Practice Test 83 - AP Biology

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

Three island bird lineages (U, V, W) and a mainland outgroup (O) were compared at a neutral locus. Pairwise nucleotide differences are: O–U = 12, O–V = 12, O–W = 12; U–V = 2, U–W = 6, V–W = 6. Assume the outgroup indicates the ancestral state and that fewer differences reflect more recent divergence among the island lineages. Which statement is best supported?
A phospholipid bilayer (no proteins) is exposed to equal concentrations of CO $_2$ (44 Da, nonpolar) and urea (60 Da, polar uncharged). Both are small molecules. Which prediction best matches their relative rates of crossing the membrane by simple diffusion?
In an experiment, cells are exposed to ligand L that activates receptor R and increases cytosolic cAMP. A FRET sensor reports PKA activity. With L alone, PKA activity peaks at 30 s and returns to baseline by 3 min. With L plus a drug that blocks receptor internalization, PKA activity remains elevated for 10 min. Ligand binding affinity is unchanged. Which explanation is best supported by these data?
In a species of rabbit, fur color depends on a pigment-producing enzyme. Genetically identical rabbits were raised in either a cool room (15°C) or a warm room (30°C). Rabbits in the cool room developed darker fur on their ears and paws, while rabbits in the warm room remained lighter in those regions. DNA sequencing of the enzyme's coding region was identical in both groups. When warm-room rabbits were later kept at 15°C, newly grown fur in extremities became darker. Which explanation best accounts for the temperature-dependent fur coloration?
In an in vitro transcription assay using purified eukaryotic RNA polymerase II and general transcription factors, a DNA template containing a promoter and gene produces a primary RNA transcript. When the same RNA is incubated with a nuclear extract, a shorter RNA product appears, and sequencing reveals that internal segments present in the primary transcript are absent while the flanking segments are joined. No nucleotides are added to the RNA ends in this experiment. Which explanation best accounts for the shorter RNA product formed after incubation with nuclear extract?
A lab constructs liposomes made only of phospholipids. The hydrophobic core limits diffusion of polar and charged solutes. Consider two solutes: argon gas (Ar), which is small and nonpolar, and hydrogen peroxide (H2O2), which is small but polar due to oxygen atoms and hydrogen bonding. Neither solute carries a net charge. No proteins are present. Which solute would most likely have higher permeability across the liposome membrane?
A lab compared stability of RNA strands under two conditions relevant to early Earth: freshwater pools with low salt and seawater-like high salt. Equal RNA samples were incubated at the same temperature. After 24 hours, more intact RNA remained in the low-salt condition than in the high-salt condition. No enzymes were present. Which conclusion is best supported about environments that could favor an RNA-based early system?
Two populations of flowering plants occur on opposite sides of a mountain pass. Wind can carry pollen across the pass, and genetic markers show occasional migrants each generation. In a contact zone near the pass, plants that result from cross-pollination between the two populations produce fewer viable seeds than plants produced within either population. Over time, flowering traits in the contact zone shift so that each population's peak flowering time overlaps less with the other population. Which process most directly explains the reduced overlap in flowering time?
Cells are stimulated with ligand L that activates receptor R and triggers production of second messenger M. M binds and activates kinase K, which phosphorylates multiple copies of substrate S per minute. A mutant kinase K binds M normally but phosphorylates S at a much lower rate. Ligand binding and M production are unchanged. Which observation would most likely accompany this mutation?
A scientist tracks a single chromosome during mitosis. At metaphase, the chromosome is duplicated, and its sister chromatids are joined at the centromere. Each sister chromatid's kinetochore is attached to microtubules from opposite spindle poles, and the chromosome is aligned at the metaphase plate. The spindle checkpoint is satisfied, and anaphase begins. Which change in chromosome structure occurs at the start of anaphase?
In a large population of wildflowers, flower color is determined by alleles $R$ and $r$ . At the start of a season, researchers sample 1,000 plants and find genotype frequencies: $RR=0.36$ , $Rr=0.48$ , and $rr=0.16$ . The population is geographically isolated, mating is random with respect to flower color, and no differences in survival or reproduction among the three genotypes are detected. No new mutations affecting the locus are observed during the season. Which conclusion is best supported about Hardy-Weinberg equilibrium for this gene in the sampled population?
A plant's xylem contains narrow tubes filled with water. Water molecules are polar and form hydrogen bonds with each other, creating cohesion. Water molecules can also form hydrogen bonds with polar groups in the cell wall materials lining xylem, creating adhesion. In narrow tubes, these intermolecular attractions influence how a continuous column of water can be maintained without breaking into separate droplets. The behavior depends on hydrogen bonding among water molecules and between water and the tube surface. Which feature best explains how water can remain as a continuous column in xylem?
A student hydrolyzes an unknown polymer and obtains monomers that each contain a phosphate group, a five-carbon sugar, and one of four different nitrogen-containing rings. The original polymer had a repeating covalent linkage between the phosphate of one monomer and the sugar of the next. Which feature best explains how monomer structure supports the polymer's general function at the molecular level?
A phospholipid bilayer is selectively permeable: its interior is composed of hydrocarbon tails, creating a nonpolar region. In an experiment, two solutes are added outside a cell: solute M is small and nonpolar, and solute N is small but carries a full positive charge. Neither solute uses transport proteins. Which outcome best matches how these solutes interact with the bilayer?
This observation of a change in beak depth is evidence of continuing evolution because it demonstrates that...
A researcher studies two cell types from the same human: a melanocyte and a lens epithelial cell. Genomic DNA tests show both contain the tyrosinase gene and a crystallin gene. RNA measurements reveal high tyrosinase mRNA in melanocytes and high crystallin mRNA in lens cells. The researcher concludes the difference is due to gene regulation. Which explanation best accounts for the cell-type-specific mRNA levels?
A scientist uses a plasmid expression vector that places an inserted gene downstream of a bacterial promoter and ribosome-binding site. The gene is inserted in two separate constructs: Construct 1 has the coding sequence in the same orientation as the promoter, and Construct 2 has the coding sequence reversed. Both constructs are transformed into identical bacteria. Which outcome best predicts protein production from the constructs?
Plant cells with internal solute concentration 0.40 M are placed in Solution Y containing 0.60 M nonpenetrating solute. The plasma membrane is permeable to water but not to the solute. Cells initially appear turgid, and the external solution concentration stays constant. Consider only osmotic water movement across the membrane at the cellular level. Which outcome is most likely after several minutes?
In a tetrad during prophase I, one nonsister chromatid from the maternal homolog and one nonsister chromatid from the paternal homolog break and rejoin, exchanging corresponding segments. After meiosis II, two gametes contain chromatids with exchanged segments, and two gametes contain chromatids that match the original homologs. Which description best identifies the direct chromosomal outcome that produces genetic variation among these gametes?
A researcher compares RNA made by RNA polymerase in bacteria versus eukaryotes. In bacteria, the primary RNA transcript matches the final mRNA sequence for a gene with no introns. In eukaryotes, the primary transcript for a similar-length gene includes additional internal sequences that are absent from the mature mRNA. Both organisms use DNA as the template. The researcher wants to explain the molecular basis for the extra internal sequences present only in the eukaryotic primary transcript. Which explanation best accounts for this difference?

Practice tests