38. An enzyme exhibits normal Michaelis-Menten kinetics at pH 7 but displays sigmoidal kinetics at pH 9. Which explanation is most plausible?
(A) Enzyme concentration doubled
(B) Competitive inhibitor formed
(C) Product inhibition dominates
(D) Substrate precipitation occurred
(E) Subunit ionization alters allosteric cooperativity NTHU115

39. A researcher subjects four distinct biological structures (a functional globular protein, a segment of DNA, a cellulose fiber, anda molecule of amylopectin) to an environment where all non-covalent interactions (hydrogen bonds, ionic bonds, van der Waals forces, and hydrophobic interactions) are completely disrupted, but all covalent bonds (peptide bonds, glycosidic linkages, and phosphodiester linkages) remain intact. Which resulting observation is the LEAST accurate regarding the stable molecular components remaining after this treatment? (A) The globular protein, such as transthyretin, would be converted into a nonfunctional, linear polypeptide chain, retaining its unique amino acid sequence. (B) The secondary structure elements (such as a-helices and β-pleated sheets) within the polypeptide backbone of the globular protein would no longer exist. (C) The two complementary strands of the DNA double helix would separate entirely, resulting in two distinct, single polynucleotide strands. (D) The amylopectin molecule (a storage polysaccharide) would be cleaved into hundreds of individual glucose monomers. (E) A cellulose molecule would retain its straight, unbranched configuration, although it would no longer be grouped into strong microfibrils.

40. The Na+/K+ pump is a critical electrogenic transport protein that uses ATP to actively move ions against their concentration gradients, contributing to the membrane potential of animal cells. This integral protein is synthesized in the Endoplasmic Reticulum (ER) and modified through the Golgi apparatus before insertion into the plasma membrane. The completed pump moves 3 Nat ions out of the cell and 2 K+ ions into the cell. Considering the required sidedness of the plasma membrane, which component of the Na+/K+ pump is exposed to the ER lumen during the initial stages of synthesis? (A) The segment of the protein that releases the terminal phosphate group derived from ATР. (B) The region of the protein that initially binds the three Nat ions. (C) The binding domain that triggers the release of the two K+ ion into the cytoplasm. (D) The nonpolar a-helical stretches that span the lipid bilayer. (E) The binding site that has a high affinity for K+ ions when the pump has been phosphorylated.

41. A highly active muscle cell is subjected to extreme mechanical work, causing a sharp, sustained acceleration in the rate of ATP hydrolysis. The initial free energy released by this process is used primarily for mechanical work, such as muscle contraction. Considering the subsequent shift in the concentrations of ATP, ADP, and inorganic phosphate (Pi) within the cell, which statement best describes the immediate, self-regulating response mediated by allosteric control mechanisms? (A) The accumulation of inorganic phosphate (Pi) acts as a competitive inhibitor, competing with substrate molecules for the active sites of catabolíc enzymes, thus slowing down ATP synthesis. (B) The reduction in ATP concentration stabilizes the inactive form of key catabolic enzymes, causing the overall pace of catabolism to decrease. (C) The remaining ATP binds to its specific regulatory sites on anabolic enzymes, inhibiting them, while simultaneously promoting the activity of catabolic enzymes. (D) The rising concentration of ADP functions as an allosteric activator, stabilizing the active conformations of key catabolic enzymes and thereby accelerating the overall rate of ATP regeneration. (E) The decrease in ATP concentration forces the cell's metabolic pathways closer to equilibrium, which increases the amount of work the cell can perform, according to the Free Energy principle (AG).

42. Pepsin is a digestive enzyme that functions optimally at a pH of 2, an environment highly concentrated with H* ions. In contrast, most enzymes found in the human body have optimal pH values ranging from 6 to 8. The extreme acidity of pH 2 would typically cause most enzymes to denature, leading to a sharp drop in activity, because the high concentration of H+ ions disrupts the weak interactions crucial for stabilizing the protein's functional shape. Which statement below best describes the mechanism by which Pepsin is specifically adapted to maintain its maximum catalytic activity in the stomach environment? (A) Pepsin uses strong covalent bonds (peptide bonds) to form its active site, which cannot be broken by H+ ions, unlike the weak bonds found in other enzyme active sites. (B) Pepsin's △G (Free Energy change) for its specific catabolic reaction shifts to a highly negative value at pH 2, thereby increasing the spontaneous rate of the reaction independent of structure. (C) The structure of Pepsin minimizes the use of the most electronegative elements. (Oxygen and Nitrogen) in its regulatory sites, making it insensitive to competitive inhibition by H+ ions. (D) Pepsin has evolved to significantly raise the overall activation energy (EA) barrier of its substrate at neutral pH, forcing the reaction to only proceed effectively when thermal energy is supplied by the acidic environment. (E) The amino acid sequence of Pepsin results in a unique three-dimensional configuration that prevents the high concentration of H+ ions from disrupting the crucial weak interactions (such as ionic bonds and hydrogen bonds) necessary for its active shape.

