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MECEE-BL Nepal practice questions

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Question 1Biology

Which organelle is the principal site of aerobic ATP synthesis in a eukaryotic cell?

  1. Lysosome
  2. Ribosome
  3. Golgi apparatus
  4. Mitochondrion Correct answer.

Explanation

The mitochondrion houses the electron transport chain and ATP synthase on its inner membrane, and the citric acid cycle occurs in its matrix. Oxidative phosphorylation there produces most of the ATP in aerobic eukaryotes.

Question 2Biology

In the nephron of the human kidney, the bulk of glucose reabsorption from the filtrate occurs in which segment?

  1. Descending limb of the loop of Henle
  2. Collecting duct
  3. Proximal convoluted tubule Correct answer.
  4. Distal convoluted tubule

Explanation

The proximal convoluted tubule has a brush border rich in sodium-glucose cotransporters, so virtually all filtered glucose is reclaimed there under normal conditions. Later segments have little capacity for glucose reabsorption.

Question 3Biology

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

  1. Aorta
  2. Pulmonary artery
  3. Pulmonary vein Correct answer.
  4. Superior vena cava

Explanation

The pulmonary vein returns blood that has been oxygenated in the alveolar capillaries to the left atrium. It is the exception to the rule that veins carry deoxygenated blood.

Question 4Biology

Which plant tissue is chiefly responsible for the transport of sucrose from source leaves to sink organs?

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

Explanation

Phloem sieve tubes with companion cells conduct assimilated sucrose by pressure flow from sources to sinks. Xylem, by contrast, conducts water and dissolved minerals upward.

Question 5Biology

Which molecule is the final electron acceptor in the electron transport chain of aerobic respiration?

  1. Pyruvate
  2. Oxygen Correct answer.
  3. NAD+
  4. FAD

Explanation

Oxygen accepts electrons at complex IV and combines with protons to form water, keeping the chain running. Without oxygen the chain backs up and NADH cannot be reoxidised.

Question 6Biology

A cross between two heterozygous pea plants (Tt × Tt) produces offspring. What is the expected phenotypic ratio for the dominant trait?

  1. 3:1 Correct answer.
  2. 1:1
  3. 1:2:1
  4. 9:3:3:1

Explanation

A monohybrid cross of two heterozygotes yields genotypes TT, Tt, Tt and tt in a 1:2:1 ratio. Because T is dominant, the three genotypes with at least one T give a 3:1 phenotypic ratio.

Question 7Biology

Which nitrogenous waste is the principal product of protein catabolism excreted by adult humans?

  1. Uric acid
  2. Creatinine
  3. Urea Correct answer.
  4. Ammonia

Explanation

Hepatic deamination of amino acids yields ammonia, which the liver converts to urea through the urea cycle. Urea is far less toxic and is excreted in urine.

Question 8Biology

Which structure in the human male reproductive system stores spermatozoa and allows their maturation before ejaculation?

  1. Prostate gland
  2. Epididymis Correct answer.
  3. Seminal vesicle
  4. Vas deferens

Explanation

Spermatozoa leaving the testis pass into the epididymis, where they acquire motility and are stored. During ejaculation they are propelled through the vas deferens.

Question 9Biology

A person with blood group A receives a transfusion of packed red cells from a group B donor. Which reaction is expected?

  1. Agglutination of donor cells by recipient anti-B antibodies Correct answer.
  2. Agglutination of recipient cells by donor anti-A antibodies
  3. Haemolysis caused by the Rh antigen only
  4. No reaction, because both are ABO groups

Explanation

Group A plasma contains anti-B antibodies, which recognise the B antigen on donor red cells and cause agglutination and complement-mediated haemolysis. ABO compatibility therefore requires that donor antigens match recipient antibodies.

Question 10Biology

During which phase of mitosis do sister chromatids separate and move toward opposite poles?

