3) a forward-looking statement. The common mistake is to treat vectors or cladistics as mere lists without linking them to functional outcomes or evolutionary insights.

Model answer

(a) Recombinant DNA technology is the laboratory method of artificially joining DNA fragments from different biological sources to create novel genetic combinations that can be introduced into host organisms for desired functions. It relies on three core steps: isolation of target DNA, insertion into a vector, and introduction into a host cell.

Vectors used in DNA cloning are carrier molecules that ferry foreign DNA into host cells. They are classified by origin and capacity:

• Plasmids (e.g., pBR322, pUC19): small, circular, double-stranded DNA from bacteria; carry selectable markers (ampicillin resistance) and multiple cloning sites; capacity ~10 kb.
• Bacteriophage λ vectors (e.g., λgt10): engineered phage genomes that accept ~20 kb inserts; high-efficiency transduction into E. coli.
• Cosmids: plasmid–cos site hybrids; combine plasmid convenience with phage λ’s packaging signal; accommodate ~40–50 kb inserts.
• Artificial chromosomes (YACs, BACs): YACs (yeast, 100–1000 kb) and BACs (bacteria, 100–300 kb) enable cloning of entire genes or operons for functional genomics.

(b) Applications of cladistics in modern taxonomy are rooted in the principle that shared derived characters (synapomorphies) reflect common ancestry, replacing phenetic similarity with phylogenetic relationships.

• Phylogenetic reconstruction: cladograms built from morphological, molecular (DNA barcoding, 16S rRNA), or combined datasets resolve evolutionary histories; e.g., the avian family tree places falcons closer to parrots than to songbirds.
• Nomenclature reform: the PhyloCode (International Code of Phylogenetic Nomenclature) uses clade names (e.g., Dinosauria, Aves) anchored in cladistic definitions, ensuring stability amid new fossil finds.
• Conservation prioritization: species are ranked by evolutionary distinctiveness (EDGE metrics) to guide resource allocation; the platypus (monotreme lineage) scores high despite low threat status.
• Forensic and epidemiological tracking: SARS-CoV-2 variants (Delta, Omicron) are classified via cladistics to map transmission pathways and inform vaccine updates.

(c) Differentiation between mitosis and meiosis

Parameter	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Sexual reproduction; gamete formation
Divisions	Single division	Two successive divisions (Meiosis I & II)
Daughter cells	Two diploid (2n), genetically identical	Four haploid (n), genetically diverse
Chromosome number	Conserved (2n → 2n)	Reduced (2n → n)
Genetic variation	None (except rare mutation)	Crossing-over, independent assortment, random fertilization
Occurrence	Somatic cells	Germ cells (ovaries, testes)

Conclusion: Recombinant DNA technology has revolutionized medicine and agriculture, while cladistics provides a rigorous framework for taxonomy and conservation. Understanding mitosis versus meiosis underpins advances in reproductive biology and cancer therapeutics, underscoring the need for precise mechanistic knowledge in applied biology.

Q4. (a) Do mitochondria contribute in the process of inheritance of characters? Explain with suitable example. 15 (15 marks)

How to approach this question

The directive word is explain, so the examiner is testing your ability to link mechanism with outcome and provide a concrete example. A top answer must (1) define mitochondrial inheritance, (2) describe the replication and transmission mechanism, and (3) give a real-world human or model-organism example. The common mistake is to equate mitochondrial inheritance with Mendelian nuclear inheritance and overlook the strictly maternal route.

Model answer

Mitochondria contribute to the inheritance of certain characters through a strictly maternal, non-Mendelian pathway called mitochondrial or cytoplasmic inheritance. Unlike nuclear genes, mitochondrial DNA (mtDNA) is transmitted exclusively via the oocyte cytoplasm, so traits encoded by mtDNA appear only when the mother carries the mutation.

The process begins with the semi-conservative replication of the 16.6 kb circular mtDNA molecule inside the mitochondrial matrix. Each daughter mitochondrion receives a random subset of the mother’s mtDNA copies; during oogenesis, this replicative segregation can lead to a mixture of mutant and wild-type genomes (heteroplasmy). When the proportion of mutant mtDNA exceeds a critical threshold in a tissue, bioenergetic failure manifests as a clinical phenotype.

A classic example is Leigh syndrome, caused by the m.8993T>G mutation in the MT-ATP6 gene. Affected mothers, even with low heteroplasmy, transmit the mutation to all offspring; sons do not pass it to the next generation. This pattern—vertical transmission through females only, unaffected males, and variable expressivity—distinguishes mitochondrial inheritance from autosomal dominant traits.

Thus, mitochondria do shape inheritance, but only for a discrete set of energy-related characters via a maternal route, underscoring the need to integrate organelle biology with classical genetics in medical and evolutionary contexts.