43. During the mitotic phase, the enzyme separase is responsible for cleaving the cohesin proteins that hold sister chromatids together, a step crucial for the initiation of anaphase. If a cell possesseda dominant mutation causing separase to be constitutively active and begin cleaving all cohesins prematurely during prometaphase (before the M checkpoint criteria were met), which outcome is the most likely and immediate consequence? (A) The resulting daughter cells would skip the G1 phase and immediately enter the S phase due to premature activation of maturation-promoting factor (MPF). (B) The cell would successfully complete mitosis, but the spindle poles would fail to move apart due to inactive non-kinetochore microtubules. (C) The M checkpoint would fall, and the liberated chromatids would segregate randomly, resulting in genetically unequal daughter cells. (D) The nuclear envelope would reform immediately, triggering early telophase and preventing the remaining spindle microtubules from attaching to kinetochores. (E) Cytokinesis would initiate during prophase, leading to the formation of multiple, small nuclei within a single parent cell.

44. In an E. coli cell undergoing rapid replication, a newly identified chemical agent completely and specifically inhibits the function of DNA ligase. Assuming all other replication proteins (Helicase, Primase, DNA pol I, and DNA pol III) are fully functional, what structural consequence would be immediately observable following the completion of the first round of DNA synthesis? (A) Both parental DNA strands would remain permanently associated, halting the replication process before the replication fork could open fully. (B) The leading strand would fail to elongate beyond the initial RNA primer because DNA pol III requires DNA ligase to begin continuous synthesis. (C) The lagging strand would consist of multiple Okazaki fragments that are fully synthesized DNA segments but lack covalent bonds between them. NTHU115 (D) The ends of the circular bacterial chromosome would shorten significantly because the replication machinery cannot replace the terminal RNA primers. (E) The concentration of thymine dimers would increase dramatically due to the inability of the DNA replication complex to proofread mismatched bases.

45. A eukaryotic cell synthesizes an mRNA molecule where the poly-A tail is normal, but a specific mutation prevents the 5' cap from binding the necessary translation initiation factors. Assuming the mRNA is successfully exported to the cytoplasm, which step of protein synthesis would be most immediately and severely affected? (A) The formation of peptide bonds in the P site would cease, halting elongation. (B) The mRNA would be rapidly degraded by hydrolytic enzymes in the cytoplasm. (C) tRNA molecules would fail to be charged with their corresponding amino acids. (D) Release factors would prematurely bind to the AUG codon, causing early termination. (E) The small ribosomal subunit would fail to bind to the mRNA or scan for the AUG start codon.

46. Which of the following statements most accurately reflects the known composition and mobility mechanisms of the repetitive DNA sequences that account for the largest proportion of the human genome? (A) Transposons (DNA intermediate) are the most abundant mobile element, responsible for 44% of the genome, moving via the cut-and-paste mechanism. (B) Simple sequence DNA (short tandem repeats, STRs) constitute 17% of the genome and primarily facilitate unequal crossing over to drive gene duplication. (C) The Alu elements (10% of the genome) are the largest type of retrotransposon and encode their own reverse transcriptase for movement. (D) Retrotransposons are the most abundant category, and unlike transposons, their movement inherently involves synthesizing a complementary DNA strand from an RNA intermediate, thus leaving a copy at the original location. (E) Transposable elements are included in the 1.5% of the genome that codes for proteins because they all encode the transposase or reverse transcriptase enzymes necessary for their own movement.

47. The evolutionary adaptation of C4 photosynthesis minimizes the photorespiratory deficit by ensuring a high local concentration of CO2 near the enzyme rubisco. This mechanism relies on the spatial segregation of tasks, primarily in the bundle-sheath cells, whose chloroplasts are specialized to contain Photosystem I (PS I) but often lack functional Photosystem II (PS II). How does the absence of Photosystem II (PS II) in the bundle-sheath chloroplasts contribute to minimizing photorespiration, even though the cell is operating under conditions of intense light? (A) The resulting reliance on cyclic electron flow generates the low ratio of ATP to NADPH required to drive the reduction of CO2 and outcompete the photorespiration pathway. (B) The four-carbon compound (e.g., malate) delivered from the mesophyll cells is designed to directly activate PS I, allowing it to generate the necessary high-energy electrons without relying on water. (C) The inhibition of H₂O splitting, which normally occurs at PS II, prevents the release of O2 gas directly into the local environment where the Calvin cycle and rubisco are confined. (D) The PS I-only system utilizes an alternate terminal electron acceptor, which binds to O2 and converts it into water, thus chemically removing the substrate for photorespiration. (E) The bundle sheath cells primarily serve to regenerate the enzyme PEP carboxylase, a process that is independent of O2 concentration and requires only the minimal ATP produced by cyclic flow.

48. A specific locus in a large, sexually reproducing deer mouse population is analyzed. Researchers find that the locus exhibits high nucleotide variability (many base-pair differences across individuals) but extremely low gene variability (less than1% heterozygosity). Assuming this population is large and mating is random, which explanation best accounts for the coexistence of these two variability measurements? (A) The high nucleotide variability is solely maintained by directional selection favoring one homozygous genotypе. (B) Genetic drift has caused the rapid fixation of the most common allele, simultaneously eliminating all new nucleotide mutations. (C) The majority of the nucleotide differences occur in noncoding regions (introns) or represent silent substitutions, resulting in few distinct functional alleles (low gene variability). (D) Balancing selection is actively maintaining two or more forms in the population, contradicting the observed low heterozygosity. (E) Gene flow is consistently introducing new alleles that are immediately selected against before they can be replicated within the population. NTHU115

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