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Practice Test 74 - AP Biology

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

  1. A rabbit population has allele AA at frequency 0.40 at a fur locus. Over five generations, 15% of the population each generation consists of immigrants from a nearby population where p(A)=0.90p(A)=0.90. No differences in survival or reproduction among genotypes are detected, and population size remains large. After five generations, p(A)p(A) rises to 0.63. Which process is most likely responsible for the increase in allele AA frequency?
  2. A eukaryotic gene can produce two different mature mRNAs from the same primary transcript. In isoform 1, exon 2 is joined to exon 3. In isoform 2, exon 2 is absent and exon 1 is joined directly to exon 3. The DNA sequence of the gene is identical in both cell types, and transcription start and stop sites are unchanged. Which explanation best accounts for the production of the two different mature mRNAs?
  3. A checkpoint protein binds and inhibits a G1/S cyclin–Cdk complex. When a signaling kinase phosphorylates the checkpoint protein, the inhibitor is targeted for rapid removal by a ubiquitin ligase, freeing cyclin–Cdk activity. A mutation prevents the checkpoint protein from binding the ubiquitin ligase but does not affect phosphorylation. Which effect is most likely in the mutant cells?
  4. In an agricultural watershed, fertilizer application increases, and nitrate concentrations in a nearby estuary rise from 0.2 mg/L to 1.5 mg/L. Soon after, seagrass coverage declines while jellyfish blooms become more common. Which explanation best accounts for these changes?
  5. A researcher maps the ends of newly produced eukaryotic RNAs and finds that cleavage occurs downstream of an AAUAAA sequence in the transcript. In untreated cells, a stretch of adenosines is present after the cleavage site. In cells where the AAUAAA sequence is mutated in the gene, cleavage at the usual position is reduced, and the downstream adenosine stretch is also reduced. Which explanation best accounts for the coupled changes in cleavage and adenosine addition?
  6. A population of prairie flowers varies in stem height. After grazing mammals become common, very tall stems are eaten frequently, while very short stems are shaded by surrounding vegetation and set fewer seeds. Flowers with intermediate stem height produce the most seeds that survive to adulthood. Across seven generations, the mean height changes little, but the frequency of extreme heights decreases. Which type of selection best explains the change in stem height distribution?
  7. A lipid-soluble signal M is added outside animal cells. In one experiment, cells respond even when a membrane receptor is blocked by an antibody. In another experiment, cells fail to respond when cytosolic proteins are removed from a cell extract, despite M being present. Which of the following best explains these results?
  8. A bird species has two populations separated by a desert. During an unusually wet decade, a narrow corridor of shrubs connects the populations, and some interbreeding occurs, producing viable hybrids. Over subsequent generations, the corridor disappears and contact becomes rare again. When contact happens, individuals preferentially mate with their own population's song type, and hybrid matings occur less often than expected from encounter rates. Which process most directly explains the increase in prezygotic isolation during intermittent contact?
  9. A student tests an enzyme at 10°C and 30°C with the same enzyme concentration and saturating substrate. The rate is 2 units/min at 10°C and 9 units/min at 30°C. The enzyme remains folded at both temperatures. Which explanation best accounts for the higher rate at 30°C? Assume temperature mainly affects molecular motion without disrupting the enzyme's tertiary structure.
  10. A plant cell's chloroplasts and mitochondria both contain internal membranes that compartmentalize reactions. In chloroplasts, a proton gradient forms across the thylakoid membrane; in mitochondria, a gradient forms across the inner membrane. Which feature best explains how these gradients can drive ATP synthesis in both organelles?
  11. Chloroplast thylakoids are illuminated while suspended in a solution containing ADP and Pi. ATP is produced under illumination. When the surrounding solution is buffered so that the pH outside the thylakoids becomes equal to the pH inside the thylakoid lumen, ATP production decreases even though light intensity and electron transfer through photosystems remain unchanged. The thylakoid membrane remains intact and impermeable to protons except through ATP synthase. This focuses on how light-driven electron transfer establishes a proton gradient used to power ATP formation.
  12. In a desert mouse population, coat color ranges from very light to very dark. On a landscape with both pale sand patches and dark rock patches, owls capture mostly medium-colored mice, while very light mice survive better on sand and very dark mice survive better on rock. After many generations, the population shows fewer medium coats and more of both extremes. Which pattern best illustrates the selection acting on coat color?
  13. A coral reef and a nearby seagrass bed were sampled for fish species. The reef contained 25 species with moderate evenness across species. The seagrass bed contained 10 species, with one species representing 60% of individuals. Sampling effort and total individuals counted were equal. Which inference is best supported about biodiversity between habitats?
  14. A grassland is sampled in two years. Year 1: 15 plant species are recorded, with no single species exceeding 20% cover. Year 2: 15 plant species are recorded, but one species accounts for 65% cover and several species are rare. Total plant cover is similar across years. Which conclusion is best supported about biodiversity from Year 1 to Year 2?
  15. A cell's cytosol has a solute concentration of 0.26 M. The cell is placed in Solution Q containing 0.18 M nonpenetrating solute; water crosses the membrane freely. After reaching equilibrium, the cell's volume is greater than its initial volume. Which conclusion is most likely about Solution Q relative to the cell?
  16. A cell is exposed to a polar drug at higher concentration outside than inside. The drug enters only when a specific transporter is expressed. Uptake is unaffected by ATP depletion and by collapsing Na+^+ and H+^+ gradients. The uptake rate increases with external drug concentration but plateaus at high concentration. Which mechanism best explains drug entry?
  17. In muscle cells, epinephrine binds a β-adrenergic GPCR, activating Gαs and adenylyl cyclase to raise cAMP. cAMP activates PKA, which phosphorylates glycogen phosphorylase kinase (GPK). Activated GPK phosphorylates glycogen phosphorylase (GP), increasing glucose-1-phosphate release. A student adds a nonhydrolyzable GTP analog that locks Gαs in its GTP-bound state, then adds epinephrine. Compared with epinephrine alone, GP activity increases more and remains elevated longer. Which step is most directly affected by the nonhydrolyzable GTP analog?
  18. A researcher designs two detergents to disrupt membranes. Detergent P has a single hydrocarbon tail and a large polar head, while detergent Q has two hydrocarbon tails and a polar head similar in size to phospholipid heads. In water, amphipathic molecules arrange to shield hydrophobic regions from water. Single-tailed amphipaths often form micelles, whereas two-tailed amphipaths more readily form bilayers because of their shape and packing. Which statement best predicts how detergent P behaves in water compared with detergent Q?
  19. A field ecologist counts rabbits in a fenced grassland each spring: Year 1 = 40, Year 2 = 80, Year 3 = 140, Year 4 = 175, Year 5 = 185. Over the same period, grass biomass per square meter declines and then stabilizes, and predator numbers remain low and constant. No rabbits enter or leave the fence. The ecologist notes that the annual increase in rabbits becomes smaller after Year 3 even though births continue. Assume counting methods are consistent each year. Which pattern best illustrates the population's growth across the five years?
  20. A cell is exposed to a lipid-soluble steroid at higher concentration outside than inside. The steroid appears in the cytosol rapidly, and uptake is unaffected by inhibitors of ATP production and by blocking membrane proteins. No vesicles are observed. Which mechanism best explains steroid entry into the cell?

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