Q5. What is nerve impulse? How is the action potential generated and propagated across the nerve membrane? 15 (c) Explain the concept of body axis formation during embryonic development with suitable example. (15 marks)

How to approach this question

The directive word “Explain” asks for a mechanistic account of two distinct phenomena: (i) the generation and propagation of the nerve impulse across the axonal membrane and (ii) the establishment of body axes during embryonic development. Examiners test understanding of membrane electrophysiology (resting potential, action potential, refractory periods, saltatory conduction) and developmental patterning (morphogen gradients, maternal effect genes, Hox genes). A top answer must be structured in two parts—one for each sub-question—with crisp definitions, step-wise events, and one illustrative example for each. The common mistake is to mix the two themes or to omit quantitative values (e.g., –70 mV, +30 mV) and key proteins (voltage-gated Na⁺/K⁺ channels, Na⁺-K⁺ ATPase).

Model answer

Nerve impulse: A nerve impulse is an electrical signal generated and propagated along the axonal membrane of a neuron. It arises from transient changes in membrane permeability to Na⁺ and K⁺ ions driven by selective ion channels and the Na⁺-K⁺ pump.

Generation of action potential:

• Resting membrane potential (RMP): Neuronal membrane is polarized at ≈ –70 mV due to the Na⁺-K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in) and selective permeability of K⁺ leak channels.
• Threshold stimulus: A depolarizing stimulus to ≈ –55 mV opens voltage-gated Na⁺ channels, initiating the “all-or-none” action potential.
• Depolarization: Rapid Na⁺ influx drives membrane potential to +30 mV, reversing polarity.
• Repolarization: Voltage-gated K⁺ channels open; K⁺ efflux restores potential toward –70 mV.
• Hyperpolarization: Excess K⁺ efflux overshoots to ≈ –90 mV.
• Refractory periods: Absolute refractory period (Na⁺ channels inactivated) prevents re-excitation; relative refractory period (hyperpolarized state) requires stronger stimulus.

Propagation: In unmyelinated fibres, continuous conduction occurs; in myelinated fibres, saltatory conduction jumps between nodes of Ranvier, accelerating impulse speed up to 120 m/s.

Body axis formation: During embryogenesis, anterior-posterior (AP) and dorsal-ventral (DV) axes are established by maternal effect genes and zygotic morphogen gradients.

• Drosophila AP axis: Maternal bicoid mRNA localizes at anterior; nanos at posterior. Bicoid protein gradient activates hunchback, defining head structures; nanos represses caudal, restricting posterior fate.
• DV axis in vertebrates: BMP4 gradient from ventral ectoderm induces epidermis; its inhibition by chordin/noggin from dorsal mesoderm permits neural plate formation—Spemann organizer in Xenopus.

Thus, axis formation integrates gene regulatory networks and morphogen gradients to pattern embryonic tissues.

Q6. (a) Name the proteins and their functions involved in skeletal muscle contraction. Unlike skeletal muscles, why do cardiac muscles not fatigue? (b) Describe the mechanism of generation, differentiation and activation of T-cells. (c) Define teratogenesis. Illustrate how teratogens interfere with early embryonic development. (15 marks)

How to approach this question

The directive word “Name…why…Describe…Define…Illustrate” splits the question into four sub-parts that test (i) factual recall of muscle proteins and their roles, (ii) conceptual reasoning on cardiac vs. skeletal muscle physiology, (iii) stepwise immunology of T-cell development, and (iv) developmental biology of teratogenesis. A top answer must organise each part with clear headings, precise protein names/functions, mechanistic steps, and concrete teratogen examples. The common mistake is to mix up actin-myosin-troponin-tropomyosin interactions or to list T-cell activation without the thymic selection stages.

Model answer

(a) Proteins and functions in skeletal muscle contraction

• Actin – Forms the thin filament; provides binding sites for myosin heads during cross-bridge cycling.
• Myosin – Forms the thick filament; ATPase activity hydrolyses ATP to drive power-stroke and filament sliding.
• Troponin complex – TnC binds Ca²⁺, TnI inhibits actin-myosin interaction, TnT anchors troponin to tropomyosin; together they regulate exposure of myosin-binding sites on actin.
• Tropomyosin – In resting state blocks myosin-binding sites on actin; shifts position upon Ca²⁺-troponin binding to permit contraction.

Why cardiac muscle does not fatigue: Cardiac myocytes possess abundant mitochondria for sustained aerobic respiration, prolonged refractory period preventing tetany, and rely on extracellular Ca²⁺ influx (via L-type channels) rather than rapid sarcoplasmic reticulum depletion, thereby avoiding metabolic exhaustion.

