NEB Grade XII Biology 2083 (2026) Set P — Answer Key, Important Questions & Free PDF

Biology is the one subject in the NEB Grade XII science stream where the exam paper itself is structured differently from every other subject — and this 2083 Set P paper (Sub.Code 2021’P’) makes that structure unmissable from the very first line: “Please complete first Section I (Botany) and then Section II (Zoology).” Unlike Physics or Chemistry, where a single continuous paper covers the whole syllabus, Biology splits cleanly into two halves, each carrying its own set of MCQs, short-answer questions, and long-answer questions, totaling 75 marks combined. For students, this means effective preparation has to be genuinely bidirectional — strong Botany alone, or strong Zoology alone, will only get you halfway to a good score.

What’s striking about this particular paper is how heavily it leans into applied, scenario-based questions rather than pure definitions. A gardener pinching off an apical bud to encourage lateral branching (Q5), a 17-year-old who can read but can’t see the whiteboard (Q14), a diabetic patient who’s unusually susceptible to candidiasis (Q22), and a question that explicitly asks students to identify respiratory disorders “prevalent in your community” (Q21) — these are not the kind of questions you can answer by reciting a textbook paragraph verbatim. They require connecting a concept (auxin’s role in apical dominance, refractive errors and eyeball shape, immune compromise and opportunistic infections, environmental health) to a real situation, and then explaining the underlying mechanism.

This paper also rewards strong diagram skills more than most. Across both sections, there are at least four questions (Q7, Q10, Q19, Q20, Q23) that explicitly require labeled diagrams — pollen development, the C3 cycle, oogenesis, a digestive/excretory organ figure, and the human brain. If your revision has focused mostly on written answers, working through this paper is a good forcing function to practice drawing biological structures accurately and labeling them correctly under time pressure, which is exactly what the actual board exam demands.

Below, you’ll find the complete MCQ answer key for both Botany and Zoology sections with explanations, a breakdown of the topics covered across both sections, a curated list of important questions for your final exam, FAQs with detailed explanations for the trickier conceptual questions, and the complete downloadable question paper. Whether you’re balancing your revision time between Botany and Zoology, practicing diagram-based answers, or working through the applied/scenario questions that this paper favors, this guide should give you a thorough foundation.

Quick Navigation

• Download Full Question Paper (PDF)
• MCQ Answer Key — Botany and Zoology
• Topics Covered in This Paper
• Important Questions for Your Final Exam
• FAQs with Detailed Explanations
• How to Use This Paper Effectively

View the Full Question Paper

Get the complete NEB Grade XII Biology 2083 Set P paper as a PDF — includes the pollen development figure (Q7), the digestive/excretory system figure (Q20), and all other diagrams referenced in the questions below.

MCQ Answer Key — NEB Biology Grade XII 2083, Set P

Group A carries 11 marks total: 5 marks from Botany (Q1-5) and 6 marks from Zoology (Q12-17), all 1 mark each. The MCQ sheet is provided 30 minutes after the exam begins, as noted on the paper.

Quick explanations for the conceptual ones: Q1 — in grafting terminology, the “stock” is the lower part of the graft that has the root system and provides support, while the “scion” is the upper part grafted onto it that develops into the shoot system. Q4 — “criss-cross inheritance” describes the characteristic zigzag pattern of X-linked recessive traits, where an affected father passes the trait (via the X chromosome) to his daughter (who becomes a carrier, since she also receives a normal X from her mother), and she in turn can pass it to her sons — the trait appears to “cross” from father to daughter to grandson. Q5 — apical dominance is maintained by auxin produced at the shoot tip, which suppresses the growth of lateral (axillary) buds. Removing the apical bud removes this auxin source, releasing lateral buds from suppression and allowing them to grow — hence “reduced auxin allows lateral buds to grow.” Q12 — tendons connect muscle to bone and need to withstand high tensile forces without stretching much, which is achieved through a dense, parallel arrangement of collagen fibres (in contrast to ligaments or elastic structures, which contain more elastin). Q14 — a person who can read (near vision is fine) but struggles to see distant objects (the whiteboard) has myopia (nearsightedness). This occurs when the eyeball is elongated (or the lens is too strong), causing the image of distant objects to focus in front of the retina rather than on it. Q16 — psychotropic drugs affect the mind/mental state; mescaline (a hallucinogen derived from peyote cactus) is a classic psychotropic substance, whereas antihistamines are not primarily psychoactive in the recreational/abuse sense intended here. Q17 — Hepatitis B vaccine is a recombinant DNA vaccine, produced by inserting the gene for the Hepatitis B surface antigen (HBsAg) into yeast cells, which then produce the antigen alone (without any live or whole virus), significantly reducing the risk of adverse reactions compared to traditional vaccines like BCG, OPV, or rabies vaccines.

