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Practice Test 81 - AP Biology

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

  1. A scientist measures DNA content in individual cells. Some cells have 1× DNA content with unreplicated chromosomes, while others have 2× DNA content with sister chromatids present. A subset of 2× cells is observed with condensed chromosomes and aligned chromatids at the metaphase plate. No cells show DNA content greater than 2×. Which phase is most consistent with the 2× cells that have condensed, aligned chromosomes?
  2. A sedimentary rock sequence contains three distinct fossil layers. In the oldest layer are only marine invertebrates. In a middle layer are marine invertebrates plus fish fossils. In the youngest layer are fish fossils plus amphibian-like tetrapod fossils with both gills and weight-bearing limb bones. The layers are undisturbed and dated using radiometric methods from volcanic ash between layers. Which conclusion is best supported by this fossil sequence?
  3. In corn, smooth kernels (K) are completely dominant to wrinkled kernels (k). Two plants are crossed: one is heterozygous smooth (Kk) and the other is also heterozygous smooth (Kk). Each offspring inherits one allele from each parent. Which proportion of offspring is expected to have the homozygous recessive genotype?
  4. A student compares two meiotic cells from the same individual. Cell 1 shows no chiasmata; Cell 2 shows several chiasmata between homologous chromosomes during prophase I. Both cells complete meiosis and produce haploid gametes. The student predicts Cell 2 will produce a greater variety of allele combinations along a single chromosome than Cell 1. Which process best supports this prediction at the chromosomal level?
  5. Two droplets are placed on identical waxy plant cuticles. Droplet 1 is water; droplet 2 is a liquid made of nonpolar molecules of similar size. Water molecules are polar, with partial charges that allow hydrogen bonding between neighboring molecules. These hydrogen bonds create strong cohesion, so water molecules at the surface experience a net inward attraction that resists spreading. The nonpolar liquid lacks hydrogen bonding, so its molecules separate more readily at the surface and spread more on the waxy cuticle. Which statement best explains why the water droplet remains more rounded?
  6. In 500 organisms, 200 are AA, 200 Aa, 100 aa. HWE-predicted counts:
  7. Cells in a dish release signal S. When S is neutralized by a specific antibody added to the medium, the same cells show reduced response compared with untreated controls. The cells are known to express the receptor for S on their own surfaces. Which of the following best explains the signaling mode involved?
  8. A eukaryotic pre-mRNA contains a short exon flanked by two introns. A mutation deletes the branch point adenine within the upstream intron but does not change the 5′ splice site or 3′ splice site sequences. When transcripts are examined, the upstream intron is frequently not removed, and the downstream intron is removed normally. Which explanation best accounts for the splicing defect observed for the upstream intron?
  9. A breeder selects only sheep with the finest wool fibers to reproduce. Fiber fineness is influenced by alleles W (finer) and w (coarser). At generation 0, f(W)=0.55f(W)=0.55. After repeated selection for fineness, f(W)=0.90f(W)=0.90 at generation 7. Which outcome is most likely in the population's phenotype distribution for fiber fineness at generation 7 compared with generation 0?
  10. A scientist measures nutrient uptake in two cube-shaped cells with side lengths of 3 units and 9 units. A nutrient enters only by passive diffusion across the plasma membrane, and the external nutrient concentration is held constant. Both cells start with the same internal nutrient concentration and have similar cytoplasmic usage per unit volume. Which explanation best accounts for why the 9-unit cell reaches a lower internal nutrient concentration at steady state than the 3-unit cell?
  11. Two populations of a flowering plant occur on opposite sides of a mountain ridge. Wind can carry pollen across the ridge, but field observations show that flowering begins about 6 weeks earlier on the east side than on the west side because of consistent temperature differences. Genetic data indicate divergence in allele frequencies, and controlled crosses show that if pollen is transferred by hand at the same time, fertilization and seed development occur normally. In natural conditions, however, pollen transfer rarely overlaps with receptive flowers across the ridge. Which isolating mechanism most directly reduces gene flow between the populations?
  12. A small population of 40 mice is founded in an isolated barn by a random sample from a much larger regional population where allele B has frequency 0.50. In the barn population's first generation, allele B frequency is 0.25, and it remains near 0.25 for several generations with no immigration. Survival and reproduction do not differ among genotypes in the barn. Which process most likely produced the initial allele-frequency difference between the barn and regional populations?
  13. A researcher adds Ligand Z (hydrophobic steroid-like molecule) to two cell types. In Cell Type 1, a fluorescently tagged receptor is mostly cytosolic before ligand addition; after ligand addition, the receptor-ligand complex is detected in the nucleus within minutes. In Cell Type 2, the same receptor is engineered to remain anchored in the plasma membrane, and ligand addition produces no detectable early intracellular second messenger changes. Which of the following best explains the initial reception step for Ligand Z in Cell Type 1?
  14. A macrophage readily engulfs bacteria by extending its plasma membrane around the particle and forming an internal vesicle. The cell's cortex contains abundant actin filaments that rapidly assemble and disassemble. Which feature best explains the membrane changes required for engulfment?
  15. In a eukaryotic nucleus, a gene's transcription start site is unchanged, but a mutation deletes the TATA box in the promoter region. When equal amounts of genomic DNA are present, the amount of nascent RNA detected near the 5′ end of the gene is greatly reduced compared with wild type. The RNA sequences that are produced match the gene's template strand complement. Which explanation best accounts for the reduced RNA production?
  16. In a grassland ecosystem, producers capture 50,000 kJ of energy as new plant biomass during one growing season. Herbivores consume plants and convert 5,000 kJ into new herbivore biomass, and carnivores convert 400 kJ into new carnivore biomass. Which conclusion is best supported about energy transfer between these trophic levels?
  17. In a synaptic terminal, action potentials open voltage-gated Ca2+^{2+} channels, increasing cytosolic Ca2+^{2+} and triggering vesicle fusion. Ca2+^{2+} also activates a Ca2+^{2+}-dependent protease that cleaves a channel-associated protein, reducing Ca2+^{2+} channel open probability during repeated firing. Inhibiting the protease increases Ca2+^{2+} entry during a train of action potentials. Which outcome best represents the feedback's effect on the signaling response?
  18. Two groups of identical animal cells (internal solute concentration 0.25 M) are placed into separate solutions. Group X is placed in 0.25 M glucose; Group Y is placed in 0.25 M sucrose. The membrane is permeable to water and glucose but not to sucrose. After 15 minutes, Group X cells are larger than at the start, while Group Y cells show little change. Which outcome is most likely to explain why Group X swelled?
  19. A protein contains multiple α-helices stabilized by hydrogen bonds between the backbone carbonyl oxygen and backbone amide hydrogen. A lab adds a chemical that disrupts hydrogen bonding without breaking covalent bonds. The protein remains a single polypeptide chain but loses its normal shape and binding specificity. Which feature best explains the observed loss of function?
  20. A bird population shows variation in song frequency. In an urban area, low-frequency songs are masked by traffic noise, and very high-frequency songs attract fewer mates. Birds with intermediate song frequencies produce the most offspring. Across nine generations, the mean song frequency remains similar, but extreme frequencies become less common. Which explanation best accounts for the pattern in the population?

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