Practice Test 87 - AP Biology

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

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1. To test whether clay minerals could aid early polymer formation, researchers mixed RNA nucleotides with montmorillonite clay in water. After incubation, they detected longer RNA chains than in a no-clay control with the same nucleotides and conditions. Early Earth had abundant mineral surfaces in sediments. Which conclusion is best supported by the evidence?
2. Two solutes contact a pure phospholipid bilayer with no proteins: glycerol (92 Da, polar uncharged) and Cl$^-$ (35.5 Da, charged). Both are small, but one crosses far more slowly. Which explanation best accounts for the difference in permeability across the bilayer's hydrophobic core?
3. At replication forks, helicase unwinds DNA and DNA polymerase synthesizes new strands by adding nucleotides to the 3′ end, guided by complementary base pairing. In a mutant cell line, helicase functions normally, but single-strand binding proteins cannot bind to exposed DNA. The templates therefore re-form hydrogen bonds with each other more frequently after unwinding. Which result is most likely for replication in the mutant compared with wild type?
4. In a species with $2n=12$, a student tracks one homologous pair through meiosis. At the end of meiosis I, each daughter cell contains one member of the homologous pair, and each chromosome still has two sister chromatids. Which outcome is most likely for that homologous pair during meiosis II in a normal cell?
5. A DNA segment is transcribed to RNA. In the RNA product, positions corresponding to adenine on the DNA template strand contain uracil. The RNA is single-stranded, but it can still form short double-stranded regions through intramolecular base pairing. The difference in one nitrogenous base between DNA and RNA changes which complementary base is used during pairing. Which feature best describes the base-pairing rule that applies in RNA during transcription and folding?
6. A wildfire removes most shrubs from a chaparral ecosystem. In the first rainy season after the fire, soil nitrate in runoff increases and stream algal blooms become more frequent downstream. Which consequence is most likely in the stream ecosystem?
7. A researcher adds a nonhydrolyzable ATP analog that binds a CDK's active site but cannot be used for phosphate transfer. Cyclin binding and CDK phosphorylation states are unchanged, but kinase activity toward substrates is reduced. Cells accumulate before entering S phase. Which change would most likely allow progression into S phase while the ATP analog remains present?
8. A plant breeder crosses and saves seeds only from the shortest 10% of mustard plants each generation to fit greenhouse shelves. Height is influenced by a locus with alleles T (taller) and t (shorter). In generation 1, $f(t)=0.30$. After eight generations, most plants are short and very tall plants are rare. No gene flow occurs. Which explanation best accounts for the change observed across generations?
9. A ligand L activates GPCR R, leading to Gα-GTP activation of adenylyl cyclase and increased cAMP. cAMP activates PKA, which phosphorylates enzyme X, increasing product formation. In a mutant cell line, L binding and cAMP increase are normal, but enzyme X phosphorylation is greatly reduced. Adding a membrane-permeable cAMP analog does not restore X phosphorylation. Which defect best explains the mutant phenotype?
10. Two city parks each contained the same species of squirrel and similar tree cover. Park A averaged 8 squirrels per hectare, and Park B averaged 32 squirrels per hectare. Over one month, both parks experienced the same temperature range and no major storms. Park B had more observed cases of a mite infestation and a lower average body mass than Park A. Which factor most likely represents density-dependent population regulation?
11. A researcher compares two RNA molecules of similar length. RNA X contains extensive regions where sequences within the same strand are complementary and form intramolecular base pairs, producing hairpin loops stabilized by hydrogen bonds. RNA Y has few complementary regions and remains mostly unpaired and extended in solution. Both RNAs have ribose sugars and phosphodiester-linked backbones. The researcher adds a compound that disrupts hydrogen bonding without breaking covalent bonds. Which RNA's overall shape will change more after treatment, and why?
12. In a large laboratory population of bacteria, allele M is absent at generation 0. A single DNA replication error creates allele M in one cell. No migration occurs, and allele M does not change growth rate relative to the ancestral allele. After many generations, allele M is present at low frequency (about 0.002) rather than remaining at zero. Which factor most directly explains the appearance of allele M in the population?
13. A researcher stains cells with a fluorescent dye proportional to DNA content and counts fluorescence intensity. One group of cells shows a single peak at 2C, and microscopy shows intact nuclei and no visible condensed chromosomes. Another group shows a single peak at 4C, and microscopy shows condensed chromosomes with sister chromatids still joined. The 4C group does not show separated chromatids or two nuclei. Based on these observations, the 4C group is most likely paused at a checkpoint during mitosis. Which checkpoint is most consistent with the data?
14. A culture of aerobic cells is shifted from normal oxygen conditions to very low oxygen. Glycolysis in the cytosol converts glucose to pyruvate and produces a small amount of ATP by substrate-level phosphorylation while reducing NAD+ to NADH. Under normal oxygen, NADH is oxidized by the mitochondrial electron transport chain, allowing continued glycolysis. When oxygen becomes limiting, electrons cannot be passed efficiently to the final electron acceptor, and NAD+ regeneration by the electron transport chain slows. Which outcome is most likely in the cells shortly after oxygen drops?
15. In a dividing cell, DNA replication has completed and the cell enters M phase. During prometaphase, spindle microtubules attach to kinetochores, and by metaphase all chromosomes are properly bi-oriented. The spindle checkpoint is satisfied, allowing progression into anaphase. Which event is most likely to occur next in this cell?
16. In two identical ponds, ecologists introduced 50 and 200 juvenile sunfish, respectively. After 8 weeks, both ponds had similar water temperature and dissolved oxygen, but the 200-fish pond showed lower average mass gain and more fin damage from aggressive interactions. No new predators were observed, and no storms occurred during the study period. Which outcome is most likely caused by a density-dependent factor regulating the sunfish population?
17. A student compares DNA replication and transcription. In replication, both DNA strands are copied to produce two double-stranded DNA molecules, and DNA polymerase uses DNA nucleotides (A, T, C, G) added to a 3′ end. In transcription, one DNA strand serves as a template to build an RNA molecule using ribonucleotides (A, U, C, G). A cell is treated with a drug that prevents incorporation of uracil-containing nucleotides. Which process is most directly inhibited by the drug?
18. During eukaryotic mRNA maturation, the 3′ end of a pre-mRNA is cleaved downstream of a specific sequence in the transcript, and then a poly(A) tail is added by poly(A) polymerase using ATP as the adenine source. A mutant cell line has a defective cleavage factor, so cleavage at the 3′ end occurs inefficiently, while transcription proceeds normally through the gene. Compared with wild type, which change is most likely observed in RNA molecules produced from this gene?
19. A plant is placed near a window so light comes from one side. Over the next 24 hours, the stem bends toward the light source while the plant remains rooted in place. When the pot is rotated 180°, the stem gradually bends in the new direction of the light. No new leaves form during the observation period. Which response best explains the stem bending toward the light?
20. A peptide hormone H binds receptor RTK on liver cells, causing receptor dimerization and autophosphorylation of tyrosines. An adaptor protein binds the phosphotyrosines and activates Ras (GDP→GTP). Ras-GTP activates kinase K1, which phosphorylates K2, which phosphorylates K3. K3 phosphorylates enzyme E, increasing glycogen breakdown within minutes. A phosphatase P removes phosphate groups from K3 and E. In cells with a loss-of-function mutation in P, H produces a larger and longer-lasting increase in glycogen breakdown than in wild type. Which explanation is best supported by these results?

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