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Question 1Biology (syllabus up to Class 12 CBSE India)

In the human heart, which structure directly prevents the backflow of blood from the left ventricle into the left atrium during ventricular systole?

  1. The bicuspid (mitral) valve Correct answer.
  2. The tricuspid valve
  3. The semilunar valve of the aorta
  4. The valve of the inferior vena cava

Explanation

The left atrium and left ventricle are separated by the left atrioventricular valve, also called the bicuspid or mitral valve. During ventricular systole the rising pressure closes this valve, preventing blood from re-entering the atrium. The tricuspid valve guards the right side, and the aortic semilunar valve prevents backflow from the aorta into the ventricle.

Question 2Biology (syllabus up to Class 12 CBSE India)

Which enzyme is responsible for unwinding the DNA double helix ahead of the replication fork during DNA replication?

  1. DNA polymerase III
  2. DNA ligase
  3. Helicase Correct answer.
  4. Primase

Explanation

Helicase hydrolyses the hydrogen bonds between complementary base pairs, separating the two strands so that each can act as a template. DNA polymerase adds nucleotides to a primer, primase synthesises the RNA primer, and ligase joins Okazaki fragments. Unwinding specifically is the job of helicase.

Question 3Biology (syllabus up to Class 12 CBSE India)

A plant cell is placed in a hypotonic solution. Which outcome is expected?

  1. The cell becomes turgid as water enters and turgor pressure rises Correct answer.
  2. The cell bursts immediately because it lacks a cell wall
  3. The cell undergoes plasmolysis as the protoplast pulls away from the wall
  4. The cell shrinks because water leaves the vacuole

Explanation

In a hypotonic solution the external solute concentration is lower than that inside the cell, so water moves in by osmosis. The rigid cellulose wall resists expansion, so internal turgor pressure rises and the cell becomes turgid rather than bursting. Plasmolysis and shrinkage occur in hypertonic solutions.

Question 4Biology (syllabus up to Class 12 CBSE India)

Which blood cells are primarily responsible for cell-mediated immunity in humans?

  1. T lymphocytes Correct answer.
  2. Platelets
  3. B lymphocytes
  4. Erythrocytes

Explanation

Cell-mediated immunity depends on T lymphocytes, which directly attack infected or abnormal cells and coordinate other immune responses. B lymphocytes produce antibodies and mediate humoral immunity. Erythrocytes transport oxygen and platelets aid clotting, so neither participates in specific immunity.

Question 5Biology (syllabus up to Class 12 CBSE India)

In a dihybrid cross between two heterozygotes (RrYy × RrYy) with independent assortment, what fraction of the offspring is expected to show the recessive phenotype for both traits?

  1. 9/16
  2. 1/4
  3. 1/16 Correct answer.
  4. 3/16

Explanation

Each trait behaves as a monohybrid cross, giving a 3:1 ratio, so the probability of the recessive phenotype for one trait is 1/4. For two independent traits the probabilities multiply: 1/4 × 1/4 = 1/16. This corresponds to the rryy class in the classic 9:3:3:1 ratio.

Question 6Biology (syllabus up to Class 12 CBSE India)

Which part of the nephron is the principal site of glucose reabsorption from the filtrate?

  1. The proximal convoluted tubule Correct answer.
  2. The collecting duct
  3. The descending limb of the loop of Henle
  4. The distal convoluted tubule

Explanation

The proximal convoluted tubule reabsorbs almost all glucose, amino acids and much of the sodium and water from the filtrate. Its cells have abundant microvilli and mitochondria to support active transport. The loop of Henle, distal tubule and collecting duct handle water and ion balance rather than glucose recovery.

Question 7Biology (syllabus up to Class 12 CBSE India)

Which of the following is a characteristic feature of gymnosperms that distinguishes them from angiosperms?

  1. They possess seeds enclosed within a fruit
  2. They produce flowers with a perianth
  3. They have a double fertilisation mechanism
  4. They bear naked seeds on megasporophylls Correct answer.

Explanation

The defining feature of gymnosperms is that their ovules are not enclosed in an ovary, so the resulting seeds remain naked on the megasporophyll or cone scale. Angiosperms enclose seeds within fruits and show double fertilisation. Gymnosperms also lack true flowers with a perianth.

Question 8Biology (syllabus up to Class 12 CBSE India)

During glycolysis, one molecule of glucose is converted into how many net molecules of ATP and pyruvate?