Topics Covered in This Paper

This Set P paper draws from across both the Botany and Zoology portions of the Grade XII syllabus. Here’s the breakdown:

• Plant Propagation and Grafting (Q1): terminology — stock, scion, propagule, callus.
• Plant Physiology — Osmosis and Water Relations (Q2): plasmolysis, deplasmolysis, endosmosis, imbibition.
• Plant Anatomy (Q3, Q6): leaf tissues (mesophyll, xylem, aerenchyma, sclerenchyma), internal structure of a monocot root.
• Genetics — Inheritance Patterns (Q4, Q8): X-linked inheritance and criss-cross inheritance, chromosomal mutations (aneuploidy vs. euploidy), 2n+1 (trisomy) conditions.
• Plant Hormones (Q5): auxin’s role in apical dominance and lateral bud suppression.
• Reproduction in Plants (Q7): pollen grain development, microspore to pollen tube development, double fertilization-related structures.
• Cellular Respiration (Q8 OR): glycolysis / EMP (Embden-Meyerhof-Parnas) pathway, ATP and NADH yield.
• Biotechnology and Applied Botany (Q9): plant tissue culture — applications, limitations, and implementation strategies for developing countries like Nepal.
• Photosynthesis (Q10): dark reaction / C3 cycle (Calvin cycle), chemical equation of photosynthesis.
• Molecular Biology (Q11): DNA vs. RNA as genetic material, DNA chemical composition, Watson-Crick model, DNA functions.
• Cell Division / Meiosis (Q11 OR): crossing over during prophase I, genetic variation.
• Animal Tissues (Q12, Q18): connective tissues — bone vs. cartilage, tendons and collagen.
• Developmental Biology / Embryology (Q13): frog embryonic development, coelom formation, germ layers.
• Human Eye and Vision Defects (Q14): myopia, hypermetropia, presbyopia and their causes.
• Circulatory System (Q15): coronary circulation, heart chambers.
• Drugs and Addiction (Q16): psychotropic drugs, opiates, hallucinogens.
• Immunology and Vaccines (Q17): recombinant DNA vaccines, types of vaccines (BCG, OPV, Hepatitis B, Rabies).
• Reproductive System (Q19): oogenesis — stages and schematic representation.
• Organ Transplantation (Q19 OR): advantages, disadvantages, ethical and medical considerations.
• Digestive and Endocrine Systems (Q20): pancreas as a mixed gland (exocrine and endocrine functions), liver, gallbladder, sphincter of Oddi at the hepatopancreatic ampulla.
• Respiratory System and Public Health (Q21): respiratory disorders, prevention strategies, environmental and lifestyle factors.
• Microbiology and Infectious Diseases (Q22): candidiasis — causative agent (Candida albicans), transmission, symptoms, prevention, and its association with diabetes.
• Nervous System (Q23): autonomic nervous system (sympathetic vs. parasympathetic), brain anatomy, brain ventricles.
• Endocrine System (Q23 OR): adrenal gland anatomy, adrenal cortex hormones (mineralocorticoids, glucocorticoids, androgens) and associated disorders.

Important Questions for Your Final Exam

These topics from Set P are high-value because they combine definitions with diagrams or applied reasoning — exactly the combination NEB Biology papers consistently reward.

• Internal structure of a monocot root — labeled diagram with all tissue layers
• Development of pollen grain from microspore — stages with labeled diagrams
• Aneuploidy vs. euploidy — definitions and chromosomal mutation types (trisomy, monosomy)
• Glycolysis (EMP pathway) — flow chart of phases, net ATP and NADH yield per glucose molecule
• Plant tissue culture — applications, limitations, and strategies for developing countries
• C3 cycle (Calvin cycle) — three major phases with flow chart, chemical equation of photosynthesis
• DNA vs. RNA as genetic material — comparative justification
• DNA structure — chemical composition, Watson-Crick double helix model with labeled diagram, functions
• Crossing over during prophase I of meiosis — labeled diagrams and effect on genetic variation
• Bone vs. cartilage — structural and functional comparison
• Oogenesis — stages with schematic diagram
• Advantages and disadvantages of organ transplantation
• Pancreas as a mixed gland — exocrine and endocrine functions, sphincter of Oddi
• Common respiratory disorders — causes, symptoms, and prevention methods
• Candidiasis — causative agent, transmission, symptoms, prevention, and link to diabetes
• Sympathetic vs. parasympathetic nervous system — functional differences
• Human brain — labeled diagram and brain ventricles
• Adrenal cortex — hormone groups, specific functions, and associated disorders (e.g., Addison’s disease, Cushing’s syndrome)
• Apical dominance and the role of auxin in plant growth regulation
• X-linked inheritance and criss-cross inheritance patterns
• Myopia, hypermetropia, and presbyopia — causes and corrective measures
• Plasmolysis and deplasmolysis — cellular water relations

Frequently Asked Questions

What is the difference between the “stock” and “scion” in grafting (Q1)?

