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Practice Test 58 - AP Biology

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

  1. An enzyme has two acidic residues in its active site that must be deprotonated to bind substrate. Activity is high at pH 9 and low at pH 5 under identical conditions. Which explanation best accounts for this pattern?
  2. In a sequencing lab, a double-stranded nucleic acid sample shows consistent A–T and G–C pairing. When the sample is heated, strands separate; upon cooling, they rejoin only when their 5′→3′ directions are opposite. The technician notes that the sugar-phosphate backbones remain intact while hydrogen bonds break and reform between bases. Which structural feature best explains why reannealing requires opposite 5′→3′ orientations of the two strands?
  3. A muscle cell contains mitochondria with densely folded inner membranes compared with a nearby connective tissue cell. Both cells have similar outer mitochondrial membrane surface area. Which outcome is most likely associated with the muscle cell's mitochondrial inner membrane structure?
  4. In a cell, ligand activates an RTK that stimulates phosphorylation of a membrane transporter, increasing transporter activity and importing amino acid X. Rising cytosolic X binds an allosteric site on the transporter and decreases its transport rate even though the transporter remains phosphorylated. A transporter mutant that cannot bind X shows continued high import after ligand addition. Which change would most likely result from this mutation?
  5. A plant species has two populations that overlap geographically. Cross-pollination experiments show that pollen from population A lands on stigmas of population B and germinates, but pollen tubes usually fail to reach ovules; the reciprocal cross shows the same pattern. Within-population crosses produce seeds at high rates. Field observations show pollinators visit both populations, and flowering times overlap. Allele frequencies differ between populations at multiple loci. Which reproductive barrier most directly limits gene flow between the populations?
  6. In a signaling assay, Ligand A binds a cell-surface receptor and rapidly increases cytosolic IP3_3. When cells are treated with a drug that inhibits phospholipase C (PLC), Ligand A still binds the receptor but IP3_3 does not increase. No changes in receptor abundance are detected during the experiment. Which of the following best predicts the role of PLC in early transduction for this pathway?
  7. Two populations of the same marine invertebrate release gametes into the water. They live in the same bay and spawn on the same nights. Laboratory trials show that eggs from population 1 are rarely fertilized by sperm from population 2, and vice versa, even when gamete concentrations are high. Genetic analysis indicates divergence in proteins on sperm surfaces and egg membranes. Which isolating mechanism most directly prevents gene flow between the populations?
  8. A yeast population was grown with a fixed amount of sugar. Cell counts were: 0 hr = 500, 2 hr = 900, 4 hr = 1,600, 6 hr = 2,400, 8 hr = 2,550, 10 hr = 2,560. Measurements showed sugar concentration steadily declined and was near zero by 8 hours. Which pattern best illustrates the yeast population growth across the 10 hours?
  9. In a herd of cattle, a breeder uses artificial insemination from only the top 5% of bulls for muscle mass each year. Muscle mass is influenced by alleles M (higher mass) and m (lower mass) at a locus. Over 10 years, the frequency of allele M rises from 0.30 to 0.75 in calves. Which statement best describes why allele M increased in frequency?
  10. A phosphatase that removes phosphate groups from mitotic substrates becomes active when Cdk1 activity falls. In normal cells, APC/C-mediated destruction of M-cyclin lowers Cdk1 activity, allowing the phosphatase to reverse mitotic phosphorylation. A mutation makes the phosphatase constitutively active even when Cdk1 remains high. Which effect is most likely in cells with the constitutively active phosphatase?
  11. A unicellular organism is observed under two conditions. In Condition 1, the cell remains small (diameter ~8 µm). In Condition 2, the cell grows larger (diameter ~16 µm) without changing membrane composition. In both conditions, the cell relies on diffusion across the plasma membrane for CO₂ removal, and CO₂ production occurs at the same rate per unit volume. The larger cell accumulates more CO₂ internally. Which explanation best accounts for this observation?
  12. In a meiosis simulation, homologous chromosomes pair and form tetrads. The instructor highlights non-sister chromatids exchanging corresponding segments at a visible chiasma during prophase I. After meiosis I and II, two of the four gametes contain chromatids with a segment from the other homolog, while the other two gametes match the original parental chromatids. Assume genes on that chromosome remain in the exchanged segment and no mutations occur. Which outcome best explains how this event generates genetic variation among gametes at the chromosomal level?
  13. At a replication fork, DNA polymerase synthesizes new DNA 5′→3′ by extending from a 3′ end, using each parental strand as a template. Because the templates are antiparallel, one new strand is synthesized continuously and the other discontinuously as fragments. A mutation causes DNA polymerase to add nucleotides only very slowly but does not affect strand separation. Which outcome is most likely at replication forks in the mutant cells?
  14. A desert plant population has heritable variation in leaf waxiness (low, medium, high). During a 20-year drought, plants with low waxiness produced 0.7 viable seeds per plant, medium waxiness produced 1.3, and high waxiness produced 2.4. Over these generations, the mean waxiness increased and the allele associated with high waxiness rose from 0.25 to 0.58. Which outcome best describes the evolutionary change in this population?
  15. Which sequence has self-complementary ends that could form a hairpin?
  16. Two species of barnacles settle on the same intertidal rocks. In the upper intertidal zone, species X is common and species Y is rare. In the lower intertidal zone, species Y is common and species X is present only when species Y is experimentally removed. When species Y is removed from the lower zone, species X increases in abundance there, while wave exposure and temperature remain similar. Which interaction best explains the pattern in the lower intertidal zone?
  17. A river has two reaches of similar length. In Reach 1, macroinvertebrates included 14 taxa with no single taxon exceeding 20% abundance. In Reach 2, 5 taxa were found and one taxon comprised 85% abundance. Water temperature was similar in both reaches. Which conclusion is best supported about biodiversity in the river?
  18. A population of snails shows heritable variation in shell banding pattern caused by alleles M (banded) and m (unbanded). A new predator is introduced that more easily detects banded snails against the local substrate. After several generations, the frequency of m increases. Which statement best explains how variation affected the evolutionary trajectory of the snail population?
  19. In a cat breed, tail length shows incomplete dominance: allele T produces long tails, allele t produces short tails, and heterozygotes (Tt) have medium tails. Two medium-tailed cats are crossed (Tt × Tt). Which prediction best matches the offspring?
  20. Pepsin activity is tested with identical enzyme and protein substrate concentrations at 37°C. In solution A (pH 2), the reaction rate is high; in solution B (pH 7), the reaction rate is very low. The difference is attributed to pH effects on amino acid side-chain charges that stabilize the active site. Which explanation best accounts for pepsin's low activity at pH 7?

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