Practice Test 41 - AP Biology

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

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1. An enzyme is tested with two molecules: Substrate X and Substrate Y. With equal enzyme and substrate concentrations, the initial rate is high for X but near zero for Y. Both molecules are stable under assay conditions, and no inhibitor is added. The enzyme's activity returns when X is reintroduced after exposure to Y. Which explanation best accounts for the difference in reaction rates?
2. In a eukaryotic cell, the nucleus is separated from the cytosol by a nuclear envelope with pores that allow selective transport. A fluorescently labeled cytosolic enzyme is too large to pass through the pores and remains outside the nucleus. When the nuclear envelope is experimentally removed, the enzyme enters the former nuclear region and begins catalyzing reactions on nuclear substrates, altering the balance of those substrates in the cell. Which feature best explains how compartmentalization normally contributes to cellular control?
3. At a replication fork, the lagging strand is synthesized as short DNA fragments that begin at multiple primers, while the leading strand is synthesized continuously. Both strands are extended 5′→3′ by adding nucleotides to a 3′ end. A mutation reduces the frequency of primer synthesis without affecting helicase activity or polymerase catalytic rate. Which effect is most likely on DNA synthesis at the fork?
4. In a flowering plant, petal color is determined by codominant alleles $P^R$ and $P^W$. $P^R$ $P^R$ plants have red petals, $P^W$ $P^W$ plants have white petals, and $P^R$ $P^W$ plants have red-and-white striped petals. A striped plant is crossed with another striped plant. Which outcome best predicts the proportion of striped offspring?
5. In cultured muscle cells, ligand L binds membrane receptor R and increases intracellular $\mathrm{Ca^{2+}}$. R activates G protein Gα, which activates adenylyl cyclase to produce cAMP. cAMP activates protein kinase A (PKA), which phosphorylates channel C, increasing $\mathrm{Ca^{2+}}$ influx. A phosphodiesterase (PDE) hydrolyzes cAMP to AMP. When cells are treated with L plus a PDE inhibitor, cAMP rises 4× higher than with L alone, and $\mathrm{Ca^{2+}}$ influx remains elevated longer. Which change would most likely reduce the duration of the $\mathrm{Ca^{2+}}$ response to L without preventing initial receptor binding?
6. A finch population shows beak sizes in three common classes: small, medium, and large. In a habitat with mostly very small seeds and very large seeds, small- and large-beaked finches produce more fledglings than medium-beaked finches. Over five generations, the frequency of medium beaks drops while both small and large beaks increase. Which pattern best illustrates the selection acting on beak size?
7. Cells with internal solute concentration 0.50 M are placed in a solution containing 0.40 M nonpenetrating solute. The plasma membrane is permeable to water but not to the solute, and the external solute concentration remains constant during observation. All cells start with the same volume and lack a cell wall. Considering only osmosis, which outcome is most likely after several minutes?
8. In a large population, allele $M$ has frequency 0.3 and allele $m$ has frequency 0.7. The population is in Hardy-Weinberg equilibrium. Which expected proportion of individuals carry at least one $M$ allele?
9. A clonal line of houseplants was grown in identical pots, but half received nitrogen-rich fertilizer and half received nitrogen-poor fertilizer. After 4 weeks, nitrogen-rich plants had higher chlorophyll content and darker leaves. DNA sequencing of a chlorophyll-synthesis gene was identical between treatments. When nitrogen-poor plants were later given nitrogen-rich fertilizer, newly produced leaves were darker than older leaves on the same plants. Which explanation best accounts for the fertilizer-dependent leaf color differences?
10. In a neuron, voltage-gated Ca2+ channels are clustered at the presynaptic terminal membrane. Calcium-binding proteins and vesicle fusion proteins are also concentrated in this same region. When Ca2+ channels are experimentally dispersed along the axon membrane, the same total Ca2+ influx occurs, but neurotransmitter release at the synapse decreases. Which feature best explains how localized compartmentalization increases efficiency of this cellular process?
11. A researcher isolates mitochondria from muscle cells and measures pH across the inner mitochondrial membrane. The intermembrane space is more acidic than the matrix. When a chemical that makes the inner membrane permeable to H+ is added, ATP production decreases even though electron transport proteins are still present. Which conclusion is best supported about how compartmentalization influences ATP production?
12. A red blood cell is moved from 300 mOsm solution to 150 mOsm solution. The membrane is permeable to water but not to the major solutes in either solution. Aquaporins are present, and cellular ATP levels are unchanged during the first minute. After transfer, the cell's volume increases measurably. Which outcome is most likely during the first minute, and what transport explains it?
13. A protein-coding gene is compared among four primates. Species P and Q differ at 2 of 1,000 nucleotides, P and R differ at 18 of 1,000, and P and S differ at 45 of 1,000. The same pattern occurs across several unlinked genes. No evidence suggests unusual mutation rates in any lineage. Which conclusion is best supported by these molecular comparisons?
14. Researchers sequence a 900-base-pair region of a mitochondrial gene from three species: Species X and Y differ at 9 sites, Species X and Z differ at 90 sites, and Species Y and Z differ at 92 sites. The gene region is confirmed to be orthologous in all three species. No other traits are considered. Which inference is best supported by these sequence comparisons about evolutionary relationships?
15. A plasmid vector and a DNA insert are cut with the same restriction enzyme, generating complementary sticky ends. The fragments are mixed with DNA ligase, then used to transform bacteria. After plating on antibiotic-containing agar, many colonies appear. A colony PCR using primers flanking the plasmid's multiple cloning site yields either a 300-bp product (empty vector) or a 900-bp product (vector plus insert). Which result best supports successful ligation of the insert into the plasmid in a colony?
16. To test whether compartments could enhance reactions, researchers encapsulated ribonucleotides and a catalytic RNA inside fatty-acid vesicles. Product formation was higher inside intact vesicles than in an equal-volume solution without vesicles. The only difference was compartmentalization. Which claim is best supported about protocells?
17. In snapdragons, flower color shows incomplete dominance: allele R produces red pigment and allele W produces no pigment. RR plants have red flowers, WW plants have white flowers, and RW plants have pink flowers. A student crosses two pink-flowered plants (RW × RW) and grows 160 offspring under the same conditions. Assuming fertilization is random and the trait is controlled by this single gene, which outcome best predicts the numbers of red, pink, and white flowers among the offspring?
18. In a temperate lake, an introduced planktivorous fish increases and reduces large zooplankton density by 70%. Phytoplankton biomass then doubles. Which consequence is most likely for the lake ecosystem?
19. A cell in meiosis is observed with homologous chromosomes paired and connected by chiasmata, forming tetrads. Later, homologous chromosomes move to opposite poles while sister chromatids remain attached. Which stage of meiosis is most consistent with the observed separation of homologous chromosomes while sister chromatids stay together?
20. In transcription, the new nucleotide enters with three phosphates. The 3'-OH attacks the:

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