  1. Prophase
  2. Anaphase Correct answer.
  3. Metaphase
  4. Telophase

Explanation

In anaphase the cohesin linking sister chromatids is cleaved and spindle microtubules shorten, pulling chromatids to the poles. This follows metaphase alignment and precedes telophase nuclear reformation.

Question 11Biology

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

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

Explanation

With q = 0.2, p = 0.8, and the heterozygote frequency is 2pq = 2 × 0.8 × 0.2 = 0.32. The homozygote frequencies are p² = 0.64 and q² = 0.04.

Question 12Biology

Which hormone is released from the posterior pituitary and increases water reabsorption in the collecting duct?

  1. Atrial natriuretic peptide
  2. Aldosterone
  3. Insulin
  4. Antidiuretic hormone Correct answer.

Explanation

Antidiuretic hormone, made in hypothalamic neurons and released from the posterior pituitary, inserts aquaporin-2 channels into collecting-duct membranes. This raises water reabsorption and concentrates the urine.

Question 13Biology

Which feature distinguishes the light reactions from the Calvin cycle in photosynthesis?

  1. The light reactions occur on thylakoid membranes and generate ATP and NADPH Correct answer.
  2. The Calvin cycle occurs on thylakoid membranes and splits water
  3. The light reactions occur in the stroma and fix carbon dioxide
  4. The Calvin cycle generates oxygen as a by-product

Explanation

Photosystems and ATP synthase are embedded in the thylakoid membranes, where light-driven electron transport produces ATP and NADPH. The Calvin cycle uses these products in the stroma to fix carbon dioxide.

Question 14Biology

A mutation changes a codon from UAC to UAA in the middle of a coding sequence. What is the most likely consequence for the polypeptide?

  1. Premature termination of translation Correct answer.
  2. A frameshift affecting all downstream codons
  3. No change because the genetic code is degenerate
  4. Substitution of one amino acid by another

Explanation

UAA is one of the three stop codons, so the ribosome will terminate translation at that point. The resulting polypeptide is truncated and usually non-functional.

Question 15Biology

Which statement correctly describes the role of restriction endonucleases in recombinant DNA technology?

  1. They cut DNA at specific palindromic recognition sequences Correct answer.
  2. They join DNA fragments by forming phosphodiester bonds
  3. They amplify DNA exponentially through thermal cycling
  4. They synthesise cDNA from an RNA template

Explanation

Restriction endonucleases recognise specific short palindromic sequences and cleave the sugar-phosphate backbone there. The resulting sticky or blunt ends can then be joined to a vector.

Question 16Biology

In a food chain, only about ten percent of the energy at one trophic level is transferred to the next. What is the main reason for this loss?

  1. Most energy is lost as heat through respiration and in uneaten material Correct answer.
  2. Predators convert all ingested energy into new biomass
  3. Energy flows back to producers through decomposers
  4. Energy is destroyed when organisms are eaten

Explanation

Assimilated energy is largely dissipated as heat during respiration, and further energy remains in uneaten parts and excretory wastes. Only a small fraction is stored as new biomass available to the next level.

Question 17Biology

Which adaptation of xerophytic plants most directly reduces water loss through transpiration?

  1. Large thin leaves with many stomata
  2. Aerial roots absorbing atmospheric moisture
  3. Deep taproots reaching the water table
  4. Sunken stomata in pits lined with hairs Correct answer.

Explanation

Sunken stomata create a humid microclimate in the pit that slows diffusion of water vapour out of the leaf. Hairs around the pit further trap moist air.

Question 18Biology

Which component of the human immune system provides a rapid, non-specific first line of defence against pathogens?

  1. Phagocytic neutrophils and macrophages Correct answer.
  2. Plasma cells secreting specific antibodies
  3. Cytotoxic T lymphocytes
  4. Memory B lymphocytes

Explanation

Neutrophils and macrophages engulf pathogens by phagocytosis without recognising a specific antigen, forming part of innate immunity. Specific antibody and memory responses belong to adaptive immunity and take longer to develop.

