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Practice Test 3 - AP Biology

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

  1. A mammalian gene has a CpG-rich promoter. When promoter cytosines are methylated, a methyl-binding protein recruits chromatin remodeling factors that increase nucleosome packing, and mRNA levels are low. In a treated cell population, promoter methylation increases while transcription decreases. Which change would most likely increase transcription without altering the gene's coding sequence?
  2. In smooth muscle cells, ligand Z binds a GPCR that activates Gs, increasing cAMP and activating PKA. PKA phosphorylates myosin light-chain kinase (MLCK), decreasing MLCK activity and reducing contraction. A drug that blocks PKA prevents MLCK phosphorylation and contraction remains high after Z addition. Which manipulation would most likely restore Z-dependent reduction in contraction while PKA remains blocked?
  3. An enzyme is tested with two molecules: Substrate X and Substrate Y. With equal enzyme and substrate concentrations, the initial rate is high for X but near zero for Y. Both molecules are stable under assay conditions, and no inhibitor is added. The enzyme's activity returns when X is reintroduced after exposure to Y. Which explanation best accounts for the difference in reaction rates?
  4. In an experiment, ligand Z binds to a receptor and causes rapid opening of an ion channel, producing a measurable influx of Na+^+. A competitive antagonist is added that occupies the receptor's ligand-binding site without activating the receptor. After antagonist treatment, Z no longer triggers Na+^+ influx. Which of the following best explains the antagonist's effect on reception?
  5. In a salt marsh, periwinkle snails graze on cordgrass leaves. Researchers placed cages that excluded snails from some plots for 6 weeks. Exclusion plots developed thicker cordgrass stems and greater leaf cover than uncaged plots. In uncaged plots, more bare sediment was visible and cordgrass leaves showed grazing scars. No other herbivores were observed, and soil salinity and flooding frequency were similar across plots. Which interaction best explains the difference between caged and uncaged plots?
  6. In cultured liver cells, the peptide hormone H binds a specific protein on the outer surface of the plasma membrane. Within 10 seconds of adding H, cytosolic cAMP increases. When cells are pretreated with a drug that prevents the receptor from activating a nearby G protein, H still binds the receptor but cAMP does not increase. No changes in total receptor amount are detected during the experiment. Which of the following best explains how the signal is initiated after H binds its receptor?
  7. After a severe drought, a grassland's flowering plant cover drops by 60% and insect pollinator counts drop by 50% the following month. Which consequence is most likely for the ecosystem over the next growing season?
  8. In yeast, pheromone F binds a GPCR on the plasma membrane. Shortly after binding, the G protein's beta-gamma subunits dissociate from the alpha subunit and interact with a downstream effector at the membrane. If a mutation prevents dissociation of the subunits, F binding still occurs but the effector is not activated. Which of the following best explains the earliest blocked step?
  9. In a classroom investigation, students shine a bright light on a fish in a tank. Within seconds, the fish swims to the darker side and remains there while the light is on. When the light is turned off, the fish resumes swimming throughout the tank. The water temperature and oxygen level remain constant during the test. Which response best explains the fish's movement when the light is turned on?
  10. An investigator compares diffusion of a nutrient into two spherical cells with the same membrane permeability. Cell S has radius 3 µm; Cell T has radius 9 µm. Both are placed in the same nutrient concentration, and both metabolize the nutrient at the same rate per unit volume. After reaching steady conditions, Cell T has a lower nutrient concentration in its central cytoplasm than Cell S. Which explanation best accounts for the lower central concentration in Cell T?
  11. In an insect population, wing color is determined by a single gene with two alleles: WDW_D (dark) and WLW_L (light). After a new bird predator becomes common, mark–recapture data over 10 generations show that dark-winged insects have higher survival and produce more offspring than light-winged insects. The frequency of WDW_D increases from 0.48 to 0.86, while WLW_L decreases accordingly. Which pattern best illustrates the evolutionary mechanism changing allele frequencies in this population?
  12. A researcher compares the forelimb bones of a bat, whale, cat, and human. Each forelimb contains a humerus, radius, ulna, carpals, metacarpals, and phalanges arranged in the same relative order, but the bones differ in length and shape. The researcher also notes that insect wings lack these bones and are made of cuticle extensions. Which conclusion is best supported by the forelimb evidence?
  13. A wax is composed of long-chain fatty acids ester-linked to long-chain alcohols, producing molecules that are largely nonpolar and lack charged or highly polar groups. A thin waxy coating on a leaf surface reduces water loss. Which statement best explains, at the molecular level, how the wax coating limits evaporation from the leaf?
  14. Scientists tested protocell formation by adding fatty acids to warm water containing dissolved salts similar to those expected in some early Earth pools. Within minutes, microscopy showed spherical vesicles with bilayer-like boundaries. When a fluorescent dye that cannot cross lipid membranes was added outside the vesicles, fluorescence remained outside; when dye was present before vesicle formation, fluorescence was observed inside many vesicles. Which conclusion is best supported by these observations about early steps in life's origin?
  15. In an experiment, cells are treated with ligand Y, which binds a receptor tyrosine kinase (RTK) in the plasma membrane. Within 1 minute, the receptor becomes phosphorylated on specific tyrosines. A drug that prevents receptor dimerization is added before Y; Y still binds, but receptor phosphorylation is not detected. Which of the following best explains why phosphorylation fails to occur?
  16. In a cultured animal cell, DNA replication completes during S phase, and the cell enters G2 with duplicated chromosomes. The cell is then exposed to a drug that prevents microtubules from attaching to kinetochores. When the cell enters M phase, chromosomes condense and the nuclear envelope breaks down, but kinetochores remain unattached. The spindle assembly checkpoint monitors attachment and tension before anaphase begins. After 45 minutes in mitosis, the cell still shows all sister chromatids paired at their centromeres and aligned poorly near the cell center. Which outcome is most likely if the checkpoint remains active?
  17. Some hypotheses propose that RNA served as an early genetic system because it can store information and catalyze reactions. In an experiment, a ribozyme population was provided RNA templates and nucleotides. After multiple cycles, ribozymes that copied templates more accurately and faster became more common in the population, even though no proteins were present. Which conclusion is best supported by this evidence about early life origins?
  18. A yeast population was grown in a closed flask with a fixed amount of sugar. Cell density rose rapidly for 10 hours, then growth slowed and cell density leveled off from hours 14–24. Measurements showed sugar concentration decreased steadily and was very low by hour 14. No cells were added or removed during the experiment. Which explanation best accounts for the slowing and leveling of population growth after hour 10?
  19. A mutant kinase in a cascade cannot bind ATP. Which event is directly blocked?
  20. A marine fish species occupies a coastline with no physical barriers. In the northern region, adults spawn in early spring; in the southern region, adults spawn in late summer. Water temperature and day length differ by latitude, and long-term surveys show little overlap in spawning dates. Genetic sampling reveals increasing allele-frequency differences with latitude. When northern and southern adults are brought into the same aquarium, they still release gametes only at their region-typical times. Which mechanism most directly reduced gene flow and promoted speciation in this system?

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