Practice Test 53 - AP Biology

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

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1. Two double-stranded DNA samples have equal length but different base composition. Sample 1 is 60% G+C, and Sample 2 is 60% A+T. When heated under identical conditions, Sample 1 requires a higher temperature to separate into single strands. The researcher attributes the difference to the number and arrangement of hydrogen bonds formed by specific base pairs in the double helix. Which feature best explains the higher melting temperature of Sample 1?
2. In a photosynthetic cell, the thylakoid membrane separates the thylakoid lumen from the stroma. Light-driven electron transport increases proton concentration in the thylakoid lumen relative to the stroma. When the thylakoid membrane is experimentally punctured, the proton difference rapidly disappears and ATP synthesis decreases. Which feature best explains how this compartmentalization supports photosynthetic ATP production?
3. Glycogen is a storage polysaccharide in animal cells composed of glucose monomers joined mainly by α-1,4 glycosidic bonds with frequent α-1,6 branch points. The many branch ends expose numerous terminal glucose residues to the aqueous cytosol, increasing the number of sites where enzymes can simultaneously interact with the polymer surface. Which statement best describes how glycogen's structure affects its cellular behavior?
4. A DNA molecule is treated with a chemical that specifically breaks hydrogen bonds but does not break covalent bonds. DNA consists of two antiparallel strands held together by hydrogen bonds between complementary bases (A-T and G-C), while each strand's sugar-phosphate backbone is held together by covalent phosphodiester bonds. After treatment, the sample is analyzed and shows intact single strands but loss of the double-helix structure. Which bonds were primarily disrupted by the treatment?
5. A signaling molecule W binds receptor R on the plasma membrane. In one experiment, cells are treated with a reagent that cross-links R molecules, preventing them from moving laterally in the membrane. After treatment, W still binds to R, but the cellular response is greatly reduced. Which of the following best explains the reduced response?
6. A researcher uses quantitative PCR (qPCR) with a fluorescent dye that binds double-stranded DNA. Two samples start with different amounts of the same target DNA sequence but use identical primers and cycling conditions. Sample A reaches a fluorescence threshold at cycle 18, while Sample B reaches the same threshold at cycle 24. Which inference best accounts for this difference?
7. In cattle, coat color is codominant. Allele $C^R$ produces red hair and allele $C^W$ produces white hair. Homozygous $C^RC^R$ cattle are red, homozygous $C^WC^W$ cattle are white, and heterozygous $C^RC^W$ cattle are roan, showing both red and white hairs. A breeder crosses a roan bull with a white cow ($C^RC^W \times C^WC^W$). Which offspring phenotype ratio is most likely?
8. Cells are stimulated with ligand L, activating a receptor that triggers a kinase cascade leading to phosphorylation of cytosolic substrate S. Adding a broad phosphatase inhibitor increases both the peak level and duration of S phosphorylation. Adding a kinase inhibitor eliminates S phosphorylation. No changes in ligand binding are detected under any condition. Which change would most likely decrease the duration of S phosphorylation without affecting its initial peak?
9. A cell contains a homologous chromosome pair with alleles G and g at one locus and H and h at a second locus on the same chromosome. The arrangement is Gh on one homolog and gH on the other. During prophase I, the homologs synapse and a crossover occurs between the G/g locus and the H/h locus on nonsister chromatids. After meiosis, some gametes carry GH and gh chromosomes, combinations not present on either original homolog. Which meiotic process best explains the appearance of these new allele combinations on single chromosomes?
10. In photosynthetic cells, the thylakoid membrane separates the thylakoid lumen from the chloroplast stroma. During the light reactions, H+ accumulates in the lumen, and ATP synthase uses the H+ gradient across the thylakoid membrane to produce ATP in the stroma. If the thylakoid membrane is punctured, the H+ gradient collapses and ATP production decreases. Which feature best explains how compartmentalization affects energy conversion here?
11. An RNA molecule is isolated and found to contain regions that fold back on themselves, creating short double-stranded stems separated by single-stranded loops. Base composition analysis shows frequent A–U and G–C pairs within the stems. When specific bases in a stem are mutated so that complementary pairing is disrupted, the RNA no longer maintains the same folded shape under the same conditions. Which feature best explains how RNA nucleotide structure supports formation of these stems and loops?
12. In an experiment on endocytosis, internalized particles enter early endosomes that gradually become more acidic than the cytosol. A drug prevents endosomal acidification without stopping vesicle formation. After treatment, many particles fail to dissociate from their receptors inside endosomes, and recycling back to the plasma membrane slows. Which feature best explains how compartmentalization affects this process?
13. Cells EE and FF are similar in composition and are placed in identical environments. Cell EE is small and spherical; Cell FF is much larger but remains spherical. Both rely on diffusion across the plasma membrane for phosphate uptake, and phosphate is incorporated into molecules at the same rate per unit volume. Cell FF shows a larger drop in external phosphate concentration near its surface, yet internal phosphate is lower than in Cell EE. Which explanation best accounts for the lower internal phosphate in Cell FF?
14. A liver cell is observed to contain extensive stacks of membrane sacs studded with ribosomes, located near the nucleus. Transport vesicles frequently bud from these membranes and move toward the Golgi apparatus. Which feature best explains this cell's ability to produce and export large amounts of a protein hormone?
15. A bacterial cell is placed in a solution containing an antibiotic that prevents formation of peptidoglycan cross-links. Soon after, many cells burst in the same solution where untreated bacteria remain intact. Which feature best explains the bursting of treated cells?
16. In snapdragons, flower color shows incomplete dominance: allele R produces red, allele W produces white, and heterozygotes (RW) are pink. A gardener crosses two pink plants (RW × RW). Which offspring outcome is most consistent with this inheritance pattern?
17. A student compares a carbohydrate polymer made only of C, H, and O to a nucleic acid polymer that also contains N and P. The nucleic acid's repeating units include a phosphate group with P bonded to multiple oxygens and a nitrogen-containing base. The phosphate groups create a negatively charged backbone that can interact with positively charged ions in solution, influencing how the polymer behaves in water. The carbohydrate polymer lacks these phosphate groups and is uncharged at the backbone. Which feature best explains why nucleic acids often attract cations more strongly than carbohydrates do?
18. A cell contains peroxisomes with enzymes that convert fatty acids into smaller molecules while producing hydrogen peroxide (H2O2) as a byproduct. Catalase inside peroxisomes breaks down H2O2. When peroxisomal membranes are disrupted, H2O2 levels rise in the cytosol and other cellular proteins become oxidatively damaged. Which feature best explains how compartmentalization affects cellular function here?
19. In mitosis, APC/C activation requires a co-activator that binds APC/C only when inhibitory phosphorylation on the co-activator is removed. A kinase keeps the co-activator phosphorylated until all kinetochores are attached. A drug inhibits the phosphatase that removes the inhibitory phosphate from the co-activator. Which result is most likely after drug treatment?
20. Which statement best describes the relationship between a gene and its alleles in a diploid organism?

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