In grafting, two plant parts are joined to grow as one plant. The stock (also called rootstock) is the lower portion that includes the root system and provides the foundation — it’s the part already established in the soil. The scion is the upper portion, a cutting from a different plant, that is attached to the stock and develops into the shoot system (stems, leaves, flowers, fruits) of the resulting grafted plant. The stock provides water and nutrient uptake, while the scion determines the variety of fruit, flower, or foliage produced.

For Q4, why is “criss-cross inheritance” the correct term and not “Y-linked inheritance”?

Y-linked inheritance refers to traits carried on the Y chromosome, passed directly from father to son (since only males have a Y chromosome) — it never passes through daughters at all. The scenario described — father to grandson through a normal carrier daughter — is the hallmark of X-linked recessive inheritance, where the pattern is called “criss-cross” because the trait appears to zigzag: an affected father passes his single X chromosome (carrying the recessive allele) to all his daughters, making them carriers (since they have a normal X from their mother too). These carrier daughters can then pass the recessive X to their sons, who — having only one X chromosome — will express the trait. The “criss-cross” pattern describes this father-to-daughter-to-grandson trajectory, which is characteristic of conditions like color blindness and hemophilia.

How do you differentiate between aneuploidy and euploidy in Q8?

Euploidy refers to a condition where an organism has an exact multiple of the normal haploid chromosome number for its species (e.g., diploid 2n, triploid 3n, tetraploid 4n — all are euploid conditions, even if they differ from the typical 2n). Aneuploidy, by contrast, refers to a condition where the chromosome number deviates from an exact multiple of the haploid number — typically by the gain or loss of one or a few individual chromosomes, not whole sets. In this question, a plant with 2n=14 normally would be euploid at 2n=14, but a mutant with 2n+1=15 chromosomes has gained a single extra chromosome (not a whole additional set) — this is trisomy, a specific type of aneuploidy. The key distinguishing principle: euploidy involves changes in whole chromosome sets (multiples of n), while aneuploidy involves changes in individual chromosome numbers that don’t correspond to whole-set multiples.

What is the net ATP and NADH yield from glycolysis (Q8 OR)?

Glycolysis (the EMP pathway) converts one molecule of glucose into two molecules of pyruvate. The pathway has two phases: an energy investment phase, where 2 ATP are consumed (used to phosphorylate glucose intermediates), and an energy payoff phase, where 4 ATP and 2 NADH are produced. The net gain is therefore: ATP = 4 produced − 2 consumed = 2 ATP (net), and NADH = 2 NADH (no NADH is consumed in glycolysis, so this is also the net figure) — both per molecule of glucose. The major phases in a flow chart would be: (1) glucose phosphorylation and splitting into two G3P molecules (energy investment, −2 ATP), (2) oxidation of G3P to pyruvate with ATP and NADH generation (energy payoff, +4 ATP, +2 NADH), giving a net of +2 ATP and +2 NADH per glucose.

What are the main limitations of plant tissue culture in developing countries like Nepal (Q9)?

The primary limitations include: high initial setup costs for sterile laboratory facilities, specialized equipment (laminar flow hoods, autoclaves, growth chambers), and culture media chemicals, which are often imported and expensive; the need for skilled, trained personnel familiar with aseptic techniques; high energy costs for maintaining controlled temperature and lighting conditions for plant growth chambers; and the risk of contamination, which can result in significant losses of time and resources. For effective implementation in Nepal, suitable measures could include establishing centralized regional tissue culture laboratories that serve multiple farming communities (reducing per-farmer costs), government subsidies or partnerships with agricultural universities to reduce equipment and training costs, prioritizing tissue culture for high-value cash crops (like potato, banana, or orchids) where the investment yields clear economic returns, and training local technicians through partnerships with international agricultural research organizations to reduce long-term dependency on foreign expertise.

What are the three major steps of the C3 cycle (Calvin cycle) in Q10?