Question 19Biology

An enzyme-catalysed reaction shows a rising rate with substrate concentration until a plateau is reached. Which explanation fits the plateau?

  1. The reaction has become endergonic
  2. All active sites are saturated with substrate Correct answer.
  3. The substrate has been used up completely
  4. The enzyme has been permanently denatured

Explanation

At saturation every enzyme active site is occupied, so adding more substrate cannot increase the number of catalytic events per unit time. The rate then depends on the enzyme's turnover number.

Question 20Biology

Which process produces genetic variation in offspring by exchanging segments between homologous chromosomes?

  1. DNA replication during S phase
  2. Independent assortment at metaphase II
  3. Crossing over during prophase I Correct answer.
  4. Cytokinesis after telophase I

Explanation

Crossing over between non-sister chromatids of homologous chromosomes in prophase I creates recombinant chromatids. This reshuffles alleles along a chromosome, generating new combinations.

Question 21Biology

A woman who is a carrier of an X-linked recessive disorder marries an unaffected man. What proportion of their sons is expected to be affected?

  1. 1/4
  2. 0
  3. All sons
  4. 1/2 Correct answer.

Explanation

Sons receive their single X chromosome from the mother, and a carrier mother transmits the mutant allele to half her sons on average. Those sons have no normal allele to mask it, so they are affected.

Question 22Biology

In the lac operon of Escherichia coli, what happens when lactose is present and glucose is absent?

  1. The repressor is inactivated and CAP-cAMP strongly activates transcription Correct answer.
  2. The repressor is inactivated but CAP-cAMP cannot bind the promoter
  3. Transcription occurs at a low basal rate regardless of the repressor
  4. The repressor binds the operator and transcription is blocked

Explanation

Lactose (as allolactose) binds the repressor and prevents it from binding the operator, while low glucose raises cAMP so CAP-cAMP binds upstream and enhances RNA polymerase recruitment. The operon is therefore strongly expressed.

Question 23Biology

A patient's blood shows elevated urea, elevated creatinine and a low glomerular filtration rate. Which interpretation is most consistent with these findings?

  1. Excessive water reabsorption in the loop of Henle
  2. Increased protein digestion in the small intestine
  3. Reduced renal clearance of nitrogenous waste Correct answer.
  4. Enhanced urea synthesis by skeletal muscle

Explanation

A low glomerular filtration rate means less urea and creatinine are filtered, so both accumulate in plasma. Together with the raised urea this points to impaired renal excretory function rather than altered digestion or water handling.

Question 24Biology

Two genes are located close together on the same chromosome. In a test cross of a double heterozygote, which result indicates linkage rather than independent assortment?

  1. A large excess of parental classes and few recombinant classes Correct answer.
  2. Equal numbers of all four phenotypic classes
  3. A 9:3:3:1 ratio among the four classes
  4. Only recombinant classes appear among the offspring

Explanation

Linked genes tend to be inherited together, so parental allele combinations predominate and recombinants arise only from crossing over between them. The recombinant frequency therefore falls well below the fifty percent expected under independent assortment.

Question 25Biology

Which change would most increase the oxygen-binding affinity of haemoglobin in a working muscle capillary?

  1. A fall in pH
  2. A rise in carbon dioxide concentration
  3. A rise in temperature
  4. A rise in pH with low carbon dioxide Correct answer.

Explanation

Alkaline conditions with low carbon dioxide shift the oxygen dissociation curve left, increasing affinity so haemoglobin holds oxygen more tightly. The Bohr effect works in the opposite direction in active tissues.

Question 26Biology

Which polysaccharide is the principal storage carbohydrate of plants and yields only glucose on complete hydrolysis?

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

Explanation

Starch is the storage polysaccharide of plants and is a polymer of alpha-glucose units, so hydrolysis releases glucose only. Cellulose is also a glucose polymer but is structural, while glycogen and chitin are not plant storage forms.

