Practice Test 7 - 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. A germ cell was labeled so that maternal homologs fluoresced green and paternal homologs fluoresced red. At the end of meiosis I, each daughter cell contained a mix of green and red chromosomes across different pairs. No recombination was measured. Which statement best reflects the genetic variation introduced by meiosis in this observation?
2. A neuron is tested for excitability while extracellular Na$^+$ is reduced, with osmolarity maintained by replacing Na$^+$ with an impermeant cation. The threshold for firing increases, and action potentials that do occur have reduced peak amplitude. Which mechanism best explains these observations?
3. In a cell-free system (150–250 words), investigators reconstituted the oxidative PPP using purified enzymes and supplied saturating G6P. When NADP$^+$ was limiting, NADPH production was low; when NADP$^+$ was increased, NADPH production rose proportionally until plateau. ATP and ADP were absent from the mixture. Entry step: G6P + NADP$^+$ $\xrightarrow{\textit{G6PD}}$ 6-phosphogluconolactone + NADPH Which outcome is most consistent with the observed dependence on NADP$^+$ availability?
4. In a physiology lab, two blood samples are compared at the same $P_{O_2}$. Sample 1 has normal 2,3-BPG levels; Sample 2 has experimentally increased 2,3-BPG in red cells. Which result is most expected for oxygen delivery to peripheral tissues?
5. A lab used a fluorescent reporter for cyclin B to track mitotic exit. In control cells, cyclin B signal dropped sharply at anaphase onset and remained low through telophase. In cells treated with a proteasome inhibitor, cyclin B remained high, chromosomes separated incompletely, and cells failed to decondense chromosomes or re-form nuclei efficiently. Based on the study, which conclusion is most consistent with the data?
6. In thyroid follicular cells, investigators track transport of two hormones: TSH (a glycoprotein peptide) and T3 (a thyroid hormone). In serum-like medium containing albumin, T3 is mostly protein-bound, whereas TSH remains largely unbound. Cells are exposed to equal total concentrations of TSH and T3. TSH produces a rapid increase in intracellular cAMP, while T3 produces no immediate change in cAMP but increases transcription of a metabolic gene after several hours. Which statement best accounts for these observations?
7. Researchers are engineering a cytosolic enzyme that functions as a homodimer (two identical subunits). Activity assays show that wild-type enzyme activity increases steeply with protein concentration, consistent with dimer formation being required for catalysis. A mutant enzyme is expressed at similar levels and shows normal circular dichroism consistent with preserved secondary structure, but it remains mostly monomeric by size-exclusion chromatography and has low activity. Which change in the protein's quaternary structure would most likely affect its function in this system?
8. Investigators measured cyclin levels in paired samples from patients: adjacent non-tumor tissue vs. tumor tissue. They found that cyclin E protein was markedly elevated in tumors, while cyclin D was unchanged. In a functional assay, tumor cells entered S phase even under low growth factor conditions that normally keep cells in G1. Based on the data and known checkpoint logic, which conclusion is most consistent with cell cycle regulation in these tumors?
9. Investigators infuse isotonic saline intravenously into healthy volunteers. Shortly after infusion, plasma volume increases and atrial stretch increases. What outcome is most consistent with the given physiological response that returns extracellular fluid volume toward baseline?
10. In Arabidopsis thaliana, researchers examined recombination near a centromere-proximal region containing two linked markers (M and N). Plants were heterozygous at both loci and produced pollen under two conditions: control and a treatment that increased chromatin compaction in early prophase I without altering DNA replication timing. The treated plants showed a decreased frequency of recombinant pollen haplotypes for M–N, while segregation of an unlinked marker on another chromosome remained Mendelian. Which interpretation is most consistent with meiosis-specific mechanisms generating genetic variation in this experiment?
11. In a brainstem reflex circuit, an afferent sensory neuron makes an excitatory synapse onto an interneuron, which then makes an inhibitory synapse onto a motor neuron. In response to a brief sensory stimulus, the motor neuron normally shows a transient hyperpolarization (inhibitory postsynaptic potential, IPSP) starting 6 ms after the stimulus. A drug is applied that selectively blocks GABA$_A$ receptors on the motor neuron but does not affect GABA release or GABA$_B$ receptors. Which function is most consistent with the observed change in motor neuron response after drug application?
12. A lab compares two 18-nt DNA oligonucleotides annealed to the same complementary strand. Oligo X differs from Oligo Y only by replacing three A–T base pairs with three G–C base pairs in the center of the duplex. Under identical buffer conditions, Oligo X shows a higher melting temperature. Based on base pairing principles, which statement best accounts for this result?
13. A researcher mutates a protein to introduce a new salt bridge on the surface at pH 7.4. They place $R = \text{CH}_2\text{-COO}^-$ at one site. Which residue at a nearby site is most consistent with forming the intended ionic interaction?
14. During repetitive firing, a neuron shows a progressive decrease in action potential upstroke velocity ($dV/dt$) while resting potential remains slightly depolarized. No change in extracellular Na+ is measured. Which statement best describes the role of ion channels in action potential propagation that explains the reduced upstroke velocity?
15. An experiment measures iron absorption in the duodenum under two conditions: normal luminal pH (~6) versus higher luminal pH (~8) after reduced gastric acid delivery. Provided definition: many dietary iron forms are more soluble at lower pH, which can increase availability for absorption. Which result would be expected when luminal pH increases to ~8?
16. Researchers monitored lac operon expression using β-galactosidase activity after adding lactose to E. coli grown either with or without glucose. In wild-type cells, lactose induced high activity only when glucose was absent. A mutant strain expresses a constitutively active adenylate cyclase that maintains high intracellular cAMP even in glucose. Which observation is most consistent with the regulatory mechanism described?
17. In a dehydration study, investigators administer a drug that blocks angiotensin II receptors (ARBs). Subjects still show increased plasma osmolality and increased ADH, but their ability to maintain blood pressure during dehydration is reduced. Based on the information, which outcome is most likely regarding renal handling of Na$^+$ and water during dehydration with ARB treatment?
18. In a zebrafish (Danio rerio) breeding experiment, a female heterozygous for a dominant fin phenotype (Ff) is crossed to a male with the recessive phenotype (ff). Embryos are collected early and genotyped; however, a subset show monosomy for the chromosome carrying the F locus. Which prediction is most consistent with the chromosomal basis of inheritance for those monosomic embryos?
19. An experimental therapy aims to restore checkpoint control in a cancer line with intact p53 but low p21 expression due to promoter methylation. Treatment with a DNA-demethylating agent increases p21 mRNA and decreases CDK2 activity. After treatment, cells show a larger G1 population and reduced colony formation. Which conclusion is most consistent with these results about cell-cycle regulation in cancer?
20. A storm colonizes a small, isolated island with 12 founding mice from a mainland population where allele A at a coat-color locus has frequency 0.50. No further migration occurs. After one generation on the island, allele A frequency is measured at 0.83, with no evidence that either coat color affects survival or mating success. Based on the scenario, which outcome is most consistent with genetic drift?

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