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Practice Test 49 - AP Biology

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

  1. A flowering plant occurs on both sides of a river within the same region. Pollinators can cross the river, and pollen transfer between sides is observed. However, plants on the east side flower in early spring, while plants on the west side flower in midsummer. Over time, allele frequencies at several loci differ between sides, and controlled crosses between east and west plants rarely produce seeds because flowers are not simultaneously receptive. Which isolating mechanism most directly limits gene flow between the populations?
  2. An enzyme catalyzes an endergonic reaction A→BA \rightarrow B in the cytosol only when ATP is present. Measurements show that ATP is converted to ADP during the reaction, and a transient phosphorylated intermediate form of A can be detected. The phosphorylated intermediate has higher free energy than A and then converts to B while releasing inorganic phosphate. The overall coupled process proceeds with a net decrease in free energy. Which explanation best accounts for how ATP enables the formation of B from A?
  3. A population of insects varies in timing of adult emergence, from early to late in the season. A parasitoid wasp is most abundant mid-season, reducing reproductive success of insects emerging at intermediate times. Over many generations, early and late emergence phenotypes each increase in frequency, while intermediate emergence declines. Which pattern best illustrates the selection acting on emergence timing?
  4. A short DNA segment is shown on one parental strand as 3′-T A C G G A-5′. During replication, the complementary daughter strand is synthesized by adding DNA nucleotides according to base pairing, producing a strand antiparallel to the template. DNA polymerase adds nucleotides to a 3′ end, so the new strand is written 5′→3′. Which sequence represents the newly synthesized complementary strand (5′→3′)?
  5. A bacterial population is grown in a flask with antibiotic X. At the start, 1% of cells carry a resistance allele (R) on the chromosome. After 24 hours (about 12 generations) in the same antibiotic concentration, 38% of cells carry R; after 48 hours, 72% carry R. The population size remains large, and the culture is well mixed. Which observation best demonstrates evolution occurring in this bacterial population in real time?
  6. In photoreceptor cells, light activates rhodopsin, which activates a G protein that activates phosphodiesterase (PDE). PDE decreases cGMP, causing cGMP-gated Na+^+ channels to close and the membrane to hyperpolarize. A mutation reduces PDE activity; after a flash of light, cGMP decreases only slightly and Na+^+ channels remain mostly open. Which outcome is most likely in the mutant compared with wild type after the same light flash?
  7. A lab compares two plant storage polysaccharides. Sample P contains long, unbranched α-1,4 glucose chains that coil into helices. Sample Q contains α-1,4 chains with occasional α-1,6 branches, producing a more open, branched structure. Both are placed in water, and iodine is added; iodine molecules can fit into helical cavities but bind poorly to highly branched regions. Which statement best predicts the iodine color intensity for P compared with Q?
  8. In plant cells, cellulose consists of unbranched glucose chains with β-1,4 glycosidic bonds that keep alternating glucose units flipped, allowing many hydroxyl groups to align and form extensive hydrogen bonds between adjacent chains. These parallel chains pack into microfibrils that resist stretching when the cell takes up water. A mutant plant produces a polysaccharide made of the same glucose monomers but with α-1,4 bonds, creating helical, less-linear chains that hydrogen-bond less effectively between polymers. Which structural feature best explains why cellulose microfibrils provide greater tensile strength than the mutant polymer?
  9. Researchers tested whether short nucleic acids could persist under early-Earth-like ultraviolet exposure. Single-stranded RNA degraded faster than RNA protected inside fatty-acid vesicles, while unprotected RNA showed little remaining after the same exposure time. Which conclusion is best supported about the role of compartments?