The C3 cycle (named because the first stable product is a 3-carbon compound) occurs in the stroma of the chloroplast and has three major phases: (1) Carboxylation — CO2 combines with RuBP (ribulose bisphosphate, a 5-carbon molecule) in a reaction catalyzed by the enzyme RuBisCO, forming an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound. (2) Reduction — the 3-PGA molecules are reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH (produced in the light reactions). Some G3P molecules exit the cycle to be used in glucose synthesis. (3) Regeneration — the remaining G3P molecules are used, with additional ATP, to regenerate RuBP, allowing the cycle to continue. The overall chemical equation of photosynthesis is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2.

Why is DNA considered the universal genetic material while RNA generally is not (Q11)?

Several properties make DNA better suited as the primary genetic material: DNA is chemically more stable than RNA, primarily because DNA contains deoxyribose (lacking the reactive 2′-OH group present in ribose) and uses thymine instead of uracil, both of which make DNA less prone to hydrolysis and chemical degradation. DNA’s double-stranded structure provides a built-in template for accurate self-repair and replication — if one strand is damaged, the complementary strand can serve as a template for repair, something single-stranded RNA generally cannot do. DNA also has lower mutation rates due to this stability and repair capacity, which is important for an organism’s genetic material to remain consistent across generations. RNA, being chemically less stable and typically single-stranded, is well-suited for its roles as a temporary messenger (mRNA), structural/catalytic molecule (rRNA, tRNA), but its instability makes it unsuitable as a long-term repository of genetic information — though some viruses do use RNA as their genetic material, accepting higher mutation rates as a trade-off (which can actually be advantageous for rapid viral evolution).

How does the absence of crossing over affect genetic variation (Q11 OR)?

Crossing over during prophase I of meiosis I involves the exchange of genetic segments between homologous chromosomes, creating new combinations of alleles on each chromosome (recombinant chromosomes) that differ from either parental chromosome. If crossing over did not occur, the chromosomes passed to gametes would remain exactly as inherited from each parent — only the existing combinations of alleles (as they were originally arranged on each parental chromosome) could be passed on. This would significantly reduce genetic variation among offspring, since the only source of variation would then be the independent assortment of whole chromosomes during meiosis (which chromosome from each homologous pair ends up in which gamete), not the shuffling of individual genes along each chromosome. Over generations, reduced genetic variation would limit a population’s capacity to adapt to changing environments and could increase the prevalence of harmful allele combinations remaining linked together.

How do bone and cartilage compare in Q18?

Bone is a hard, rigid connective tissue composed of a mineralized matrix (primarily calcium phosphate and calcium carbonate, giving it hardness) embedded with collagen fibers, organized into a system of osteons (Haversian systems) containing osteocytes in lacunae. Bone is highly vascular (has blood vessels) and capable of self-repair and remodeling throughout life. Cartilage, by contrast, is a softer, more flexible connective tissue with a matrix made primarily of chondroitin sulfate and collagen/elastin fibers, containing chondrocytes in lacunae. Cartilage is avascular (lacks blood vessels), receiving nutrients by diffusion, which makes it heal much more slowly than bone. Functionally, bone provides structural support, protection for organs, and serves as a lever system for movement, while cartilage provides flexible support, cushioning at joints, and forms the template for bone development during embryonic growth (endochondral ossification).

Why is the pancreas called a “mixed gland” (Q20ii)?

The pancreas is termed a “mixed gland” because it has both exocrine and endocrine functions. Its exocrine portion (the bulk of the pancreas, organized into acini) secretes digestive enzymes (such as trypsin, lipase, and amylase) and bicarbonate into the pancreatic duct, which empties into the duodenum to aid digestion — this is the exocrine function, where secretions are released into a duct system. Its endocrine portion, the islets of Langerhans, scattered throughout the pancreas, secrete hormones (insulin from beta cells, glucagon from alpha cells) directly into the bloodstream to regulate blood glucose levels — this is the endocrine function, where secretions go directly into the blood rather than through a duct. Having both types of secretion within the same organ is what earns it the “mixed gland” designation.

What sphincter is located at the hepatopancreatic ampulla (Ampulla of Vater) in Q20iv?

The sphincter of Oddi (also called the hepatopancreatic sphincter) is located at the Ampulla of Vater, where the common bile duct (from the liver and gallbladder) and the main pancreatic duct join before emptying into the duodenum. This sphincter regulates the flow of bile and pancreatic digestive juices into the small intestine, opening in response to hormonal signals (such as cholecystokinin, released when fatty food enters the duodenum) and remaining closed between meals to allow bile to be stored and concentrated in the gallbladder.

Why is candidiasis particularly common in diabetic patients (Q22)?

