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Question 1. Which of the following statements best describes the primary structure of a protein? A) The three‑dimensional folding of the polypeptide chain B) The sequence of amino acids linked by peptide bonds C) The association of multiple polypeptide subunits D) The hydrogen bonding between backbone atoms Answer: B Explanation: Primary structure is the linear amino‑acid sequence held together by peptide bonds; higher‑order structures involve folding and subunit interactions. Question 2. In the Michaelis‑Menten equation, Vmax represents A) The substrate concentration at half‑maximal velocity B) The maximum rate achieved at saturating substrate C) The affinity of enzyme for substrate D) The rate constant for product formation Answer: B Explanation: Vmax is the asymptotic maximum velocity when all enzyme active sites are saturated with substrate. Question 3. A competitive inhibitor will have which effect on a Lineweaver‑Burk plot? A) Increase the y‑intercept, unchanged slope B) Increase the slope, unchanged y‑intercept C) Increase both slope and y‑intercept
D) Decrease the slope, unchanged y‑intercept Answer: B Explanation: Competitive inhibition raises Km (thus slope = Km/Vmax) without changing Vmax, leaving the y‑intercept (1/Vmax) unchanged. Question 4. Which of the following is the immediate product of the pyruvate dehydrogenase complex? A) Acetyl‑CoA B) Oxaloacetate C) Lactate D) Citric acid Answer: A Explanation: Pyruvate is decarboxylated to acetyl‑CoA, releasing CO₂ and generating NADH. Question 5. The enzyme that catalyzes the rate‑limiting step of glycolysis is A) Hexokinase B) Phosphofructokinase‑ 1 C) Pyruvate kinase D) Aldolase Answer: B Explanation: Phosphofructokinase‑1 (PFK‑1) irreversibly phosphorylates fructose‑ 6 ‑phosphate; it is highly regulated and rate‑limiting.
C) A channel protein forming a pore across the membrane D) A lipoprotein that is embedded in the membrane core Answer: B Explanation: Peripheral proteins associate loosely with the membrane surface, often via interactions with cytoskeletal elements. Question 9. The primary active transport mechanism that moves glucose into intestinal epithelial cells is A) Sodium‑glucose cotransporter (SGLT1) B) GLUT2 facilitated diffusion C) Na⁺/K⁺‑ATPase pump D) Endocytosis Answer: A Explanation: SGLT1 couples glucose uptake to the downhill movement of Na⁺, a form of secondary active transport. Question 10. Tight junctions are most abundant in which of the following tissues? A) Cardiac muscle B) Simple squamous epithelium of the alveoli C) Stratified squamous epithelium of the skin D) Simple columnar epithelium of the intestine Answer: D
Explanation: Tight junctions seal adjacent intestinal epithelial cells, preventing paracellular leakage of luminal contents. Question 11. During which phase of the cell cycle are sister chromatids most tightly attached at the centromere? A) G₁ phase B) S phase C) G₂ phase D) M phase (metaphase) Answer: D Explanation: In metaphase, kinetochores attach microtubules to sister chromatids, holding them together at the centromere. Question 12. The transition from G₂ to M phase is primarily regulated by A) Cyclin D‑CDK4/6 complex B) Cyclin E‑CDK2 complex C) Cyclin A‑CDK2 complex D) Cyclin B‑CDK1 complex Answer: D Explanation: Cyclin B binds CDK1 (also called Cdc2) to form the maturation‑promoting factor (MPF) that triggers entry into mitosis. Question 13. In meiosis I, homologous chromosomes separate during
Answer: C Explanation: A 9:3:4 ratio is classic for duplicate recessive epistasis, where two genes can each produce the same phenotype when both are recessive. Question 16. In a pedigree, a trait that appears only in males and is transmitted from affected fathers to all daughters is most consistent with A) Autosomal dominant inheritance B) X‑linked recessive inheritance C) X‑linked dominant inheritance D) Mitochondrial inheritance Answer: C Explanation: X‑linked dominant traits affect all daughters of an affected male because daughters receive his only X chromosome. Question 17. According to Hardy‑Weinberg equilibrium, if the frequency of the dominant allele (p) is 0.7, the expected frequency of heterozygotes (2pq) is A) 0. B) 0. C) 0. D) 0. Answer: B Explanation: q = 1 – p = 0.3; 2pq = 2 × 0.7 × 0.3 = 0.42.
