Practice Test 8 - AP Biology

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

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1. A DNA polymer is treated with an enzyme that specifically breaks hydrogen bonds but does not break covalent bonds. After treatment, the sample contains two intact nucleotide chains of the same length as the original molecule. Which statement best describes what bonds were disrupted?
2. A researcher compares cytochrome c amino acids among four species (1–4). The number of differences from species 1 are: species 2 differs by 4, species 3 differs by 4, species 4 differs by 9. Additional comparisons show species 2 and 3 differ by 1, species 2 and 4 differ by 10, and species 3 and 4 differ by 10. Assume fewer differences indicate a more recent common ancestor among lineages. Which inference is best supported?
3. Two cells have plasma membranes with identical phospholipid composition and thickness. Cell X has many transmembrane carrier proteins specific for glucose; Cell Y has far fewer of these carriers. Both cells are placed in a solution with glucose concentration higher outside than inside. Glucose enters Cell X faster than Cell Y, even though glucose is polar. Which membrane feature best explains the higher glucose uptake rate?
4. A signaling molecule Q binds a membrane receptor R on Cell Z. When excess unlabeled Q is added, binding of fluorescent Q to Cell Z decreases. A different molecule, Q2, similar in size and charge to Q, does not reduce fluorescent Q binding even at high concentration. Cell Z shows no response to Q when receptor R is removed from the membrane. Which of the following best explains these observations?
5. An unknown macromolecule is composed of a long chain of repeating units, each containing an amino group (–NH2), a carboxyl group (–COOH), and a variable R group. Adjacent units are connected by covalent bonds formed when the carboxyl group of one unit reacts with the amino group of the next, releasing water. The resulting polymer can fold into specific shapes based on interactions among R groups, influencing catalytic activity and binding. Which feature best explains why this molecule is classified as a protein?
6. In a freshwater fish population, body coloration ranges from very pale to very dark. Predators more easily detect intermediate-colored fish against patchy substrates, while very pale fish blend with sand and very dark fish blend with rocks. Over ten generations, the frequency of intermediate-color phenotypes decreases, while both pale and dark phenotypes increase and the distribution becomes bimodal. Which pattern best illustrates the selection acting on body coloration?
7. Two bacterial cells communicate using a secreted molecule AHL that accumulates in the environment. In low-density culture, no response occurs. At high density, many cells respond simultaneously. If AHL is removed by frequent medium replacement, the response does not occur even at high density. Which of the following best explains this signaling behavior?
8. Two solutions contain nonpenetrating solutes: Solution 1 is 0.12 M and Solution 2 is 0.30 M. Identical cells have internal solute concentration 0.20 M, and membranes allow water movement only. A cell placed in Solution 1 increases in volume. Which outcome is most likely for a cell placed in Solution 2?
9. A diploid cell with $2n=4$ completes DNA replication before meiosis. At metaphase I, both homologous pairs align. In anaphase I, homologous chromosomes separate normally. After meiosis I, each daughter cell contains two chromosomes, each still composed of two sister chromatids. Without another round of DNA replication, meiosis II occurs. Which change in chromosome structure occurs during anaphase II?
10. A cell is placed in a solution containing a polar solute that cannot cross the lipid bilayer. When a specific membrane protein is added to the bilayer, the solute begins moving down its concentration gradient. Which feature best explains the change?
11. A fluorescent reporter indicates cyclin E–CDK2 activity. In control cells, activity rises in late G1 and peaks at the G1/S transition. When cells are treated with a phosphatase inhibitor, CDK2 remains phosphorylated at an inhibitory site, and reporter signal stays low despite normal cyclin E binding. Which change would most likely restore reporter signal in the presence of the phosphatase inhibitor?
12. In a signaling assay, a CDK inhibitor binds and inactivates multiple cyclin–CDK complexes. The inhibitor is normally sequestered by a phosphorylated docking protein during late G1, reducing inhibitor availability and increasing CDK activity. A mutation prevents phosphorylation of the docking protein, so the inhibitor remains free. Which outcome is most likely in mutant cells?
13. A receptor tyrosine kinase (RTK) binds ligand V on the cell surface. In cells expressing a mutant RTK that can bind V but cannot dimerize, V binding occurs normally but signaling does not proceed. Which of the following best explains why signaling fails?
14. A neuron releases neurotransmitter N into a synaptic cleft. On the postsynaptic cell, N binds a receptor that spans the membrane and opens an ion channel within milliseconds. When an enzyme that degrades N is added to the cleft, the postsynaptic ion flow stops quickly even though the presynaptic neuron continues releasing N. The postsynaptic cell has no direct cytoplasmic connection to the presynaptic neuron. Which outcome is most likely if the degrading enzyme concentration is increased?
15. A diploid cell ($2n=6$) undergoes meiosis. During prophase I, synapsis occurs normally, but crossing over is experimentally prevented. Homologous chromosomes still align as tetrads at metaphase I and segregate to opposite poles in anaphase I. Sister chromatids separate in meiosis II. Which outcome is most likely regarding the chromatid composition of the resulting gametes?
16. A clonal population of yeast cells was split into two flasks: one at 20°C and one at 37°C. After 6 hours, cells at 37°C produced higher levels of a heat-shock protein, measured by antibody staining, than cells at 20°C. DNA sequencing of the heat-shock gene was identical in both flasks. When 37°C cells were returned to 20°C, heat-shock protein levels decreased within hours. Which explanation best accounts for the temperature-dependent protein levels?
17. A phospholipid bilayer's hydrophobic core is a barrier to solutes that are highly polar or charged. Permeability generally decreases with increasing size and increasing polarity. Compare two carbohydrates: glucose (a monosaccharide) and maltose (a disaccharide). Both are uncharged but contain many hydroxyl groups, making them strongly polar. No transport proteins are present. Which statement best predicts their relative permeability through the bilayer?
18. In a meiosis simulation, a student tracks alleles on homologous chromosomes without considering gene expression. A heterozygous cell has alleles M/m on one homologous pair and N/n on a different pair. No crossing over occurs. The student observes four gamete types (MN, Mn, mN, mn) produced in roughly equal proportions. The chromatids remain intact, and the only variability arises from how whole homologs are distributed to daughter cells. Which meiotic event most directly produces these four gamete combinations?
19. A researcher compares two glucose polymers in animal tissues. Polymer X is highly branched, with α-1,6 glycosidic bonds at frequent branch points in addition to α-1,4 bonds along each chain. Polymer Y is unbranched and consists only of α-1,4 bonds, forming long helices. When enzymes bind to polymer ends to release glucose monomers, the rate depends on the number of nonreducing ends available for enzyme attachment. Which statement best predicts how branching affects the rate of monomer release from polymer X compared with polymer Y?
20. Two phospholipid bilayers are assembled at the same temperature. Bilayer 1 contains phospholipids with shorter fatty acid tails, while bilayer 2 contains phospholipids with longer tails; both have the same head groups and the same degree of saturation. Longer hydrocarbon chains have greater surface area and therefore more van der Waals interactions with neighboring tails, increasing packing strength. Shorter chains have fewer such interactions. Which statement best predicts the relative permeability of the two bilayers to small nonpolar molecules?

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