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Practice Test 23 - AP Biology

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

  1. A student models DNA replication by separating the two parental strands and assembling new strands by matching A with T and G with C. The student predicts that, after replication, one daughter molecule will contain both newly synthesized strands and the other will contain both parental strands. In actual cells, each daughter molecule contains one parental and one new strand. Which explanation best accounts for the observed distribution of strands?
  2. In a rabbit population, ear length is heritable. During a period of cold winters lasting 9 generations, rabbits with intermediate ear length have higher survival to reproduction and produce an average of 10 offspring, while rabbits with very short or very long ears average 4 offspring. Over the same period, the population mean ear length remains similar, but the range of ear lengths narrows. No migration is detected, and population size stays large. Which pattern best illustrates the selection acting on ear length?
  3. In a model membrane system, adding cholesterol decreases permeability to small polar molecules but has little effect on permeability to a small nonpolar molecule. When the nonpolar molecule is added at higher concentration outside, it still equilibrates across the membrane without proteins or ATP. Which mechanism best explains the nonpolar molecule's movement?
  4. A botanist compared flower structure in five plant lineages. Two lineages (X and Y) both have petals fused into a single tubular corolla and share the same unusual arrangement of vascular bundles supplying the fused petals. The other three lineages have separate petals and lack that vascular-bundle arrangement. Genetic analysis of a chloroplast gene shows X and Y differ by 3 substitutions, while each differs from the other three lineages by more than 40 substitutions. Which inference is best supported by the combined evidence?
  5. A diploid cell with 2n=62n=6 enters meiosis. At metaphase I, each homologous pair can orient toward either pole independently of the other pairs. Assume no crossing over and no nondisjunction. The cell completes meiosis I and II normally, producing four gametes. Which number of distinct combinations of maternal and paternal chromosomes is most likely possible among gametes from many such meiotic events?
  6. A gene's coding region includes a repeated sequence of three nucleotides. During DNA replication, one additional triplet is inserted into the coding sequence (e.g., one extra codon is added), while the rest of the sequence remains unchanged and in-frame. Transcription and translation occur normally. Which outcome is most likely for the resulting protein compared with the original?
  7. A coastal grass population contains individuals with either high or low salt tolerance due to different alleles at one gene. After a storm surge, soil salinity increases and remains high for several years. In the first year, survival is 70% for high-tolerance plants and 30% for low-tolerance plants. By year five, seedlings show a higher proportion of high-tolerance phenotypes than before the surge. Which statement best explains how preexisting variation contributed to evolutionary change in this population?
  8. Two sets of genetically identical rabbit kits were raised on diets with equal calories but different pigment precursors. One diet lacked tyrosine; the other contained tyrosine. After 6 weeks, rabbits on the tyrosine-containing diet had darker fur than rabbits on the tyrosine-lacking diet. DNA sequencing of the tyrosinase gene showed identical sequences in both groups. Which explanation best accounts for the fur-color difference?
  9. A small mammal population was surveyed in two similar grassland plots for one year. In Plot 1, predators were excluded with netting; in Plot 2, predators were present. Both plots had the same rainfall and plant biomass. Plot 1 increased from 40 to 85 individuals, while Plot 2 remained near 40–45. Which explanation best accounts for the difference in population growth between plots?
  10. In a population of field mice, fur color varies from light tan to dark brown due to heritable alleles at one gene. Owls hunt visually over pale sand dunes, and mice that contrast strongly with the sand are captured more often before reproducing. Over several generations with similar dune color and owl predation, which outcome is most likely for allele frequencies in the mouse population?
  11. Two cells of the same type are placed in distilled water. Water moves across the plasma membrane by osmosis. Cell M is small; cell N is larger but has the same shape and membrane permeability. The larger cell shows a slower percentage increase in volume over the first minute. Which explanation best accounts for the slower relative swelling of the larger cell?
  12. During transcription of a eukaryotic gene, RNA polymerase moves along the DNA template strand and synthesizes RNA. A student is given a short DNA template-strand sequence: 3'-T A C G G A T-5'. The student predicts the RNA sequence produced from this template in the 5' to 3' direction, before any RNA processing occurs. Which RNA sequence best matches the transcript segment synthesized from the template provided?
  13. Five bird species (J–N) were compared for a mitochondrial gene. Differences from species J are: J–K 2, J–L 2, J–M 7, J–N 8. Additional comparisons: K–L 1, K–M 7, K–N 8; L–M 6, L–N 7; M–N 2. Assuming fewer differences indicate more recent common ancestry, which inference is best supported?
  14. In endocrine cells, hormone W binds a cell-surface receptor and causes a rapid decrease in intracellular cAMP within 15 seconds. When a toxin that inactivates inhibitory G proteins (Gi_i) is added, W still binds the receptor but cAMP no longer decreases. Which of the following best explains the earliest role of Gi_i in this pathway?
  15. A kelp forest experiences a sharp decline in sea star populations due to a disease outbreak. In the following year, sea urchin density increases and kelp canopy cover decreases across multiple sites. Measurements show reduced habitat for small fish and invertebrates that live among kelp fronds. Seabirds that feed on those small fish are observed less often near affected sites. Which consequence is most likely if sea star populations remain low?
  16. A eukaryotic cell contains an organelle that divides by fission and has a double membrane. However, extensive sequencing detects no DNA inside the organelle, and its ribosomes match cytosolic ribosomes rather than bacterial ribosomes. The organelle's membrane proteins are encoded in the nucleus and resemble proteins found in the endomembrane system. Which conclusion is best supported about this organelle's origin?
  17. A newly discovered unicellular eukaryote contains an organelle that produces ATP. The organelle is surrounded by two membranes; the inner membrane contains proteins similar to bacterial electron transport proteins. The organelle also contains circular DNA and ribosomes that are smaller than cytosolic ribosomes. When treated with an antibiotic that inhibits bacterial ribosomes, synthesis of several organelle-encoded proteins decreases while cytosolic protein synthesis is unaffected. Which evidence best supports an endosymbiotic origin for this organelle?
  18. Steroid molecules share a core of four fused carbon rings and differ mainly in the functional groups attached to that ring structure. The fused rings are largely nonpolar and rigid compared with long, flexible fatty acid tails. In a membrane, a lipid with a rigid ring structure can restrict movement of nearby hydrocarbon tails by close contact, altering how easily the tails bend and rotate. Which feature best explains how a steroid can change membrane physical properties?
  19. A ligand-gated receptor channel opens upon ligand binding and allows Na+^+ influx. The increased Na+^+ activates an intracellular Na+^+-sensitive enzyme N, which produces a second messenger M. M activates kinase K, and K phosphorylates cytosolic protein V, increasing V activity. When extracellular Na+^+ is replaced with an impermeant cation, ligand binding still occurs but M levels and V activity do not increase. Which step is most immediately prevented by removing extracellular Na+^+?
  20. A river is dammed, reducing seasonal flooding that formerly deposited nutrient-rich sediment onto the floodplain forest. Over several years, leaf litter input remains similar, but tree growth rates decline. Which explanation best accounts for the decline at the ecosystem level?

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