Practice Test 5 - MCAT Biological and Biochemical Foundations of Living Systems

20 questions. Answer them in any order, then submit to see your score and every explanation.

Investigators measured initial velocities of purified human phosphofructokinase-1 (PFK-1) at 37°C, pH 7.4, with saturating ATP (5 mM) and varying fructose-6-phosphate (F6P). In the presence of 2 mM citrate, the apparent $K_{0.5}$ for F6P increased, while $V_{max}$ was unchanged. Which interpretation best describes citrate's effect on PFK-1 under these conditions?
A coding sequence contains the segment 5'-...GGA UUU CCA...-3'. A mutation changes the middle codon from UUU to UUA. The lab reference lists: UUU → Phe; UUA → Leu. Which effect is most consistent with the described mutation?
In a developmental model, a morphogen-like signal was applied uniformly to a population of progenitor cells, but only a subset differentiated into Cell Type T. Single-cell analysis showed that cells that became Cell Type T had higher baseline expression of receptor R before morphogen exposure. When receptor R was experimentally overexpressed in all cells, the fraction differentiating into Cell Type T increased. Cellular principle assessed: differential responsiveness to the same extracellular signal can arise from differences in receptor expression, leading to distinct cell fates. Which outcome would be expected during differentiation under uniform morphogen exposure?
A lab compares two inocula introduced into separate host systems: purified prion aggregates added to mouse brain slice cultures and purified viroid RNA introduced into plant protoplasts. Over time, the brain slices show increasing detergent-insoluble PrP and synaptic loss without changes in PrP mRNA, while the plant protoplasts show increasing viroid RNA and a rise in 21–24 nt small RNAs mapping to the viroid sequence. Which conclusion best links each subviral particle to its primary mode of propagation and downstream cellular effect?
In an experiment on stress physiology, subjects receive an acute infusion of a hormone secreted by the adrenal medulla. The hormone is water-soluble and produces increased heart rate and bronchodilation within seconds to minutes. In target tissues, the response is blocked by a competitive antagonist of a G protein–coupled receptor. Plasma levels of the hormone fall rapidly after infusion stops. Based on the vignette, which statement best explains the expected receptor location and signaling properties? (Assume no changes in hormone synthesis during the infusion.)
In an experiment on synaptic delay, a presynaptic action potential is evoked at time $t=0$ . The postsynaptic response begins at $t=1.5$ ms. A drug is applied that blocks SNARE complex function required for vesicle fusion but does not affect presynaptic action potentials. What outcome would be expected after SNARE blockade?
Researchers infect an E. coli strain (lacZ−, streptomycin-resistant) with a lytic bacteriophage grown previously in a donor strain (lacZ+, streptomycin-sensitive). After a short infection, the culture is plated on medium containing streptomycin and X-gal. Rare blue colonies appear among mostly white colonies. The investigators conclude the phage occasionally packages host DNA by mistake. Which statement best describes the process of viral transduction in this scenario?
In an embryology teaching lab, students observe a time-lapse of early development. At time point 1, the embryo is a blastula with a single outer cell layer surrounding a fluid-filled cavity. At time point 2, cells move inward and the embryo reorganizes into three layers with a new internal cavity continuous with the outside at a single opening. Baseline: gastrulation transforms a blastula into a gastrula and establishes ectoderm, mesoderm, and endoderm. Based on the observations, which process is most consistent with the transition from time point 1 to time point 2?
In a study of intestinal carbohydrate digestion, a participant lacks functional lactase at the brush border. After ingesting lactose, breath hydrogen rises and stool becomes watery. The concept being tested is carbohydrate structure, digestion, and osmotic effects. Which statement best explains the watery stool? Disaccharide: lactose = galactose-$$14-glucose.
A developmental biology group tracked formation of the neural tube in vertebrate embryos. They observed that a sheet of neuroepithelium (epithelial tissue whose cells give rise to nervous system structures) bent into a tube only when cells maintained tight, continuous apical contact along the midline. When apical continuity was disrupted, the sheet remained flat despite similar cell proliferation. Apical continuity was defined as uninterrupted cell-cell contact at the tissue surface facing the embryo exterior. Which outcome is most consistent with the described tissue development?
