Practice Test 11 - 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 cytosolic enzyme (EnzA) functions as a homodimer; activity requires formation of a continuous substrate channel that spans the quaternary interface. In a folding screen, wild-type EnzA shows normal catalytic rate, but a variant with a single substitution at the dimer interface shows normal monomer stability (unchanged melting temperature by DSC) yet markedly reduced $V_{max}$ with unchanged $K_m$. Based on the quaternary structural requirement, what is the most likely consequence of the substitution?
2. A patient has reduced secretion of secretin from the duodenum in response to acidic chyme. Definitions: secretin increases pancreatic bicarbonate secretion; bicarbonate neutralizes gastric acid to raise duodenal pH; many pancreatic enzymes function best near neutral pH. Which result would be expected if secretin signaling is decreased?
3. A researcher claims that because the plasma membrane is a bilayer, all membrane proteins must span the bilayer and cannot be removed without detergent. Which statement best reflects the fluid mosaic model and corrects this claim?
4. Denaturation kinetics of a purified enzyme were measured at 45°C by tracking loss of activity over time. The decay followed first-order kinetics, and the apparent rate constant $k_{obs}$ was determined under two conditions: Buffer A (pH 7.4) and Buffer B (pH 3.0). The enzyme concentration was identical in both. Based on the data, which inference about protein stability is most supported?
5. Investigators compared translation initiation on two otherwise identical mRNAs differing only in their 5' untranslated region (UTR). The mRNA with a stable 5' UTR secondary structure showed reduced protein output and fewer ribosomes per mRNA in polysome profiling. Which process is most likely involved in the reduced translation of the structured 5' UTR mRNA?
6. A bacterial isolate is resistant to an antibiotic that targets an enzyme required for folate synthesis. Enzyme assays show the resistant enzyme has normal catalytic activity but reduced antibiotic binding. Sequencing identifies a single amino acid substitution near the enzyme's active site. Which genetic change is most consistent with the resistance observed?
7. In a controlled heat exposure (38°C), two groups are compared: Group 1 is well hydrated; Group 2 is mildly dehydrated after 12 hours of fluid restriction. Both groups show similar initial increases in skin blood flow, but Group 2 exhibits a smaller increase in measured sweat rate and a greater rise in core temperature over 30 minutes. The central thermoregulatory principle is eccrine sweating for evaporative heat loss. Which outcome is most consistent with the observed difference in Group 2?
8. A 6-month-old presents with developmental delay and recurrent episodes of vomiting after high-protein feeds. Plasma shows elevated leucine, isoleucine, and valine; urine has a characteristic sweet odor. Sequencing reveals a loss-of-function mutation in the E1 subunit of branched-chain $\alpha$-ketoacid dehydrogenase (BCKDH). The biochemical concept is impaired oxidative decarboxylation leading to upstream metabolite accumulation. Based on this enzyme defect, which outcome is most likely?
9. A researcher compares two perfectly complementary 8-bp DNA duplexes. Duplex 1 has sequence 5′-ATATATAT-3′ (with its complement). Duplex 2 has sequence 5′-GCGCGCGC-3′ (with its complement). Under the same ionic strength, which conclusion about relative melting temperature ($T_m$) is most consistent with base pairing and hydrogen bonding principles?
10. A retrovirus with an RNA genome infects a population of dividing human cells. In one condition, a small-molecule inhibitor of integrase is added immediately after viral entry; in another condition, the inhibitor is added 24 hours later. Only the early-addition condition markedly reduces the number of cells with long-term viral gene expression. Which outcome would be expected during the viral replication cycle described?
11. A group tested how kinetochore tension relates to checkpoint satisfaction. Cells expressing a mutant kinetochore protein showed normal microtubule attachment frequency, but inter-kinetochore distance remained low and SAC proteins persisted at kinetochores. These cells delayed anaphase onset despite apparent attachment. Based on the study, which conclusion is most consistent with the data?
12. Investigators compared two hormones regulating calcium balance: Hormone X is a peptide secreted by the thyroid and Hormone Y is a lipid-soluble hormone secreted by parathyroid tissue. In target cells, Hormone X produced measurable effects within 2 minutes, while Hormone Y required 4–6 hours to change protein abundance. Hormone Y's effects were abolished by a drug that blocks receptor binding to DNA. Based on the vignette, which outcome is most consistent with hormone class action?
13. Researchers test whether DNA methylation regulates the eukaryotic gene CDH9. In Cell Line A, CDH9 mRNA is low and bisulfite sequencing shows heavy CpG methylation at the CDH9 promoter. In Cell Line B, CDH9 mRNA is high and the promoter is largely unmethylated. Line A is treated with a DNA methyltransferase inhibitor for 5 days; promoter methylation decreases and CDH9 mRNA increases. Which mechanism best explains the regulation observed?
14. Failure of neural crest delamination would most directly reduce?
15. A cohort of healthy volunteers underwent controlled dehydration (24 hours fluid restriction) with no change in red blood cell (RBC) mass. Venous blood was sampled at baseline and after dehydration. The analyzer reported: hematocrit increased from 42% to 47%, plasma total protein increased, and measured plasma osmolality increased. Based on the vignette, which conclusion is most consistent with blood composition changes during dehydration?
16. A retrovirus infecting human cells is engineered so its long terminal repeats (LTRs) remain intact after integration. The integrated viral DNA is found near the start site of a host proto-oncogene. Weeks later, the infected cell line shows increased transcription of that proto-oncogene compared with uninfected controls. Based on the vignette, which conclusion is most consistent with retroviral integration as the cause of the observed change?
17. A bacterial strain shows reduced translation efficiency for a gene enriched in codons ending in A or U. Sequencing reveals no coding changes, but a mutation is found in a tRNA gene: its anticodon 5'-GCI-3' (I = inosine) is replaced with 5'-GCC-3'. The affected codon family is 5'-GCU/GCC/GCA/GCG-3' (Ala). Which mechanism best explains the observed translation defect, consistent with the wobble hypothesis?
18. Researchers differentiated human induced pluripotent stem cells (iPSCs) into two lineages using brief exposure to distinct growth-factor cocktails. RNA-seq showed that Lineage A strongly expressed MYH6 and TNNT2, while Lineage B expressed ALB and TTR. Whole-genome sequencing confirmed the iPSCs and both lineages had identical DNA sequence. ATAC-seq revealed increased chromatin accessibility at the MYH6 promoter only in Lineage A and at the ALB promoter only in Lineage B. Cellular principle assessed: differential gene expression driven by chromatin state. Which mechanism best explains the development of these different cell types from the same genome?
19. A laboratory maintains a large population of fruit flies at an autosomal locus with alleles D and d. Each generation, technicians introduce 2% new individuals from a separate stock in which $p(D)$ is substantially higher. No viability differences among genotypes are detected in controlled assays, and mating within the cage is random. Hardy-Weinberg equilibrium assumes: (i) no migration (gene flow), (ii) random mating, (iii) no selection, (iv) no mutation, and (v) very large population size. Which condition would most disrupt Hardy-Weinberg equilibrium in this population?
20. A biotechnology team introduced a plasmid encoding a metabolic enzyme into a bacterial population. After 30 generations without antibiotic selection, many cells retained the enzyme activity, but plasmid DNA became undetectable in a subset; whole-genome sequencing revealed the enzyme gene integrated into the chromosome in those cells. The researchers argued that this result supports a key idea about heredity in cellular organisms. Which conclusion most directly applies cell theory to the prokaryotic observations?

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