(b) Mechanism of T-cell generation, differentiation and activation

• Generation – Haematopoietic stem cells in bone marrow migrate to thymus where they become double-negative (CD4⁻CD8⁻) thymocytes.
• Differentiation – Positive selection in thymic cortex selects thymocytes whose TCRs bind self-MHC (CD4⁺CD8⁺ stage); negative selection in medulla eliminates high-affinity self-reactive clones.
• Activation – Naïve T-cells encounter antigen-loaded dendritic cells in lymph nodes; co-stimulatory signals (CD28-B7) plus cytokine milieu (IL-2) drive clonal expansion and effector differentiation into Th1, Th2, Th17 or cytotoxic T-cells.

(c) Definition and mechanism of teratogenesis

Teratogenesis is the induction of structural or functional birth defects during organogenesis by exogenous agents called teratogens. Interference occurs via:

• Disruption of morphogen gradients – Retinoic acid alters HOX gene expression causing limb truncations (e.g., isotretinoin embryopathy).
• Cell migration defects – Alcohol-induced inhibition of L1CAM-mediated neuronal migration results in agenesis of corpus callosum.
• Cell proliferation arrest – Valproate inhibits histone deacetylases, prolonging cell cycle and causing neural tube defects.
• Extracellular matrix degradation – Cortisone induces cleft palate by reducing hyaluronic acid synthesis in palatal shelves.

Thus, teratogens hijack conserved signalling pathways (Shh, Wnt, BMP) that guide gastrulation and organ patterning, producing dose- and stage-dependent malformations.

Q7. (a) Define apoptosis. Explain the characteristics, molecular pathways and significance of apoptosis. (b) Describe the role of pancreas in blood sugar regulation. (c) Define vitamins. How are vitamins different from co-enzymes in their structure and functions? ★★★ SB27—294 (15 marks)

How to approach this question

The directive word “Define-Explain-Describe” splits the question into three parts: (a) definition plus characteristics, pathways and significance of apoptosis, (b) role of pancreas in glycaemic control, and (c) definition of vitamins and their structural-functional distinction from co-enzymes. Examiners test conceptual clarity, molecular detail, and comparative reasoning. The common mistake is to write generic statements without citing concrete examples or pathways (e.g., caspase cascade, GLUT4 translocation, vitamin functional groups).

Model answer

(a) Apoptosis

Definition: Apoptosis is a genetically programmed, energy-dependent process of controlled cell dismantling that eliminates damaged, infected or superfluous cells without triggering inflammation.

Characteristics: Cell shrinkage, chromatin condensation, membrane blebbing, formation of apoptotic bodies, and rapid phagocytosis by neighbouring cells.

Molecular pathways: (i) Extrinsic—death-receptor mediated (Fas-FasL, TNF-α binding activates caspase-8). (ii) Intrinsic—mitochondrial pathway triggered by p53, Bcl-2 family proteins (Bax/Bak permeabilize outer mitochondrial membrane releasing cytochrome c), apoptosome formation (Apaf-1 + cytochrome c + dATP) activates caspase-9. Both converge on caspase-3 executioner.

Significance: Embryogenesis (digit formation, neural pruning), immune homeostasis (deletion of self-reactive T-cells), tumour suppression (p53-induced apoptosis), and resolution of inflammation; defects cause cancer (Bcl-2 overexpression) or neurodegeneration (excess caspase activity).

(b) Pancreas and blood sugar regulation

The pancreas maintains glycaemia via endocrine islets of Langerhans. β-cells secrete insulin (fed state): promotes GLUT4 translocation in muscle/adipose, stimulates glycogenesis, inhibits gluconeogenesis. α-cells secrete glucagon (fasted state): activates glycogenolysis and gluconeogenesis via cAMP-PKA pathway in liver. Together they form a negative-feedback loop stabilising plasma glucose between 70–140 mg/dL.

(c) Vitamins vs Co-enzymes

Definition: Vitamins are organic micronutrients required in trace amounts that the body cannot synthesise (or synthesises inadequately) and must be obtained from diet.

Structural-functional distinction:

• Vitamins: Often intact organic molecules (e.g., ascorbic acid—vitamin C; cholecalciferol—vitamin D3) that serve as precursors to co-enzymes or act as antioxidants and hormone precursors.
• Co-enzymes: Modified vitamin derivatives (e.g., thiamine pyrophosphate from B1, NAD+ from niacin, FAD from B2) that bind transiently to apo-enzymes, participate directly in catalytic cycles (e.g., NAD+ in redox, TPP in decarboxylation), and are recycled.

Thus, vitamins are dietary inputs; co-enzymes are functional derivatives that directly catalyse biochemical reactions.

Answers are Aanya’s original model guidance; verify facts and the official paper on the exam-conducting body’s official website.

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