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Practice Test 21 - AP Biology

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

  1. In AaBb × AaBb, what fraction of offspring showing both dominant traits are heterozygous at both loci?
  2. In a fish species, scale color shows codominance. Allele G produces green scales and allele S produces silver scales. Heterozygotes (GS) have both green and silver scales visible. Two heterozygous fish are crossed (GS × GS). Which outcome best predicts the phenotypes among their offspring?
  3. Enzyme V catalyzes S → P. In a closed reaction vessel with fixed [E] and [S], the initial rate is high but decreases over time as P accumulates. When purified P is added at time zero to a separate reaction with the same [E] and [S], the initial rate is lower than the control. No other conditions change. Which explanation best accounts for the effect of added product on reaction rate?
  4. An enzyme's active site includes a positively charged lysine R group that stabilizes a negatively charged substrate during binding. A mutation replaces this lysine with glutamine, which is polar but uncharged. The enzyme's overall folding remains similar, but substrate affinity decreases. Which statement best describes the molecular reason for reduced affinity?
  5. In immune cells, ligand L binds receptor R and activates kinase J, which phosphorylates protein A. Phosphorylated A activates kinase K, which phosphorylates enzyme E, increasing reactive oxygen production. Protein B binds phosphorylated A and recruits a phosphatase that removes phosphate from A, reducing signal strength. Cells lacking protein B show a 3× higher peak of phosphorylated E after L addition but similar receptor occupancy. Which change would most likely mimic the effect of deleting protein B?
  6. Endocrine cells in the pancreas secrete peptide hormone H into the bloodstream. Liver cells display surface receptor S; skeletal muscle cells do not. In an experiment, H is injected and binds strongly to liver cells but not to skeletal muscle cells. A modified H variant (H*) is injected and is found inside liver cells without binding to S. Which outcome is most likely if H* is injected instead of H?
  7. A student builds a model of a nucleic acid strand with repeating sugar-phosphate units and bases attached to each sugar. The student notes that nucleotides are connected through bonds between the 3′ carbon of one sugar and the phosphate attached to the 5′ carbon of the next. This creates a strand with distinct ends labeled 5′ and 3′. Which statement best explains how this structure relates to the strand's directionality?
  8. A kinase cascade activated by extracellular growth factor signaling increases CDK activity by promoting removal of an inhibitory phosphate from a G1 CDK. In the absence of growth factor, the inhibitory phosphate remains on the CDK, and cells do not pass the G1 checkpoint. Researchers introduce a small molecule that mimics the growth factor signal downstream by directly activating the phosphatase that removes the inhibitory phosphate. Nutrients and ATP are sufficient, and DNA is undamaged. Which outcome is most likely after adding the small molecule?
  9. A cell expresses Receptor R, a transmembrane protein with an extracellular ligand-binding domain and an intracellular kinase domain. Ligand L binding leads to rapid dimerization of R and phosphorylation of specific tyrosines on the intracellular domains. A mutant R that still binds L but cannot dimerize shows no detectable tyrosine phosphorylation. Which of the following best explains why phosphorylation fails in the mutant?
  10. A biochemist notes that ATP hydrolysis releases heat into the surrounding aqueous cytosol, but the cytosol temperature changes only slightly. Water molecules are polar, with partial negative charge on oxygen and partial positive charges on hydrogen. Neighboring water molecules form hydrogen bonds, creating an intermolecular network. When heat is added, some of the energy increases molecular motion, and some is used to disrupt hydrogen bonds before the average kinetic energy rises substantially. This buffering reduces rapid temperature shifts in the solution. Which feature best explains water's ability to limit temperature change after heat release?
  11. In a cell, a ligand activates a receptor that opens a Ca2+^{2+} channel in the plasma membrane. Incoming Ca2+^{2+} activates a Ca2+^{2+}-dependent kinase that phosphorylates the same Ca2+^{2+} channel, increasing its open probability. A kinase inhibitor reduces the steep rise in Ca2+^{2+} during ligand exposure but does not affect ligand binding. Which outcome best illustrates how feedback changes pathway behavior?
  12. A meiotic cell from a diploid organism (2n = 4) is observed during metaphase I. Each homologous pair is synapsed as a tetrad, and the two tetrads align at the metaphase plate with random orientation relative to the poles. Homologs separate in anaphase I, and sister chromatids separate in anaphase II. Which number of distinct chromosome combinations is most likely possible in gametes from independent assortment alone?
  13. A eukaryotic gene is transcribed into pre-mRNA that is complementary to the template DNA strand. In a mutant cell, RNA polymerase II has an altered active site that occasionally incorporates U opposite template-strand G, but otherwise transcription proceeds. Sequencing of newly synthesized pre-mRNA shows some positions where A is expected (from pairing with template T) are unchanged, but some positions where C is expected (from pairing with template G) are instead U. Which explanation best accounts for the base changes observed in the mutant pre-mRNA?
  14. In a city park, a population of pigeons includes two alleles at a gene affecting beak depth: D (deeper) and d (shallower). After a shift in available food toward larger, harder seeds, researchers sample allele frequencies each breeding season for 5 seasons. Allele D increases from 0.33 to 0.58. Which observation best demonstrates continuing evolution in the pigeon population?
  15. In a diploid organism, homologous chromosome 7 carries alleles A and B on one homolog (AB) and a and b on the other (ab). During prophase I, the homologs synapse and form a tetrad. A single crossover occurs between the loci, and chromatids separate normally through meiosis I and II. As a result, the four gametes produced from this meiosis include two parental chromatid types and two recombinant chromatid types. Which meiotic process best explains the appearance of recombinant gametes containing Ab and aB allele combinations?
  16. An enzyme-catalyzed reaction is started with fixed enzyme and substrate concentrations. After 2 minutes, an additional amount of substrate is added, but the measured reaction rate immediately after the addition does not increase. Prior measurements showed the rate had already plateaued at the original substrate concentration. Temperature and pH are constant and no inhibitor is added. Which explanation best accounts for the unchanged rate after adding more substrate?
  17. A membrane-associated molecule contains a glycerol backbone attached to two fatty acid tails and a phosphate-containing head group. Formation of the ester bonds between glycerol and fatty acids involves dehydration reactions, but the molecule is not an extended chain of repeating monomers. The phosphate-containing region is polar, while the hydrocarbon tails are nonpolar, influencing how the molecule interacts with water. Which statement best predicts the molecule's behavior in an aqueous environment?
  18. A coastal snail population shows variation in shell thickness controlled largely by a heritable allele T (thick) versus t (thin). A crab species preys on these snails and is more successful at crushing thin-shelled individuals, reducing their survival to reproduction. Thick- and thin-shelled snails occur in the same interbreeding population, and offspring shell thickness resembles that of their parents. Over multiple years, crab abundance remains high and other conditions are similar. Which outcome is most likely in the snail population over time?
  19. A population of freshwater snails was counted monthly in a pond after a fish predator was removed. Counts were: Month 0 = 40, Month 1 = 60, Month 2 = 90, Month 3 = 135, Month 4 = 200, Month 5 = 205, Month 6 = 198. No new predators were added, and food algae became visibly scarce by Month 4. Which explanation best accounts for the population trend over time?
  20. During translation, the ribosome moves along mRNA in the 5′ to 3′ direction, decoding each codon in order. A researcher introduces an mRNA in which the start codon AUG is unchanged, but the stop codon is mutated to a sense codon that is recognized by a tRNA. The mRNA sequence downstream contains additional codons before a later stop codon. Assume initiation occurs normally and the ribosome remains bound to the mRNA. Which outcome is most likely for the translated product compared with the original mRNA?

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