  1. 4 ATP and 2 pyruvate
  2. 2 ATP and 1 pyruvate
  3. 4 ATP and 4 pyruvate
  4. 2 ATP and 2 pyruvate Correct answer.

Explanation

Glycolysis yields 4 ATP by substrate-level phosphorylation but consumes 2 ATP during the preparatory phase, giving a net gain of 2 ATP. One glucose is split and oxidised to give 2 molecules of pyruvate. The gross figure of 4 ATP ignores the investment phase.

Question 9Biology (syllabus up to Class 12 CBSE India)

A person has blood group AB. Which statement about that person's red blood cells and plasma is correct?

  1. The red cells carry antigen B only and the plasma contains anti-A
  2. The red cells carry no antigens and the plasma contains both anti-A and anti-B
  3. The red cells carry both A and B antigens and the plasma contains neither antibody Correct answer.
  4. The red cells carry antigen A only and the plasma contains anti-B

Explanation

AB individuals inherit both alleles, so their red cells display both A and B antigens. Because both antigens are recognised as self, the plasma contains neither anti-A nor anti-B antibody. A and B individuals carry one antigen and the opposite antibody.

Question 10Biology (syllabus up to Class 12 CBSE India)

In the light reactions of photosynthesis, the splitting of water during non-cyclic photophosphorylation directly provides which products?

  1. Glucose and oxygen
  2. Electrons, protons and oxygen Correct answer.
  3. NADPH and carbon dioxide
  4. ATP and NADPH

Explanation

Photolysis of water at photosystem II supplies electrons to replace those lost by P680, releases protons into the thylakoid lumen, and liberates molecular oxygen. The ATP and NADPH used in the Calvin cycle are produced by the electron transport chain, not by water splitting itself. Glucose is formed later in the dark reactions.

Question 11Biology (syllabus up to Class 12 CBSE India)

In a population at Hardy–Weinberg equilibrium, the frequency of a recessive allele is 0.2. What proportion of the population is expected to be heterozygous?

  1. 0.32 Correct answer.
  2. 0.64
  3. 0.04
  4. 0.16

Explanation

If q = 0.2, then p = 0.8. The heterozygous frequency is 2pq = 2 × 0.8 × 0.2 = 0.32. The homozygous recessive frequency q² is 0.04, and the homozygous dominant p² is 0.64.

Question 12Biology (syllabus up to Class 12 CBSE India)

Which hormone triggers ovulation in the human menstrual cycle and is also detected by ovulation predictor kits?

  1. Luteinising hormone Correct answer.
  2. Prolactin
  3. Follicle-stimulating hormone
  4. Progesterone

Explanation

A surge in luteinising hormone around mid-cycle causes the mature Graafian follicle to rupture and release the oocyte. Ovulation predictor kits detect this LH surge in urine. Progesterone supports the luteal phase, and FSH mainly stimulates follicular growth.

Question 13Biology (syllabus up to Class 12 CBSE India)

A mutation changes a single nucleotide in a codon but the amino acid incorporated into the protein remains unchanged. What is this type of mutation called?

  1. Silent mutation Correct answer.
  2. Frameshift mutation
  3. Missense mutation
  4. Nonsense mutation

Explanation

A silent mutation alters the nucleotide sequence but not the encoded amino acid, exploiting the degeneracy of the genetic code. A missense mutation changes one amino acid, a nonsense mutation creates a stop codon, and a frameshift shifts the reading frame.

Question 14Biology (syllabus up to Class 12 CBSE India)

Which structural adaptation of the mammalian small intestine most directly increases the surface area for absorption?

  1. The secretion of hydrochloric acid in the stomach
  2. The presence of villi and microvilli on the mucosal lining Correct answer.
  3. The thick muscular wall of the colon
  4. The presence of bile salts in the duodenum

Explanation

Villi are finger-like projections of the intestinal mucosa, and microvilli are further folds of the epithelial cell membrane. Together they multiply the absorptive surface enormously. Bile salts emulsify fats but do not enlarge the surface area of the gut lining.

Question 15Biology (syllabus up to Class 12 CBSE India)

In the nitrogen cycle, which group of microorganisms converts ammonia into nitrite and then into nitrate?