  10. During translation, a ribosome begins at a start codon and reads an mRNA 5′→3′ in sets of three nucleotides. A student compares two mRNAs that are identical except at codon 4: mRNA 1 has 5′-AUG GCU UAC GAA CCG-3′, and mRNA 2 has 5′-AUG GCU UAC UAA CCG-3′. In this system, UAA is a stop codon recognized by a release factor rather than a tRNA. All other codons shown are sense codons that recruit tRNAs carrying amino acids. Which outcome is most likely for translation of mRNA 2 compared with mRNA 1?
  11. A diploid cell contains one homologous chromosome pair with alleles E and e at a locus near the middle of the chromosome. During prophase I, homologous chromosomes synapse, and a crossover occurs between the locus and the centromere on two non-sister chromatids. After meiosis II, some gametes contain chromatids with E linked to a different set of nearby marker segments than in the original parental chromatids. Which process best explains this change in marker linkage?
  12. In mice, black fur (B) is completely dominant to brown fur (b). A black mouse of unknown genotype is crossed with a brown mouse known to be homozygous recessive (bb). The litter contains 8 pups: 4 black and 4 brown. Assume a single autosomal gene with complete dominance and that the sample reflects the expected ratio. Which genotype is most likely for the black parent?
  13. During G2, a CDK–cyclin complex is present but kept inactive by an inhibitory phosphate. When DNA damage is detected, signaling increases activity of the kinase that adds the inhibitory phosphate and decreases activity of the phosphatase that removes it, maintaining CDK inactivity. A mutant cell line cannot increase the inhibitory-kinase activity after DNA damage, but the phosphatase regulation is normal. Which effect is most likely in the mutant after DNA damage occurs?
  14. A student analyzes an unknown macromolecule and finds it consists of a sugar, a phosphate group, and a nitrogen-containing base. Many of these units connect into a long chain when a phosphate group on one unit reacts with a hydroxyl group on the sugar of another unit, releasing water and forming a covalent linkage in the backbone. The sequence of nitrogenous bases varies along the chain. Which feature best explains why this molecule is classified as a nucleic acid polymer?
  15. In a tundra ecosystem, plants produce 10,000 kJ of new biomass energy. Lemmings store 700 kJ as new biomass, and owls store 35 kJ as new biomass from consuming lemmings. Which conclusion is best supported about the overall producer-to-owl transfer efficiency?
  16. In muscle cells, ligand Y binds a GPCR that activates Gq, stimulating PLC to generate IP3_3. IP3_3 releases Ca2+^{2+} from the sarcoplasmic reticulum; Ca2+^{2+} binds troponin, allowing contraction. A separate feedback protein F binds the activated receptor and reduces further G protein activation after ~20 s. Cells lacking F show larger and longer Ca2+^{2+} transients after the same Y dose. Which change would most likely mimic the phenotype of cells lacking F in normal cells?
  17. In a bacterial cell, replication begins at a single origin and proceeds bidirectionally. As the helix unwinds, DNA polymerase extends new DNA only by adding nucleotides to the 3′ end, using complementary pairing to the template strand. One new strand is synthesized continuously toward each fork, and the other is synthesized discontinuously as short DNA fragments. A mutation reduces the ability of DNA polymerase to proofread newly added bases, but does not affect unwinding or primer formation. Which outcome is most likely after several rounds of replication?
  18. Cells exposed to ligand L show a rapid increase in phosphorylated protein P (P-P) that peaks at 1 minute and returns to baseline by 5 minutes. Inhibiting a specific phosphatase causes P-P to remain high at 5 minutes, while inhibiting the upstream kinase prevents any increase in P-P. The receptor and ligand-binding affinity are unchanged by either inhibitor. Which inference about pathway regulation is best supported?
  19. An invasive nitrogen-fixing shrub spreads through a nutrient-poor sand dune ecosystem. Within three years, soil nitrate increases and fast-growing grasses become more abundant while several native forbs decline. Which ecosystem-level change is most likely?
  20. A membrane antiporter exchanges Na+^+ moving into the cell with H+^+ moving out. In the experiment, Na+^+ is high outside and low inside; H+^+ becomes lower inside than outside after transport proceeds. The antiporter stops when the Na+^+ gradient is eliminated, even though ATP is still present. Which mechanism best explains H+^+ export in this system?

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