Candidiasis is caused by the yeast-like fungus Candida albicans, which is normally present in small amounts in the body (mouth, gut, skin, vaginal tract) without causing disease, kept in check by the immune system and competing normal flora. In diabetic patients, elevated blood glucose levels create a more favorable environment for Candida growth — glucose is a readily available nutrient source for the fungus, and high sugar levels in bodily secretions (such as urine, sweat, and mucosal surfaces) promote fungal proliferation. Additionally, diabetes can impair immune function (particularly neutrophil activity), reducing the body’s ability to control fungal overgrowth. The combination of an enriched nutrient environment and weakened immune defenses makes diabetic patients significantly more susceptible to candidiasis, which commonly manifests as oral thrush, vaginal yeast infections, or skin fold infections in this population.

How do sympathetic and parasympathetic nervous systems differ in Q23?

The sympathetic nervous system generally prepares the body for “fight or flight” responses — it increases heart rate, dilates pupils, redirects blood flow toward skeletal muscles and away from digestive organs, increases respiratory rate, and inhibits digestive processes. Its neurons originate from the thoracic and lumbar regions of the spinal cord, and it primarily uses norepinephrine as its neurotransmitter at target organs. The parasympathetic nervous system, by contrast, promotes “rest and digest” functions — it decreases heart rate, constricts pupils, stimulates digestive secretions and gut motility, and generally conserves energy. Its neurons originate from the brainstem (cranial nerves, especially the vagus nerve) and the sacral region of the spinal cord, and it primarily uses acetylcholine as its neurotransmitter at target organs. The two systems typically have opposing (antagonistic) effects on the same organs, allowing fine-tuned control of involuntary bodily functions depending on the body’s needs at any given moment.

What are the brain ventricles, as asked in Q23?

The brain ventricles are a series of interconnected, fluid-filled cavities within the brain that produce and circulate cerebrospinal fluid (CSF). There are four ventricles: two lateral ventricles (one in each cerebral hemisphere, the largest of the four), the third ventricle (located centrally, between the thalami, connected to the lateral ventricles via the interventricular foramina), and the fourth ventricle (located between the brainstem and cerebellum, connected to the third ventricle via the cerebral aqueduct). CSF produced by the choroid plexus within these ventricles cushions the brain and spinal cord, removes metabolic waste, and helps maintain stable intracranial pressure; it eventually drains into the subarachnoid space surrounding the brain and spinal cord.

What hormone groups does the adrenal cortex secrete, and what disorders are associated with them (Q23 OR)?

The adrenal cortex secretes three groups of steroid hormones, organized into three zones: the outer zona glomerulosa secretes mineralocorticoids (primarily aldosterone), which regulate sodium and potassium balance and blood pressure by acting on the kidneys; the middle zona fasciculata secretes glucocorticoids (primarily cortisol), which regulate glucose metabolism, suppress inflammation, and help the body respond to stress; and the inner zona reticularis secretes small amounts of gonadocorticoids (androgens), which contribute to secondary sexual characteristics. Disorders associated with these hormones include: Addison’s disease (adrenal insufficiency — underproduction of cortisol and aldosterone, causing fatigue, low blood pressure, and weight loss), Cushing’s syndrome (overproduction of cortisol, causing weight gain, particularly around the face and trunk, high blood pressure, and muscle weakness), and Conn’s syndrome (primary hyperaldosteronism — overproduction of aldosterone, causing high blood pressure and low potassium levels).

How to Use This Paper Effectively

Given the strict Botany-then-Zoology split in this paper, simulate it during practice: complete Section I (Botany) entirely — including its own MCQs, short answers, and long answers — before moving to Section II (Zoology), exactly as the instructions require (“Please complete first Section I and then Section II”). This isn’t just exam formality; it reflects how your time management needs to work, since each section has its own internal mix of question types and mark weightings.

The diagram-heavy questions (Q7, Q10, Q19, Q20, Q23) deserve dedicated practice time separate from written-answer revision. For each, practice drawing the diagram from memory, labeling every part correctly, and then writing the accompanying explanation — many students can describe a process in words but struggle to produce an accurate, properly labeled diagram under exam time pressure, and NEB allocates real marks specifically for diagram accuracy and labeling.

Finally, the applied/scenario questions (Q5, Q9, Q14, Q21, Q22) reward being able to connect a named concept to a real situation and explain the underlying mechanism in 2-4 sentences. When revising, don’t just memorize “auxin causes apical dominance” — practice explaining why removing the apical bud changes the hormonal balance and what effect that has, in your own words. This kind of mechanistic explanation is precisely what distinguishes a 4/4 answer from a 2/4 answer on questions like these.