  1. Nitrogen-fixing bacteria such as Rhizobium
  2. Denitrifying bacteria such as Pseudomonas
  3. Nitrifying bacteria such as Nitrosomonas and Nitrobacter Correct answer.
  4. Ammonifying fungi

Explanation

Nitrification is a two-step aerobic process: Nitrosomonas oxidises ammonia to nitrite, and Nitrobacter oxidises nitrite to nitrate. Rhizobium fixes atmospheric nitrogen, while denitrifiers reduce nitrate back to nitrogen gas. Ammonifying organisms release ammonia from organic matter.

Question 16Biology (syllabus up to Class 12 CBSE India)

Which type of muscle tissue is involuntary, striated and found only in the wall of the heart?

  1. Areolar connective tissue
  2. Smooth muscle
  3. Skeletal muscle
  4. Cardiac muscle Correct answer.

Explanation

Cardiac muscle is striated because it has organised sarcomeres, yet it contracts involuntarily and is confined to the heart wall. Skeletal muscle is striated but voluntary, and smooth muscle is involuntary but non-striated. Areolar tissue is connective, not contractile.

Question 17Biology (syllabus up to Class 12 CBSE India)

A student crosses a red-flowered plant with a white-flowered plant and all the F1 offspring have pink flowers. When the F1 plants are self-pollinated, the F2 shows red, pink and white in a 1:2:1 ratio. Which phenomenon explains these results?

  1. Complete dominance with a 3:1 ratio
  2. Incomplete dominance Correct answer.
  3. Pleiotropy
  4. Codominance with both parental phenotypes expressed

Explanation

In incomplete dominance the heterozygote shows an intermediate phenotype, so F1 is pink and the F2 segregates 1 red : 2 pink : 1 white. Complete dominance would give a 3:1 phenotypic ratio with no intermediate. Codominance would express both parental colours distinctly, not a blend.

Question 18Biology (syllabus up to Class 12 CBSE India)

Which statement best explains why the oxygen dissociation curve of haemoglobin shifts to the right when carbon dioxide concentration rises?

  1. Carbon dioxide converts haemoglobin into methaemoglobin
  2. Carbon dioxide lowers blood pH, reducing haemoglobin's affinity for oxygen Correct answer.
  3. Carbon dioxide binds to haem and permanently blocks oxygen binding
  4. Carbon dioxide increases the affinity of haemoglobin for oxygen

Explanation

Carbon dioxide forms carbonic acid, lowering pH and increasing hydrogen ion concentration. This acidic shift reduces haemoglobin's affinity for oxygen so it releases oxygen more readily to active tissues, an effect known as the Bohr shift. It does not permanently block or chemically convert haemoglobin.

Question 19Biology (syllabus up to Class 12 CBSE India)

In a food chain, if the energy available at the producer level is 10,000 kJ, approximately how much energy is transferred to the tertiary consumer?

  1. 1000 kJ
  2. 1 kJ
  3. 10 kJ Correct answer.
  4. 100 kJ

Explanation

Roughly 10 percent of energy passes to each successive trophic level. Producers hold 10,000 kJ, primary consumers about 1,000 kJ, secondary consumers about 100 kJ, and tertiary consumers about 10 kJ. The remaining energy is lost mainly as heat during respiration.

Question 20Biology (syllabus up to Class 12 CBSE India)

Which feature of the alveoli is most important for efficient gas exchange?

  1. A large surface area combined with a thin, moist wall and rich capillary network Correct answer.
  2. A high concentration of mucus-secreting cells
  3. Cartilage rings supporting the alveolar walls
  4. A thick, moist epithelial lining

Explanation

Alveoli provide an enormous total surface area with walls only one cell thick, kept moist and surrounded by dense capillaries. These features maximise diffusion of oxygen and carbon dioxide. Cartilage rings support bronchi, not alveoli, and a thick wall would impede diffusion.

Question 21Biology (syllabus up to Class 12 CBSE India)

A patient's urine contains large amounts of glucose despite normal blood glucose levels. Which defect in the nephron is most likely responsible?

  1. Increased filtration pressure in the glomerulus
  2. Failure of the proximal tubule to reabsorb glucose Correct answer.
  3. Damage to the loop of Henle's water permeability
  4. Excess secretion of antidiuretic hormone

Explanation

Glucose normally disappears from the filtrate because the proximal convoluted tubule reabsorbs it by active transport. If this transport fails, glucose escapes into the urine even when blood levels are normal, a renal glycosuria. The other options affect water balance rather than glucose recovery.