Question 18. Which enzyme creates the RNA primer during bacterial DNA replication? A) DNA polymerase I B) DNA polymerase III C) Primase D) Helicase Answer: C Explanation: Primase synthesizes a short RNA primer that provides a 3’‑OH for DNA polymerases to extend. Question 19. The leading strand during DNA replication is synthesized A) Continuously in the 5’→3’ direction toward the replication fork B) Continuously in the 5’→3’ direction away from the replication fork C) Discontinuously in short Okazaki fragments D) Only after the lagging strand is completed Answer: B Explanation: The leading strand is synthesized continuously in the 5’→3’ direction as the fork progresses, moving away from the fork. Question 20. In eukaryotes, the addition of a 5’‑cap to pre‑mRNA primarily serves to A) Signal for polyadenylation B) Facilitate ribosome binding during translation
Question 23. In a Southern blot, DNA fragments are separated by A) SDS‑PAGE B) Agarose gel electrophoresis under denaturing conditions C) Agarose gel electrophoresis under native conditions D) Two‑dimensional gel electrophoresis Answer: C Explanation: Southern blotting uses agarose gel electrophoresis of intact (native) DNA fragments, followed by transfer to a membrane. Question 24. Which neurotransmitter is primarily associated with the parasympathetic division of the autonomic nervous system? A) Norepinephrine B) Acetylcholine C) Dopamine D) Serotonin Answer: B Explanation: Parasympathetic post‑ganglionic neurons release acetylcholine onto target organs. Question 25. The refractory period of a neuron is primarily caused by A) Inactivation of voltage‑gated sodium channels B) Opening of voltage‑gated potassium channels
C) Depletion of neurotransmitter vesicles D) Reuptake of neurotransmitter into the synaptic cleft Answer: A Explanation: After an action potential, Na⁺ channels become inactivated, preventing immediate re‑excitation (absolute refractory period). Question 26. Which hormone is secreted by the anterior pituitary in response to hypothalamic releasing factors? A) Cortisol B) Thyroxine (T₄) C) Prolactin D) Aldosterone Answer: C Explanation: Prolactin is released from the anterior pituitary under stimulation by hypothalamic prolactin‑releasing hormone (PRH) and inhibited by dopamine. Question 27. Steroid hormones differ from peptide hormones in that they A) Bind to cell‑surface receptors only B) Require second‑messenger cascades for action C) Diffuse through the plasma membrane and bind intracellular receptors D) Are stored in secretory granules Answer: C
B) Aldosterone C) Parathyroid hormone (PTH) D) Atrial natriuretic peptide (ANP) Answer: B Explanation: Aldosterone up‑regulates ENaC channels, increasing Na⁺ reabsorption (and K⁺ secretion) in the distal nephron. Question 31. Which of the following enzymes is secreted by the pancreas in an inactive form to protect the pancreas from autodigestion? A) Lipase B) Amylase C) Trypsinogen D) Lactase Answer: C Explanation: Trypsinogen is activated to trypsin only in the duodenum by enteropeptidase, preventing pancreatic tissue damage. Question 32. The hormone gastrin primarily stimulates A) Secretion of bicarbonate from pancreatic ducts B) Gastric HCl secretion from parietal cells C) Release of secretin from duodenal S cells D) Motility of the large intestine
Answer: B Explanation: G cells in the stomach release gastrin, which directly stimulates parietal cells to secrete hydrochloric acid. Question 33. The sliding filament model of muscle contraction requires which ion to bind troponin? A) Na⁺ B) K⁺ C) Ca²⁺ D) Mg²⁺ Answer: C Explanation: Calcium binds to troponin C, causing tropomyosin to shift and expose actin’s myosin‑binding sites. Question 34. Osteoclasts resorb bone by A) Secreting alkaline phosphatase B) Producing collagen fibers C) Acidifying the resorption lacuna via H⁺‑ATPase D) Synthesizing osteoid matrix Answer: C Explanation: Osteoclasts secrete H⁺ via a vacuolar H⁺‑ATPase, dissolving hydroxyapatite and allowing matrix degradation. Question 35. Which of the following is a defining characteristic of Archaea?