Question 27Biology

Which organelle is the site of the Krebs cycle in a eukaryotic cell?

  1. Mitochondrial matrix Correct answer.
  2. Cytosolic fluid
  3. Golgi apparatus
  4. Rough endoplasmic reticulum

Explanation

The enzymes of the Krebs cycle are located in the mitochondrial matrix, except succinate dehydrogenase, which is bound to the inner membrane. Glycolysis alone occurs in the cytosol.

Question 28Biology

A cell is placed in a solution whose solute concentration is higher than that of the cytoplasm. What is the expected net movement of water?

  1. Water leaves the cell by osmosis Correct answer.
  2. Water enters the cell by osmosis
  3. Water is actively pumped out by the sodium-potassium pump
  4. Water moves equally in both directions with no net change

Explanation

The external solution is hypertonic, so its water potential is lower than that of the cytoplasm. Water therefore moves out of the cell across the membrane by osmosis, causing the cell to shrink or become plasmolysed.

Question 29Biology

Which group of organisms is characterised by having cell walls made of chitin and by being heterotrophic?

  1. Fungi Correct answer.
  2. Cyanobacteria
  3. Bryophytes
  4. Euglenoids

Explanation

Fungi are heterotrophic organisms whose cell walls contain chitin, and they absorb nutrients from their surroundings. Bryophytes are plants, cyanobacteria are photosynthetic prokaryotes, and euglenoids are mostly photosynthetic protists without chitinous walls.

Question 30Biology

Which part of the nephron is the main site of selective reabsorption of glucose and amino acids from the filtrate?

  1. Proximal convoluted tubule Correct answer.
  2. Distal convoluted tubule
  3. Descending limb of the loop of Henle
  4. Collecting duct

Explanation

The proximal convoluted tubule has a brush border and numerous mitochondria that support active transport, and it reabsorbs nearly all glucose and amino acids. The loop of Henle mainly handles water and salt gradients, and the distal tubule and collecting duct fine-tune salt and water balance.

Question 31Biology

In a monohybrid cross, an organism showing the dominant phenotype is test-crossed and half of the offspring show the recessive phenotype. What is the genotype of the tested organism?

  1. Homozygous recessive
  2. Homozygous dominant
  3. Heterozygous Correct answer.
  4. Hemizygous

Explanation

A test cross with a homozygous recessive partner reveals the unknown genotype through the offspring ratio. A 1:1 ratio of dominant to recessive offspring means the tested organism carried one recessive allele and was therefore heterozygous.

Question 32Biology

Which type of immunity is acquired when antibodies pass from a mother to her fetus across the placenta?

  1. Natural passive immunity Correct answer.
  2. Artificial passive immunity
  3. Artificial active immunity
  4. Natural active immunity

Explanation

The antibodies are received naturally rather than by medical intervention, and the recipient's own immune system is not stimulated to produce them, so the immunity is passive. Because the antibodies are transferred from mother to fetus, the term is natural passive immunity.

Question 33Biology

During which stage of meiosis does crossing over between non-sister chromatids of homologous chromosomes occur?

  1. Prophase I Correct answer.
  2. Telophase II
  3. Anaphase I
  4. Metaphase II

Explanation

Crossing over takes place during prophase I of meiosis, when homologous chromosomes pair as bivalents and form chiasmata. The later stages separate homologues and then sister chromatids without further exchange.

Question 34Biology

A couple in which both parents are heterozygous for the autosomal recessive allele causing cystic fibrosis have a child. What is the probability that the child is a healthy carrier?

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

Explanation

The cross Tt × Tt gives offspring in the ratio 1 TT : 2 Tt : 1 tt. Among all offspring, the two heterozygous carrier genotypes account for 2 of 4, that is a probability of 1/2.

Question 35Biology

Which feature of the genetic code allows a single amino acid to be specified by more than one codon?