Question 22Biology (syllabus up to Class 12 CBSE India)

In a cross between a heterozygous black guinea pig (Bb) and a white guinea pig (bb), what proportion of the offspring is expected to be black, assuming B is completely dominant?

  1. All offspring
  2. Half the offspring Correct answer.
  3. Three quarters of the offspring
  4. One quarter of the offspring

Explanation

The heterozygote Bb produces gametes B and b in equal numbers, while bb produces only b gametes. The offspring are therefore 50 percent Bb (black) and 50 percent bb (white). A test cross with a recessive parent always gives a 1:1 ratio when the dominant parent is heterozygous.

Question 23Biology (syllabus up to Class 12 CBSE India)

Which combination of events correctly describes the sequence in meiosis that generates genetic variation?

  1. DNA replication in prophase II followed by separation of sister chromatids in anaphase I
  2. Independent assortment in prophase II followed by crossing over in metaphase II
  3. Separation of homologues in anaphase II followed by cytokinesis without prior replication
  4. Crossing over in prophase I followed by independent assortment of homologues in metaphase I Correct answer.

Explanation

Crossing over between non-sister chromatids occurs during pachytene of prophase I, and the random orientation of homologous pairs at metaphase I produces independent assortment. Together these reshuffle alleles. DNA replicates before meiosis I, not in prophase II, and homologues separate in anaphase I rather than anaphase II.

Question 24Biology (syllabus up to Class 12 CBSE India)

A bacterial culture is shifted from 37 °C to 25 °C. Which immediate change in membrane lipid composition would help maintain fluidity?

  1. Complete removal of cholesterol from the membrane
  2. Increased proportion of saturated fatty acids in the phospholipids
  3. Increased proportion of unsaturated fatty acids in the phospholipids Correct answer.
  4. Increased length of the fatty acid tails

Explanation

Lower temperature tends to rigidify membranes, so cells compensate by raising the proportion of unsaturated fatty acids, whose kinked tails pack less tightly and preserve fluidity. Saturated tails and longer chains pack more tightly and reduce fluidity further. Bacteria generally lack cholesterol in their membranes.

Question 25Biology (syllabus up to Class 12 CBSE India)

In an experiment, a plant is supplied with radioactive carbon dioxide, and radioactivity is first detected in a three-carbon compound before appearing in glucose. Which conclusion is best supported?

  1. The plant is fixing carbon by the C4 pathway exclusively
  2. The radioactive carbon is incorporated into ATP during photophosphorylation
  3. The first stable product of carbon fixation is a three-carbon compound, consistent with the Calvin cycle Correct answer.
  4. Carbon dioxide is reduced directly to glucose without intermediates

Explanation

In the Calvin cycle, carbon dioxide combines with ribulose bisphosphate to give an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate, a three-carbon compound. Detection of radioactivity first in a C3 compound supports this pathway. C4 plants initially form a four-carbon acid such as oxaloacetate.

Question 26Biology (syllabus up to Class 12 CBSE India)

Which polysaccharide is the principal storage carbohydrate of plants, and is composed of amylose and amylopectin?

  1. Chitin
  2. Glycogen
  3. Cellulose
  4. Starch Correct answer.

Explanation

Starch is the main storage carbohydrate of plants, stored as granules in tissues such as seeds and tubers. It consists of two polymers: unbranched amylose and branched amylopectin, both made of alpha-glucose. Glycogen is the animal storage form and cellulose is structural.

Question 27Biology (syllabus up to Class 12 CBSE India)

Which enzyme unwinds the double helix ahead of the replication fork during DNA replication?

  1. DNA helicase Correct answer.
  2. DNA ligase
  3. Primase
  4. DNA polymerase III

Explanation

DNA helicase breaks the hydrogen bonds between complementary base pairs, separating the two strands so that each can act as a template. This creates the replication fork. Ligase joins fragments, primase lays RNA primers, and polymerase extends new strands.

Question 28Biology (syllabus up to Class 12 CBSE India)

Which blood vessel carries deoxygenated blood from the heart to the lungs?

  1. Pulmonary vein
  2. Coronary artery
  3. Pulmonary artery Correct answer.
  4. Aorta

Explanation

The pulmonary artery leaves the right ventricle and carries deoxygenated blood to the lungs, making it the standard exception to the rule that arteries carry oxygenated blood. The pulmonary vein returns oxygenated blood from the lungs to the left atrium.

