Practice Test 4 - MCAT Biological and Biochemical Foundations of Living Systems

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

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1. In a controlled cross, a true-breeding tall plant (TT) is crossed with a true-breeding short plant (tt). All F1 offspring are tall. The F1 plants are then crossed with short plants (tt). Based on Mendelian inheritance, which outcome would be expected according to Mendel's laws among the resulting offspring?
2. A study examined a peptide hormone secreted by pancreatic islet cells in response to elevated plasma glucose. In isolated hepatocytes, adding the hormone increased phosphorylation of a cytosolic kinase within 2 minutes and increased glycogen synthesis by 20 minutes. In vivo, infusion of the hormone lowered plasma glucose; subsequently, secretion of the hormone from the pancreas decreased. Which statement best explains the mechanism of action for this hormone and the observed regulation?
3. A developmental anomaly is induced in a zebrafish embryo by briefly disrupting cell adhesion during gastrulation. Histology later shows that the embryo forms an external epithelium and an internal gut-like tube, but the space between them contains few differentiated tissues and lacks organized connective tissue matrices. The investigators conclude that one germ layer formed relatively normally while another was selectively reduced. Which outcome would be expected if the middle germ layer is selectively reduced during gastrulation?
4. A neuroscience lab compared signal transmission in two cultured tissues: (i) a network of neurons (excitable cells with long processes) supported by glial cells (non-neuronal support cells), and (ii) neurons cultured with glia depleted. Electrical stimulation was delivered to one region and activity was recorded at a distant region. The glia-depleted cultures showed reduced sustained firing and increased extracellular potassium accumulation near active regions. Extracellular potassium was measured with an ion-sensitive electrode. Which statement best explains the role of glial organization in this system?
5. In a controlled experiment, subjects inhaled a low concentration of carbon monoxide (CO) for a brief period while maintaining normal ventilation. Investigators monitored pulse oximetry and arterial $P_{O_2}$. Despite symptoms of hypoxia, arterial $P_{O_2}$ remained near normal. Which finding best explains the discrepancy between near-normal arterial $P_{O_2}$ and impaired oxygen delivery?
6. In a climate-chamber study, healthy volunteers (n=12) were rapidly moved from $22^\circ$C to $10^\circ$C air for 20 minutes. Core temperature remained near baseline ($37.0\pm0.1^\circ$C), while skin temperature fell. Mean arterial pressure did not change. The primary effector response observed was increased shivering and reduced skin blood flow. Which of the following best describes the feedback mechanism illustrated?
7. A lab compared senescence markers in epithelial cells from three groups: young adults, older adults, and older adults taking a drug that reduces cell proliferation in the epithelium. The drug group showed slower telomere shortening over a year but also slower wound healing after biopsy. Which outcome is expected concerning aging?
8. A lab quantified chromosome alignment after selectively depleting the kinetochore motor CENP-E using siRNA. In depleted cells, time from nuclear envelope breakdown to metaphase plate formation increased, and a subset of chromosomes remained near spindle poles with low inter-kinetochore stretch (low tension) despite being attached to microtubules. Control cells showed rapid congression and higher inter-kinetochore stretch at metaphase. Which statement best describes the role of CENP-E during mitosis as supported by these observations?
9. A researcher studies replication across a repetitive microsatellite (e.g., CACACACA…) using an in vitro system. When a protein that stabilizes exposed single-stranded DNA is depleted, products show a higher frequency of small insertions and deletions localized to the repeat tract. Which statement is most consistent with the mechanism underlying these errors during replication?
10. A researcher compares conduction in two axons of equal diameter: Axon 1 is unmyelinated; Axon 2 is myelinated with regularly spaced nodes of Ranvier. A toxin is applied that increases the capacitance of the axonal membrane without changing ion channel densities. The toxin slows conduction much more in Axon 1 than in Axon 2. Which mechanism best explains why the myelinated axon is less affected?
11. A lactating parent reports that milk letdown becomes more reliable as breastfeeding frequency increases. Nipple stimulation precedes transient increases in oxytocin and milk ejection, which further encourages continued feeding. Which of the following best describes the feedback mechanism illustrated?
12. A lab studied the arabinose (ara) operon, which can be activated by AraC in the presence of arabinose but can repress transcription in its absence by looping DNA. Cells were grown with or without arabinose, and ara mRNA was measured. A mutant AraC cannot bind arabinose but can still bind DNA. Based on the scenario, which outcome would be expected given the gene regulation process?
13. Researchers monitored chromosome segregation in a rod-shaped bacterium using fluorescent tags. When a drug disrupted the actin-like protein MreB, cells became spherical and showed frequent mis-segregation of the circular chromosome during division, even though DNA replication initiation remained normal. No nucleus was observed. Core cell theory idea: hereditary information is transmitted to daughter cells. Based on the vignette, which structure is most critical for the prokaryote's function in reliably passing genetic material to progeny?
14. A patient-derived fibroblast line has a defect in a nuclear envelope protein required for proper nuclear pore complex distribution. The nucleus is a membrane-bound organelle that separates transcription (RNA synthesis) from cytosolic translation. These cells show accumulation of polyadenylated mRNA within the nucleus and decreased cytosolic mRNA. Based on the scenario, what is the most direct consequence of disrupted nuclear-cytosolic compartmentalization?
15. In an in vitro replication system, a researcher supplies a circular DNA template and observes that replication proceeds normally until a single-strand break (nick) is introduced ahead of the replication machinery on one template strand. After introduction of the nick, replication products include many truncated fragments, even though nucleotide concentrations and polymerase activity are unchanged. Based on the mechanism of replication, which outcome is most consistent with converting a nick into a more severe lesion during copying?
16. A lab examined volume regulation in cultured cells placed into a hypertonic medium made by adding impermeant mannitol. Within minutes, cells shrank. Over the next 30 minutes, cell volume partially recovered despite continued hypertonic conditions. The principle being tested is regulatory volume increase (RVI): cells can restore volume by increasing intracellular osmolytes via ion transport, which then drives water re-entry. Which response would be expected under the given conditions?
17. A study evaluated subjects with anemia but normal lung function. Arterial blood gases showed normal $P_{O_2}$. Despite this, tissue hypoxia was suspected during exercise. Which statement best explains this apparent discrepancy in terms of gas exchange versus transport?
18. Investigators studied limb regeneration in an adult salamander species following standardized mid-forelimb amputation. Animals were treated locally with a short interfering RNA (siRNA) targeting gene Rgx1, a marker enriched in the early blastema. Compared with scrambled siRNA, Rgx1 knockdown animals formed a wound epithelium but showed markedly reduced outgrowth over 21 days, with persistent scar-like tissue at the stump. No increase in systemic inflammatory markers was detected. Based on the vignette, which conclusion is most consistent with regeneration?
19. Researchers infused saline into the pulmonary interstitium of an isolated lung, producing mild interstitial edema without airway obstruction. Ventilation and perfusion were held constant. Which change would be expected if edema primarily impairs diffusion?
20. A biomechanics lab examined articular cartilage, a tissue rich in chondrocytes (cartilage cells) embedded in an extracellular matrix. They compared intact cartilage with cartilage in which the matrix was enzymatically softened while chondrocyte viability remained high. Under compressive loading, softened samples deformed more and recovered more slowly. Deformation was measured as thickness change over time under constant load. Which statement best explains the role of cartilage tissue organization in joint function?

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