  1. Collinearity
  2. Degeneracy Correct answer.
  3. Non-overlapping reading
  4. Universal nature

Explanation

Degeneracy means that most amino acids correspond to several synonymous codons, usually differing in the third base. This redundancy reduces the impact of some point mutations on the protein product.

Question 36Biology

Which enzyme joins two DNA fragments by forming phosphodiester bonds during gene cloning?

  1. Helicase
  2. Restriction endonuclease
  3. DNA polymerase
  4. DNA ligase Correct answer.

Explanation

DNA ligase catalyses the formation of phosphodiester bonds between adjacent nucleotides, sealing the sugar-phosphate backbone of the insert and vector. Polymerases synthesise new strands, helicases unwind duplexes, and restriction enzymes cut DNA.

Question 37Biology

A bacterial population is grown in a medium in which the only nitrogen source is atmospheric nitrogen. Which organism could grow without added nitrogen compounds?

  1. A denitrifying bacterium such as Pseudomonas
  2. A sulphate-reducing bacterium such as Desulfovibrio
  3. A nitrogen-fixing bacterium such as Rhizobium Correct answer.
  4. A nitrifying bacterium such as Nitrosomonas

Explanation

Nitrogen-fixing bacteria possess nitrogenase, which reduces atmospheric nitrogen to ammonia that can be assimilated into organic compounds. They can therefore grow when no combined nitrogen is supplied.

Question 38Biology

In the light reactions of photosynthesis, what is the immediate source of electrons that replaces those lost by the reaction centre of photosystem II?

  1. Hydrolysis of ATP
  2. Oxidation of plastoquinone
  3. Photolysis of water Correct answer.
  4. Reduction of NADP+

Explanation

Photosystem II loses excited electrons to the electron transport chain, and the resulting electron hole is filled by electrons released when water is split by the oxygen-evolving complex. This photolysis of water also releases protons and molecular oxygen.

Question 39Biology

Which structural feature of the mammalian small intestine most increases the surface area available for absorption?

  1. Taeniae coli
  2. Villi and microvilli Correct answer.
  3. Sphincter of Oddi
  4. Rugae of the stomach

Explanation

Finger-like villi project into the lumen, and each epithelial cell bears many microvilli forming a brush border, greatly multiplying the absorptive surface. The other structures belong to different parts of the digestive tract.

Question 40Biology

A person hyperventilates voluntarily for a short period. What immediate change occurs in the blood?

  1. Carbon dioxide rises and pH falls
  2. Carbon dioxide falls and pH rises Correct answer.
  3. Bicarbonate rises and pH falls
  4. Oxygen falls and pH falls

Explanation

Hyperventilation removes carbon dioxide from the blood faster than it is produced, lowering the partial pressure of carbon dioxide. Less carbonic acid forms, so the hydrogen ion concentration falls and the blood becomes more alkaline.

Question 41Biology

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

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

Explanation

Auxin, mainly indole-3-acetic acid, promotes cell elongation in shoots and, by suppressing lateral bud growth, produces apical dominance. Abscisic acid and ethylene are generally inhibitory, and cytokinins promote cell division and lateral bud growth.

Question 42Biology

In a pond ecosystem, phytoplankton are consumed by zooplankton, which are eaten by small fish, which are eaten by a heron. Which trophic level does the heron occupy?

  1. Fourth trophic level Correct answer.
  2. First trophic level
  3. Second trophic level
  4. Third trophic level

Explanation

Phytoplankton are producers at the first trophic level, zooplankton are primary consumers at the second, small fish are secondary consumers at the third, and the heron is a tertiary consumer at the fourth.

Question 43Biology

Which evidence most strongly supports the theory that mitochondria originated from free-living prokaryotes?

