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Practice Test 6 - AP Biology

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

  1. A locus shows significantly more heterozygotes than HWE predicts. Safest conclusion:
  2. A student observes plant cells under a microscope. In Solution A (0.55 M solute), the plasma membrane pulls away from the cell wall. In Solution B (0.15 M solute), the vacuole expands and the membrane presses against the wall. The membrane is permeable to water but not the solute. Which statement best predicts net water movement in Solution A?
  3. A student compares two eukaryotic cell types. Cell X has many mitochondria and an extensive network of folded inner mitochondrial membranes (cristae). Cell Y has fewer mitochondria with less folded inner membranes. Both cell types have similar plasma membrane surface area and similar numbers of ribosomes. When provided the same amount of glucose and oxygen, Cell X produces more ATP per unit time than Cell Y. Which outcome is most likely explained by the difference in mitochondrial structure?
  4. In immune cells, ligand X triggers a rapid increase in cAMP. Pretreatment with a toxin that inactivates the Gαs\alpha_s subunit prevents the cAMP increase, but a toxin that inactivates Gαi\alpha_i has no effect. Ligand X still binds its receptor in both toxin conditions. Which of the following best explains the early transduction step after ligand binding?
  5. Two membranes have the same phospholipid bilayer composition but differ in cholesterol content. Membrane A has high cholesterol; Membrane B has low cholesterol. At a low temperature, small nonpolar molecules diffuse more readily through Membrane A than through Membrane B. Cholesterol is embedded among phospholipid tails. Which membrane property change best explains the observation?
  6. A human red blood cell lacks a nucleus and most internal membrane-bound organelles. Despite this, it maintains a flexible biconcave shape while squeezing through narrow capillaries. Which cellular feature best explains this flexibility at the cellular level?
  7. In immune cells, ligand L binds a receptor tyrosine kinase (RTK) on the plasma membrane. Within 30 seconds of adding L, the receptor becomes phosphorylated on cytosolic tyrosines, and a cytosolic adaptor protein binds the phosphorylated receptor. If a mutation removes the receptor's cytosolic kinase domain, L still binds extracellularly but receptor phosphorylation is not detected. Which of the following best explains the earliest disrupted step?
  8. A digestive protease shows maximal activity at pH 2.0. When placed at pH 8.0, activity decreases to near zero, but returns to near maximal when moved back to pH 2.0. Which condition would most likely reduce activity at pH 8.0?
  9. A group of 30 small birds is observed on a cold morning (2∘C2^{\circ}\mathrm{C}) and a mild afternoon (18∘C18^{\circ}\mathrm{C}). In the morning, birds fluff feathers and huddle; in the afternoon, birds spread out and feathers lie flat. Which response best explains the short-term mechanism underlying these behavioral changes?
  10. At a replication fork, single-strand DNA-binding proteins (SSBs) bind to exposed parental DNA strands after helicase separates them. DNA polymerase then uses each parental strand as a template, extending new DNA by complementary base pairing. In a mutant extract lacking functional SSBs, the parental strands frequently re-form hydrogen bonds with each other soon after unwinding. Which outcome is most likely in the mutant extract compared with a normal extract?
  11. A coastal plant population includes individuals with either high leaf salt-gland density or low salt-gland density. A storm surge increases soil salinity for several growing seasons. Plants with high gland density have higher survival and set more seeds than plants with low gland density under these conditions. In the next generation, the proportion of seedlings with high gland density increases. Genetic analyses show both phenotypes were present before the storm surge. Which statement best explains the role of variation in this evolutionary change?
  12. A cell is heterozygous for two genes (A/a and B/b) located on the same homologous chromosome pair. In prophase I, homologs synapse and a single crossover occurs between the A and B loci on non-sister chromatids. After meiosis, some gametes carry Ab and aB combinations not present on either original homolog. Which process best explains the appearance of these recombinant gametes?
  13. In a diploid cell, homologous chromosomes pair during prophase I, forming tetrads. A crossover occurs between two non-sister chromatids, producing chromatids that contain segments derived from both homologs. The cell then proceeds through meiosis I and II, separating homologs first and sister chromatids second. The resulting gametes contain chromatids that are not identical to either original parental chromatid due to exchanged segments. Which stage and process directly created these recombinant chromatids?
  14. In a eukaryotic cell, the cytosol is near neutral pH, while lysosomes maintain an acidic lumen using proton pumps. Many lysosomal hydrolases show maximal activity only at low pH and have reduced activity at neutral pH. If a lysosomal membrane becomes leaky, protons diffuse into the cytosol and the lysosomal pH rises toward neutral. Shortly after, the rate of macromolecule breakdown inside lysosomes decreases even though the hydrolase proteins remain present. Which feature best explains how compartmentalization influences this change in cellular function?
  15. In a meiosis with no crossing over, a cell contains three homologous chromosome pairs. For each pair, either the maternal or paternal homolog can move to a given pole during anaphase I. The student focuses only on how whole homologs segregate, not on alleles within a chromosome. Which statement best explains how this process generates different combinations of chromosomes in gametes from different meioses?
  16. A bird population on an island was counted annually. For four years, the population stayed near 1,200 birds. In year 5, a hurricane reduced the population to 700 birds. In years 6–8, the population increased to 900, then 1,050, then 1,170 birds. Habitat area and food availability returned to pre-hurricane levels by year 6, and no new predators arrived. Which factor most likely caused the sharp decline in year 5?
  17. A diploid cell begins meiosis with 12 chromosomes, each consisting of two sister chromatids. After meiosis I completes cytokinesis, which description best matches each resulting cell's chromosome state?
  18. In fruit flies, gray body (G) is completely dominant to black body (g), and normal wings (N) are completely dominant to vestigial wings (n). Two flies heterozygous for both traits (GgNn) are crossed, assuming independent assortment. Offspring with black bodies and vestigial wings must be homozygous recessive at both genes. Which proportion of offspring is expected to show both recessive phenotypes?
  19. A researcher designs a short nucleic acid probe to bind a target RNA sequence in solution. The probe is made of DNA rather than RNA but still binds the RNA target strongly. Which feature best explains why a DNA probe can base-pair with an RNA target?
  20. A population of dividing cells is exposed to a chemical that prevents completion of DNA replication. Cells enter S phase and begin copying chromosomes, but replication forks stall and many regions remain unreplicated. When these cells reach the G2 checkpoint, the checkpoint detects incomplete DNA replication and blocks entry into M phase. Which result is most likely observed in the treated cells after several hours?

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