Answer: C Explanation: Echinoderms (e.g., sea stars) have a water vascular system used for locomotion and feeding. Question 38. The lytic cycle of a bacteriophage results in A) Integration of phage DNA into the host genome B) Production of lysogenic conversion proteins C) Immediate synthesis of new virions and host cell lysis D) Dormancy of the phage genome within the host Answer: C Explanation: In the lytic cycle, phage DNA is expressed, new particles assembled, and the host cell is lysed to release progeny. Question 39. Gram‑positive bacteria are distinguished from Gram‑negative bacteria by A) Presence of an outer membrane containing lipopolysaccharide B) Thick peptidoglycan layer that retains crystal violet‑iodine complex C) Periplasmic space containing β‑lactamase D) Lack of teichoic acids Answer: B Explanation: Gram‑positive cells have a thick peptidoglycan wall that holds the primary stain, appearing purple after Gram staining.
Question 40. During fertilization, the cortical reaction prevents A) Sperm from binding to the zona pellucida B) Fusion of the male and female pronuclei C) Polyspermy by hardening the zona pellucida D) Release of calcium from the endoplasmic reticulum Answer: C Explanation: The cortical reaction releases enzymes that modify the zona pellucida, forming a fertilization membrane that blocks additional sperm entry. Question 41. The morula stage of embryonic development is characterized by A) Formation of a fluid‑filled cavity B) A solid ball of cells resulting from cleavage divisions C) Differentiation of the three germ layers D) Implantation into the uterine wall Answer: B Explanation: The morula is a compact, solid cluster of blastomeres produced by successive cleavage. Question 42. Which germ layer gives rise to the epithelial lining of the respiratory tract? A) Ectoderm B) Mesoderm C) Endoderm D) Neural crest
Question 45. In a food chain, the 10 % energy rule refers to A) The proportion of energy transferred from one trophic level to the next B) The amount of energy lost as heat during cellular respiration C) The percentage of sunlight captured by photosynthetic organisms D) The efficiency of ATP synthesis in mitochondria Answer: A Explanation: Only about 10 % of the energy stored in biomass is transferred to the next trophic level; the rest is lost as heat, waste, or metabolic costs. Question 46. Which of the following best describes a commensal relationship? A) Both species benefit B) One species benefits while the other is harmed C) One species benefits and the other is unaffected D) Both species are harmed Answer: C Explanation: In commensalism, the commensal gains a benefit (e.g., shelter) while the host experiences no significant effect. Question 47. Primary succession most commonly begins on A) Abandoned agricultural fields B) Recently formed volcanic lava flows C) Old-growth forest clearings
D) River floodplains after a flood Answer: B Explanation: Primary succession starts on newly formed, barren substrates such as lava, where no soil exists initially. Question 48. In operant conditioning, a behavior that increases in frequency because it is followed by a rewarding stimulus is called A) Positive reinforcement B) Negative reinforcement C) Punishment D) Extinction Answer: A Explanation: Positive reinforcement adds a desirable stimulus after a behavior, strengthening the response. Question 49. Which of the following best illustrates a fixed‑action pattern? A) A rabbit learning to navigate a maze for food B) A male stickleback building a nest after seeing a specific cue C) A chick imprinting on the first moving object it sees D) A dog salivating to the sound of a bell after conditioning Answer: B