Question 29Biology (syllabus up to Class 12 CBSE India)

Which plant tissue is responsible for the transport of water and dissolved minerals from roots to shoots?

  1. Xylem Correct answer.
  2. Cortex
  3. Epidermis
  4. Phloem

Explanation

Xylem conducts water and dissolved minerals upward from the roots, driven mainly by transpiration pull, and its vessel elements and tracheids are dead at maturity. Phloem transports sugars and other organic solutes in both directions.

Question 30Biology (syllabus up to Class 12 CBSE India)

Which vitamin deficiency is most directly associated with prolonged bleeding and delayed clotting?

  1. Vitamin A
  2. Vitamin C
  3. Vitamin D
  4. Vitamin K Correct answer.

Explanation

Vitamin K is a cofactor for the synthesis of several clotting factors, including prothrombin, so its deficiency impairs coagulation and prolongs bleeding time. Vitamins A, C and D have other primary roles.

Question 31Biology (syllabus up to Class 12 CBSE India)

Which structure in a eukaryotic cell is the site of synthesis of ribosomal subunits?

  1. Smooth endoplasmic reticulum
  2. Nucleolus Correct answer.
  3. Lysosome
  4. Golgi apparatus

Explanation

The nucleolus is a dense region within the nucleus where ribosomal RNA is transcribed and assembled with proteins into ribosomal subunits. These subunits then pass into the cytoplasm for assembly into functional ribosomes.

Question 32Biology (syllabus up to Class 12 CBSE India)

Which plant hormone is chiefly responsible for promoting cell elongation and producing apical dominance?

  1. Cytokinin
  2. Ethylene
  3. Abscisic acid
  4. Auxin Correct answer.

Explanation

Auxin, mainly indole-3-acetic acid, promotes cell elongation and, when produced at the shoot apex, suppresses the growth of lateral buds, producing apical dominance. Abscisic acid and ethylene are generally inhibitory in these contexts.

Question 33Biology (syllabus up to Class 12 CBSE India)

Which group of organisms is characterised by a body cavity that is not lined by mesoderm-derived peritoneum?

  1. Pseudocoelomates Correct answer.
  2. Coelomates
  3. Acoelomates
  4. Deuterostomes

Explanation

In pseudocoelomates such as roundworms, the body cavity is a blastocoel remnant and is not fully lined by mesoderm, in contrast to true coelomates where the cavity is lined by mesodermal peritoneum. Acoelomates lack a body cavity altogether.

Question 34Biology (syllabus up to Class 12 CBSE India)

A cell with 2n = 24 chromosomes undergoes meiosis. What number of chromosomes is present in each daughter nucleus after meiosis I, and after meiosis II?

  1. 12 and 6
  2. 24 and 24
  3. 12 and 12 Correct answer.
  4. 24 and 12

Explanation

Meiosis I is reductional: homologous chromosomes separate, so each daughter nucleus receives 12 chromosomes, each still consisting of two chromatids. Meiosis II is equational and separates sister chromatids, so each of the four products still has 12 chromosomes, now single-chromatid.

Question 35Biology (syllabus up to Class 12 CBSE India)

In a dihybrid cross between two plants heterozygous for both genes, the observed F2 ratio deviates from 9:3:3:1 to 9:7. Which interaction best explains this result?

  1. Pleiotropy of one gene
  2. Incomplete dominance at one locus
  3. Codominance at one locus
  4. Complementary gene action Correct answer.

Explanation

A 9:7 ratio arises when dominant alleles at two different loci are both required to produce the phenotype, so all genotypes lacking a dominant allele at either locus share the mutant phenotype. This is complementary gene action, a form of epistasis.

Question 36Biology (syllabus up to Class 12 CBSE India)

A solution containing 0.9% NaCl is separated from distilled water by a membrane permeable only to water. What will be observed over time?

  1. Equal net movement of water in both directions with no change in volume
  2. Net movement of water into the distilled water until the salt solution becomes concentrated
  3. Net movement of NaCl across the membrane into the distilled water
  4. Net movement of water into the salt solution until osmotic pressure balances Correct answer.

Explanation

Water moves by osmosis from the region of higher water potential, distilled water, toward the region of lower water potential, the salt solution. The net flow continues until the hydrostatic pressure developed balances the osmotic pressure difference.

Question 37Biology (syllabus up to Class 12 CBSE India)

A person hyperventilates voluntarily for one minute. Which immediate change in blood chemistry is expected?

