Practice Test 64 - AP Biology

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

In an in vitro assay, cyclin A–CDK2 phosphorylates a substrate required for progression through S phase. A second protein binds cyclin A–CDK2 and blocks access to the substrate without removing cyclin A. When this blocker is added to cells, DNA content analysis shows fewer cells with intermediate (between 2N and 4N) DNA. Which change would most likely restore S-phase progression while the blocker remains present?
Pepsin is tested with identical protein substrate at constant temperature. At pH 2, the reaction rate is high; at pH 7, the rate is near zero. Which outcome is most likely at pH 7?
A membrane contains phospholipids whose head groups include a negatively charged phosphate. In aqueous environments, charged or polar groups form favorable interactions with water, while nonpolar hydrocarbon tails minimize contact with water. When phospholipids assemble into a bilayer, head groups face outward and tails face inward. Which feature best explains why phosphate-containing head groups orient toward the surrounding solution?
In a tissue, cells release cytokine C that diffuses locally. When an antibody that binds C is added to the extracellular fluid, neighboring cells no longer respond, but the secreting cells continue producing C. Which of the following best explains why neighboring cells stop responding?
In a diploid cell with $2n=6$ , homologous chromosomes pair as tetrads during prophase I. In one observed meiosis, homologous chromosomes (not sister chromatids) separate and move to opposite poles, and cytokinesis occurs before the next division. Which outcome is most likely at the end of meiosis I for each daughter cell's chromosome number and chromatid composition?
Cells exposed to a DNA-damaging agent activate a checkpoint kinase that phosphorylates and inhibits a Cdc25-like phosphatase. When inhibited, the phosphatase cannot remove inhibitory phosphates from Cdk1, keeping M-cyclin–Cdk1 inactive. In a follow-up treatment, a second drug blocks the checkpoint kinase's catalytic activity without affecting other kinases. The damaged DNA remains unrepaired during the observation window. Which effect would most likely occur after adding the second drug?
Enzyme M catalyzes S → P. A researcher tests two substrates, S1 and S2, each at 10 mM, with the same enzyme concentration. The initial rate with S1 is 90 µM/min, while with S2 it is 5 µM/min under identical conditions. No inhibitors are present. Which explanation best accounts for the large difference in initial rates?
A red blood cell is placed in a solution containing a detergent that disrupts membrane proteins but leaves most phospholipids intact. Shortly afterward, the cell loses its typical biconcave shape and becomes more spherical, even though the membrane remains continuous. In red blood cells, a network of cytoskeletal proteins is attached to the cytoplasmic side of the plasma membrane through specific membrane proteins. Which outcome is most likely explained by disrupting these membrane proteins?
Two similarly shaped cells are exposed to a toxin that enters by simple diffusion. Cell S has a surface area–to–volume ratio of 0.9; Cell T has a surface area–to–volume ratio of 0.3. Both are placed in the same toxin concentration for the same time. Which prediction best describes toxin concentration inside the cells after exposure?
In a vertebrate group, embryos of Species M and Species N both develop pharyngeal arches and a post-anal tail during early stages. In later development, Species M retains a tail and forms gill structures, while Species N loses the tail and develops middle-ear bones. Genetic analysis shows both species share highly similar sequences in a conserved developmental gene that regulates arch formation. Which conclusion is best supported by the embryological and molecular evidence?
In a meiosis II cell, chromosomes align individually at the metaphase plate, and then sister chromatids separate during anaphase II. Which description best predicts the immediate result of chromatid separation within that cell?
Which of the following processes ensures that evolution continues to occur even in an organism with a highly streamlined genome like M. genitalium?
A scientist compares transport in two cells that are identical except for size. Both use active transporters in the plasma membrane to import nitrate, and transporter density per unit membrane area is the same. The larger cell has 4 times the volume of the smaller cell. When both are placed in the same low-nitrate solution, the larger cell's cytoplasmic nitrate concentration rises more slowly. Which explanation best accounts for the slower rise?
A bird population shows variation in song frequency. In dense forest, males with lower-frequency songs achieve more matings than higher-frequency males. Over seven generations, the population's mean song frequency decreases steadily, while higher-frequency songs persist at low frequency. Which type of selection best explains this population-level change?
In neurons, neurotransmitter N opens receptor channel C, allowing Ca $^{2+}$ influx that activates kinase P. Active P phosphorylates C, increasing C open probability during continued N exposure. A phosphatase later removes the phosphate from C, returning open probability to baseline. Which change would most likely strengthen the feedback-driven increase in Ca $^{2+}$ influx during sustained N stimulation?
A gene produces an mRNA with a 3' untranslated region (3' UTR) that contains the sequence 5'-AUUUA-3', a motif that promotes rapid mRNA degradation. A mutation changes this motif to 5'-AGGGA-3' in the 3' UTR, while the coding region remains unchanged. Transcription rate of the gene is unchanged, and translation initiation signals in the mRNA are intact. Which outcome is most likely from this mutation?
A cell contains 0.40 M nonpenetrating solute and is placed in 0.60 M nonpenetrating solute. The membrane allows water movement but not solute movement. After 5 minutes, the cell's volume decreases. Which outcome is most likely for net water movement?
Cells are treated with ligand P2, and a rapid increase in cAMP is observed. A phosphodiesterase (PDE) activator is then added, and the cAMP increase becomes smaller despite unchanged receptor binding. Which of the following best explains the effect of increased PDE activity on early signaling output?
A leaf is exposed to light of a wavelength that is poorly absorbed by its chlorophyll pigments, while CO2 and water are abundant. Light reactions depend on pigment excitation to drive electron transfer and build a proton gradient. Which outcome is most likely compared with illumination by strongly absorbed wavelengths?
In a lab, cells are placed in a solution containing 20 mM glucose while cytosolic glucose is 2 mM. When a specific membrane protein is inhibited, glucose entry rapidly decreases, yet cellular ATP levels remain unchanged during the first minute. If the inhibitor is removed, glucose entry resumes immediately. No vesicle formation is observed, and the membrane remains intact. Which explanation best accounts for the glucose movement into the cells under normal conditions?

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