  1. Mitochondria are bounded by a double membrane and contain cristae
  2. Mitochondria produce ATP by oxidative phosphorylation
  3. Mitochondria have their own circular DNA and ribosomes resembling those of bacteria Correct answer.
  4. Mitochondria are inherited through the maternal line in many species

Explanation

The endosymbiotic theory predicts that mitochondria retain prokaryote-like genetic and translational machinery. Circular DNA and bacterial-type ribosomes are the strongest direct evidence of a prokaryotic ancestry.

Question 44Biology

A man with haemophilia A, an X-linked recessive disorder, marries a woman who is homozygous for the normal allele. What is the expected status of their children?

  1. All daughters are affected and all sons are carriers
  2. Half of the daughters are affected and half of the sons are affected
  3. All daughters are carriers and all sons are unaffected Correct answer.
  4. All sons are affected and all daughters are unaffected

Explanation

The father contributes his affected X chromosome to every daughter and his Y chromosome to every son. Daughters therefore receive one affected allele and one normal maternal allele and are carriers, while sons receive only the normal maternal X and are unaffected.

Question 45Biology

Which statement best explains why the rate of an enzyme-catalysed reaction falls at temperatures well above the optimum?

  1. The enzyme is diluted by the increased volume of the reaction mixture
  2. The substrate molecules gain kinetic energy and collide more frequently
  3. The product is removed faster than it can be formed
  4. The enzyme's tertiary structure is disrupted and its active site loses shape Correct answer.

Explanation

Above the optimum temperature, increased thermal vibration breaks the hydrogen bonds and other weak interactions that maintain the enzyme's tertiary structure. Denaturation distorts the active site so that the substrate can no longer bind effectively.

Question 46Biology

Which condition is required for a population to remain at Hardy-Weinberg equilibrium?

  1. Continuous immigration from a genetically different population
  2. Strong selection against the recessive phenotype
  3. Random mating with no mutation, migration or selection Correct answer.
  4. Frequent mutation that generates new alleles

Explanation

The Hardy-Weinberg principle assumes a large population, random mating, no mutation, no migration and no natural selection. Under these conditions allele and genotype frequencies remain constant from generation to generation.

Question 47Biology

A gene is transcribed and the primary transcript is processed in the nucleus. Which modification directly protects the 5' end of a eukaryotic mRNA molecule?

  1. Addition of a 7-methylguanosine cap Correct answer.
  2. Methylation of histone proteins
  3. Removal of introns by spliceosomes
  4. Addition of a poly-A tail

Explanation

A 7-methylguanosine cap is added to the 5' end of the primary transcript, protecting it from exonuclease attack and assisting ribosome binding. The poly-A tail protects the 3' end and aids export and stability.

Question 48Biology

Two species of finch on the same island feed on seeds of different sizes, and their beak depths differ markedly. Which term best describes this pattern?

  1. Genetic drift
  2. Convergent evolution
  3. Character displacement Correct answer.
  4. Sympatric speciation by polyploidy

Explanation

Character displacement occurs when competition for a shared resource selects for divergence in traits related to resource use. The two species partition the seed resource by beak size, reducing competition where they coexist.

Question 49Biology

A patient's arterial blood shows a low pH, a low bicarbonate concentration and a normal partial pressure of carbon dioxide. Which acid-base disturbance is most consistent with these findings?

  1. Metabolic alkalosis
  2. Respiratory acidosis
  3. Metabolic acidosis Correct answer.
  4. Respiratory alkalosis

Explanation

A primary fall in bicarbonate lowers the pH and indicates a metabolic acidosis. The normal carbon dioxide value suggests some respiratory compensation has occurred, but the low bicarbonate remains the primary abnormality.

Question 50Biology

In a dihybrid cross of two heterozygotes for independently assorting genes, what proportion of the offspring is expected to show both recessive phenotypes?

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

Explanation

Each gene in a heterozygous self-cross yields a 1/4 probability of the recessive phenotype. Because the genes assort independently, the joint probability is 1/4 × 1/4, which equals 1/16.