  1. Decreased arterial oxygen and increased pH
  2. Decreased arterial carbon dioxide and increased pH Correct answer.
  3. Increased arterial oxygen and decreased pH
  4. Increased arterial carbon dioxide and decreased pH

Explanation

Hyperventilation washes out carbon dioxide faster than it is produced, lowering arterial PCO2. Because CO2 forms carbonic acid, reduced CO2 raises blood pH, producing respiratory alkalosis.

Question 38Biology (syllabus up to Class 12 CBSE India)

Which structural feature of the mammalian nephron allows the loop of Henle to create a concentration gradient in the medulla?

  1. The descending limb is permeable to water and the ascending limb actively transports salts Correct answer.
  2. Both limbs are equally permeable to water and salts
  3. Both limbs actively secrete glucose into the interstitium
  4. The ascending limb is permeable to water and the descending limb is impermeable to water

Explanation

The descending limb is permeable to water but relatively impermeable to solutes, so water leaves as filtrate descends. The thick ascending limb is impermeable to water and actively pumps out sodium and chloride, and the resulting interstitial gradient enables urine concentration.

Question 39Biology (syllabus up to Class 12 CBSE India)

A crop plant grown in waterlogged soil develops yellowing of leaves despite adequate mineral supply in the soil. Which explanation is most plausible?

  1. Waterlogging reduces oxygen availability to roots, impairing active ion uptake Correct answer.
  2. Waterlogging blocks light reaching the leaves, preventing chlorophyll synthesis
  3. Waterlogging increases oxygen supply and causes ion toxicity
  4. Waterlogging directly destroys chlorophyll in the leaf mesophyll

Explanation

Waterlogged soil is deficient in oxygen, so root respiration and ATP production decline. Active uptake of mineral ions such as nitrate and magnesium then fails, and the resulting deficiency causes chlorosis even though ions are present in the soil.

Question 40Biology (syllabus up to Class 12 CBSE India)

Which component of the human immune system is responsible for the cell-mediated destruction of virus-infected cells?

  1. Helper T lymphocytes
  2. Mast cells releasing histamine
  3. B lymphocytes secreting antibodies
  4. Cytotoxic T lymphocytes Correct answer.

Explanation

Cytotoxic T cells recognise viral peptides presented on MHC class I molecules and kill infected cells directly. Antibodies act mainly on extracellular pathogens, while helper T cells coordinate the response rather than killing target cells.

Question 41Biology (syllabus up to Class 12 CBSE India)

A population of insects develops resistance to an insecticide after several generations of spraying. Which process best explains this observation?

  1. The insecticide induces targeted mutations that confer resistance
  2. Resistance spreads because insects learn to avoid the insecticide
  3. Individual insects gradually strengthen their resistance during exposure
  4. Pre-existing resistant variants survive and reproduce, increasing their frequency Correct answer.

Explanation

Resistance alleles arise by random mutation before exposure; the insecticide then acts as a selective pressure, so resistant individuals survive and leave more offspring. Over generations the frequency of resistant alleles rises. This is directional selection, not induced mutation.

Question 42Biology (syllabus up to Class 12 CBSE India)

Which statement about the role of the placenta in human pregnancy is correct?

  1. It permits exchange of nutrients and gases while keeping the two bloodstreams separate Correct answer.
  2. It serves only as a physical anchor and has no endocrine role
  3. It allows free mixing of maternal and fetal blood for nutrient transfer
  4. It produces all the progesterone needed throughout pregnancy

Explanation

The placental barrier separates maternal and fetal blood, so exchange of nutrients, oxygen, carbon dioxide and wastes occurs by diffusion across the villi without mixing the two circulations. The placenta also secretes hormones such as hCG, progesterone and oestrogen.

Question 43Biology (syllabus up to Class 12 CBSE India)

In a DNA sample, adenine constitutes 30% of the total bases. What percentage of the bases is guanine?

  1. 30%
  2. 20% Correct answer.
  3. 70%
  4. 40%

Explanation

By complementary base pairing, the proportion of thymine equals that of adenine, so A + T = 60% of the bases. The remaining 40% must be shared equally by guanine and cytosine, giving 20% guanine.

Question 44Biology (syllabus up to Class 12 CBSE India)

A patient with a defective pancreatic exocrine secretion shows poor digestion of fats but normal absorption of simple sugars. Which explanation is most consistent with this finding?