An experiment tests the effect of inhibiting RNA polymerase II CTD Ser5 phosphorylation (required early in transcription) on expression of a detoxification enzyme gene. After inhibitor treatment, total nascent RNA from the gene decreases, and the remaining transcripts show reduced 5' capping efficiency. Which additional change would be expected as a direct consequence of reduced Ser5 phosphorylation?
A researcher expresses a mutated nonselective cation channel in a cell line. In whole-cell recordings, opening the channel increases permeability equally to Na $^+$ and K $^+$ , with negligible permeability to Cl $^-$ . Under control conditions, the membrane potential is $-65\ \text{mV}$ . Ion concentrations (mM) are Na $^+$ in/out = 15/145 and K $^+$ in/out = 140/5. Based on the scenario, what effect would increasing this channel's permeability have on the membrane potential?
In a genetic stability assay, germ cells were exposed to a compound that specifically prevents formation of the physical connections between homologous chromosomes that normally appear after pairing. The cells still completed two divisions. Based on the information, which conclusion is most consistent with gamete formation and genetic variation in the produced gametes?
In an experimental design to test organelle-specific protein targeting, a lab fused GFP to a candidate enzyme suspected to reside in the mitochondrial matrix. Mitochondria are membrane-bound organelles with an outer and inner membrane; many matrix proteins are encoded in the nucleus and imported post-translationally via an N-terminal mitochondrial targeting sequence (MTS), an amphipathic helix recognized by import machinery. The lab created two constructs: GFP-enzyme (intact MTS) and GFP-enzyme-ΔMTS (MTS deleted). Cells expressing GFP-enzyme showed punctate fluorescence co-localizing with a mitochondrial dye, while GFP-enzyme-ΔMTS showed diffuse cytosolic fluorescence. Which outcome best exemplifies the role of the MTS in mitochondrial compartmentalization?
A patient with HIV has a markedly reduced CD4+ T-cell count. After vaccination with a protein antigen, they generate low titers of class-switched antibodies. Which statement best reflects the immune response described?
In an ex vivo perfused-lung experiment, investigators decreased alveolar surface area by selectively collapsing a fraction of alveoli while keeping the remaining alveoli ventilated and perfused. Total pulmonary blood flow and inspired O $_2$ fraction were held constant. Which result is most consistent with this manipulation?
A hospital lab tests decontamination methods for a suspected prion-contaminated instrument. After standard UV irradiation and DNase/RNase treatment, a surface swab still seeds conversion of a recombinant host protein in a misfolding assay. After treatment with a strong protein-denaturing protocol, seeding activity is markedly reduced. The implicated agent lacks detectable nucleic acid. Which outcome would be expected in patients exposed to residual contamination if infection occurs?
A genetic variant in a secreted digestive enzyme replaces a Cys with Ser. The enzyme loses activity in the intestinal lumen, but mass spectrometry confirms the full-length polypeptide is produced and secreted. Which statement best distinguishes the role of peptide bonds in primary structure from the effect of the Cys→Ser change? (Assume the wild-type enzyme contains disulfide bonds.)
To assess ketone production capacity, researchers compare fasting plasma metabolites in two groups: individuals with a confirmed mitochondrial HMG-CoA synthase deficiency (ketogenesis defect) and matched controls after 18 hours of fasting. Plasma glucose is mildly decreased in both groups. Observations: - Controls: elevated ;2-hydroxybutyrate; moderate rise in plasma FFAs - HMG-CoA synthase deficiency: low ;2-hydroxybutyrate; marked rise in plasma FFAs Based on the experimental setup, what is the most likely metabolic consequence of the mutation during fasting?
A bacterium becomes resistant to an aminoglycoside that normally binds the 30S ribosomal subunit and causes mistranslation. Sequencing of resistant clones shows a single nucleotide change in a gene encoding a 30S ribosomal protein; no new plasmids are detected. Protein synthesis rates in the absence of antibiotic are near normal. Based on these findings, which genetic change is most consistent with the resistance observed?

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