Practice Test 51 - AP Biology

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

Identical twin calves are fed either a high-fiber diet or a high-starch diet for 6 weeks. Calves on high-fiber diets show increased expression of genes for fatty-acid transport in intestinal cells and higher blood levels of short-chain fatty acids, with no DNA sequence differences detected. Which explanation best accounts for the phenotype differences between diets?
A cell begins meiosis with $2n=10$ after DNA replication. At metaphase I, homologous pairs align at the equator as tetrads. Which explanation best accounts for why the chromosome number is reduced in the cells produced after meiosis I?
A comparative study examines the pelvic bones of modern whales and terrestrial mammals. Whales have small internal pelvic bones that are not connected to hindlimbs, while a terrestrial mammal species has a pelvis attached to functional hindlimbs. Fossils of early whale ancestors show progressively reduced hindlimbs while retaining pelvic structures similar in position and composition to those of terrestrial mammals. Which inference is best supported by these observations?
A secretory gland cell is observed to have abundant rough endoplasmic reticulum (RER) with ribosomes attached, a prominent Golgi apparatus, and many small vesicles near the plasma membrane. Shortly after stimulation, the cell releases a burst of protein into the extracellular fluid without losing cytoplasm. Which outcome is most likely enabled by the arrangement of these cellular structures?
In cultured bacteria, gene X encodes a sugar transporter. A repressor protein binds a specific operator sequence overlapping the promoter and blocks RNA polymerase binding. When sugar S is added, S binds the repressor and reduces its affinity for the operator. After S addition, levels of gene X mRNA increase within minutes. Which change would most likely prevent the increase in transcription after adding sugar S?
During transpiration, water evaporates from moist cell walls inside a leaf and exits through stomata. Water molecules are polar, so they form hydrogen bonds: the partially positive hydrogen of one water molecule is attracted to the partially negative oxygen of another. When a water molecule at the air–water interface gains enough energy to leave as vapor, several hydrogen bonds must be broken. Because breaking these intermolecular attractions requires energy, evaporation removes relatively large amounts of thermal energy from the remaining liquid water and nearby tissues. Which statement best explains how hydrogen bonding contributes to evaporative cooling in leaves?
Biogeographers study a lizard lineage found on three volcanic islands that formed sequentially: Island 1 (5 million years), Island 2 (3 million years), and Island 3 (1 million years). Genetic data show the Island 3 population is most similar to Island 2, and Island 2 is most similar to Island 1. No lizards occur on the mainland, and ocean distances between islands are small enough for rare rafting events. Which conclusion is best supported by the geographic ages and genetic pattern?
To model prebiotic nucleotide formation, scientists exposed a mixture of simple carbon- and nitrogen-containing compounds to ultraviolet (UV) light under low-oxygen conditions. After exposure, the mixture contained detectable RNA nucleobases; an identical mixture kept in the dark produced much lower amounts. Early Earth lacked an ozone layer, increasing surface UV. Which conclusion is best supported by the evidence?
During translation, a ribosome begins at the start codon and reads mRNA in groups of three nucleotides (codons). A student provides an mRNA segment: 5′-AUG GCU UAC GAA UGA-3′. The tRNA carrying methionine binds the start codon, and each subsequent codon is matched by a tRNA with a complementary anticodon, adding one amino acid per codon. Translation ends when a stop codon enters the ribosome, and no tRNA binds that codon. Assume this segment is in-frame and is translated without skipping or editing. Which outcome is most likely for the polypeptide produced from this mRNA segment?
A eukaryotic gene has an intron with the 3′ splice acceptor site ending in the DNA sequence 5′-...AG-3′ (coding strand). A mutation changes this acceptor site to 5′-...AA-3′, while the coding exons remain unchanged. The pre-mRNA is transcribed normally. Which outcome is most likely for the mature mRNA?
In a cell, a ligand activates a GPCR that increases intracellular cAMP. cAMP activates PKA, which phosphorylates a regulator of G-protein signaling (RGS) protein, increasing RGS activity. RGS accelerates GTP hydrolysis on G $\alpha$ , reducing further adenylyl cyclase activation. When RGS phosphorylation is blocked, cAMP levels remain elevated longer after ligand addition. Which outcome best illustrates the effect of this feedback loop?
A student compares two cube-shaped cells with equal membrane composition in the same environment. Cell A has side length $2\,\mu m$ and Cell B has side length $6\,\mu m$ . Both rely on diffusion across the plasma membrane for oxygen entry, and both use oxygen at the same rate per unit volume. After a short time, Cell B becomes oxygen-limited sooner than Cell A. Which explanation best accounts for why Cell B becomes oxygen-limited sooner?
A plant leaf is exposed to constant light intensity while CO $_2$ concentration is suddenly decreased. Light reactions continue transferring electrons through thylakoid membrane complexes, producing ATP and reducing NADP $^+$ to NADPH. The Calvin cycle in the stroma uses ATP and NADPH when CO $_2$ is available to incorporate carbon into organic molecules. Which outcome is most likely after CO $_2$ decreases, assuming light intensity remains unchanged?
In cattle, coat color shows codominance. Allele $C^R$ produces red hairs and allele $C^W$ produces white hairs. C^RC^R cattle are red, C^WC^W cattle are white, and C^RC^W cattle are roan with both red and white hairs. A roan bull is crossed with a white cow. Which outcome best predicts the phenotypes of their calves?
In a eukaryotic cell, enzymes that break down fatty acids are located inside a peroxisome, while many cytosolic proteins are outside it. The peroxisome membrane restricts diffusion of these enzymes and concentrates fatty acid substrates inside the organelle. When the peroxisome membrane is experimentally made leaky, fatty acid breakdown rate decreases even though total enzyme amount is unchanged. Which feature best explains how compartmentalization increased the original reaction efficiency?
In a desert annual plant population, two alleles at a flowering-time locus occur: F (earlier flowering) and f (later flowering). After several years of shorter spring rains, researchers sample seeds each generation. Over 6 generations, allele F increases from 0.27 to 0.63, with similar population sizes and no new seed introductions. Which observation best demonstrates continuing evolution in this plant population?
In cultured liver cells, hormone H binds receptor R and activates kinase K, which phosphorylates enzyme E to increase second messenger M. As M accumulates, M binds an allosteric site on K and decreases K's catalytic activity without changing K abundance. When H is held constant, M rises quickly and then reaches a stable plateau. Which outcome best illustrates how this feedback affects pathway behavior after initial stimulation?
Two proteins have identical amino acid compositions but different sequences. Each polypeptide forms peptide bonds to create a primary structure, then folds via side-chain interactions into a specific tertiary structure. Protein A binds ligand L with high specificity; Protein B does not bind L. No post-translational modifications occur. Which statement best explains why binding differs between A and B?
In cardiac myocytes, a ligand activates a GPCR that stimulates PLC to generate IP $_3$ . IP $_3$ opens ER Ca $^{2+}$ channels to release Ca $^{2+}$ into the cytosol. Cytosolic Ca $^{2+}$ binds the IP $_3$ receptor and decreases its open probability, reducing further Ca $^{2+}$ release despite continued IP $_3$ presence. A mutation that prevents Ca $^{2+}$ binding to the IP $_3$ receptor increases the duration of Ca $^{2+}$ release. Which change would most likely result from this mutation?
In a coastal plant population, leaf waxiness varies due to heritable alleles W (high wax) and w (low wax). During frequent salt-spray events, plants with low wax lose more water and produce fewer seeds. Salt spray intensity remains high for decades. Which explanation best accounts for the expected change in allele frequencies?

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