  1. Fats are absorbed in the stomach while sugars are absorbed in the small intestine
  2. Fats are digested by salivary amylase while sugars need pancreatic lipase
  3. Simple sugars need no hydrolysis and are absorbed directly, whereas fats require lipase and bile salts Correct answer.
  4. Simple sugars are absorbed by active transport that does not require pancreatic enzymes

Explanation

Monosaccharides such as glucose can be absorbed without prior luminal hydrolysis, whereas triglycerides require pancreatic lipase to yield fatty acids and monoglycerides, assisted by bile salts. Loss of exocrine pancreatic secretion therefore impairs fat digestion far more than sugar absorption.

Question 45Biology (syllabus up to Class 12 CBSE India)

In an experiment, a freshwater protozoan is placed in sea water. Which response would most help it maintain its internal osmotic balance?

  1. Increasing the permeability of its membrane to water
  2. Actively pumping in additional salts to match the medium
  3. Reducing contractile vacuole activity to conserve water Correct answer.
  4. Increasing contractile vacuole activity to expel excess water

Explanation

Sea water is hypertonic to the freshwater protozoan, so water tends to leave the cell. Reducing contractile vacuole activity, which normally expels surplus water in hypotonic surroundings, helps conserve water and limit shrinkage.

Question 46Biology (syllabus up to Class 12 CBSE India)

Which feature of the genetic code explains why a single base substitution often produces no change in the encoded protein?

  1. The code contains no stop codons
  2. The code is overlapping, so one base affects several codons
  3. The code is universal across all organisms
  4. The code is degenerate, with several codons specifying one amino acid Correct answer.

Explanation

Degeneracy means that more than one codon can specify the same amino acid, particularly in the third position, so many substitutions are silent. Universality and non-overlapping organisation are separate properties of the code.

Question 47Biology (syllabus up to Class 12 CBSE India)

A respirometer records that germinating seeds consume oxygen and release carbon dioxide in equal volumes. Which respiratory substrate is most consistent with this observation?

  1. Carbohydrate Correct answer.
  2. Fat
  3. Protein
  4. Organic acid

Explanation

When carbohydrate is fully oxidised aerobically, the respiratory quotient, the ratio of CO2 released to O2 consumed, is close to 1.0. Fats and proteins have lower quotients because they are more reduced and require more oxygen per carbon oxidised.

Question 48Biology (syllabus up to Class 12 CBSE India)

A mutation in a bacterial promoter region weakens the binding of the sigma factor of RNA polymerase. Which outcome is most likely?

  1. Elongation of the transcript slows after initiation
  2. DNA replication at the origin is blocked
  3. Initiation of transcription at that promoter becomes less frequent Correct answer.
  4. Translation of the mRNA stalls at the ribosome

Explanation

Sigma factor recognises promoter sequences and recruits RNA polymerase for transcription initiation. Weakening this interaction reduces the frequency of initiation at that promoter, so fewer transcripts are made, while elongation and translation machinery are unaffected.

Question 49Biology (syllabus up to Class 12 CBSE India)

In a pedigree, an unaffected couple has an affected daughter and two unaffected sons. Which mode of inheritance is most likely, and why?

  1. Y-linked, because only one sex is affected
  2. X-linked recessive, because only daughters can be affected
  3. Autosomal recessive, because unaffected carrier parents can have an affected child of either sex Correct answer.
  4. Autosomal dominant, because the parents must also be affected

Explanation

Two unaffected parents producing an affected child indicates that the trait is recessive and both parents are heterozygous carriers. The affected daughter rules out X-linked recessive inheritance, since an affected girl would need an affected father. Autosomal recessive inheritance fits both observations.

Question 50Biology (syllabus up to Class 12 CBSE India)

A plant is kept in continuous darkness with adequate water and minerals. Which process will continue for some time but eventually decline because of the lack of light reactions?

  1. Photolysis of water in photosystem II
  2. Carbon fixation by RuBisCO using ATP and NADPH Correct answer.
  3. Absorption of mineral ions by root hairs
  4. Reduction of NADP+ in the Calvin cycle

Explanation

Carbon fixation by RuBisCO uses ATP and NADPH generated by the light reactions, so it can continue briefly on accumulated pools but declines in darkness. Photolysis of water requires light and stops immediately, and NADP+ reduction is itself a light-driven process.