Practice Test 91 - AP Biology

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

A phylogenetic analysis uses sequences from 20 nuclear genes to compare three mammal species: Aardwolf, Hyena, and Wolf. Across genes, aardwolf and hyena share more derived nucleotide substitutions with each other than either shares with wolf. Fossil evidence places early hyena-like carnivores in Africa before the earliest known fossils of wolves in Eurasia. Which conclusion is best supported by the molecular and fossil evidence together?
In epithelial cells, a ligand binds a membrane receptor and quickly increases cytosolic IP3. When a phospholipase C (PLC) inhibitor is added, the ligand still binds the receptor but IP3 does not increase. All other cellular components are unchanged. Which of the following best explains PLC's role in the early signaling pathway?
Human ABO blood type is controlled by three alleles: $I^A$ , $I^B$ , and $i$ . $I^A$ and $I^B$ are codominant, and both are dominant to $i$ . A person with type AB ( $I^A I^B$ ) has a child with a person with type O ( $ii$ ). Which blood type(s) can their child have?
A eukaryotic gene contains two alternative polyadenylation (poly(A)) sites in its final exon. In one condition, cleavage and poly(A) addition occur at the upstream site; in another condition, cleavage occurs at the downstream site. The transcription start site and all splice sites are unchanged between conditions, and the same exons are included in both mature RNAs. The upstream poly(A) site is 300 nucleotides earlier than the downstream site within the same exon. Which outcome best accounts for the difference between the two mature RNA molecules?
A signaling module at the G2/M checkpoint integrates two inputs: (1) an activating phosphatase that removes inhibitory phosphate from Cdk1, and (2) an inhibitory kinase that adds the inhibitory phosphate. In an experiment, the inhibitory kinase is hyperactive, but the phosphatase activity is also increased twofold. Active M-cyclin–Cdk1 depends on the balance of these opposing activities. Which change would most likely shift the balance toward mitotic entry?
In a comparative anatomy lab, students observe that the forelimbs of humans, cats, whales, and bats each contain a humerus, radius, ulna, carpals, metacarpals, and phalanges arranged in the same relative order, although the bones differ in length and thickness. Embryological observations show that these limb bones develop from similar tissue buds in early embryos of all four species. Fossils of early tetrapods show a similar limb-bone pattern. Which inference is best supported by these observations about evolutionary relationships?
In a plant leaf, Cell A releases a small peptide signal into the extracellular fluid. Nearby Cell B responds within seconds only if it has membrane protein R. When Cell B is treated with a protease that removes extracellular protein domains, the peptide is still present outside the cell but Cell B no longer responds. A different nearby cell lacking R shows no response under any condition. Which of the following best explains the loss of Cell B's response after protease treatment?
In a yeast cell, ligand Y binds a membrane receptor that normally causes a rapid rise in cytosolic Ca $^{2+}$ . A mutant strain has a receptor that still binds ligand Y but shows no Ca $^{2+}$ increase. When researchers add a drug that directly opens Ca $^{2+}$ channels in the endoplasmic reticulum membrane, both wild-type and mutant cells show the Ca $^{2+}$ increase. The receptor's extracellular domain is unchanged in the mutant, but a single amino acid substitution occurs in the cytosolic domain. Which prediction is most consistent with how the mutation disrupts early signal transduction?
Two polysaccharides are composed only of glucose. Polymer X has mostly α-1,4 glycosidic bonds with occasional α-1,6 branches. Polymer Y has only β-1,4 glycosidic bonds and is unbranched. In water, Polymer X forms compact granules, while Polymer Y forms long, straight chains that associate into bundles. Which statement best explains Polymer Y's tendency to form bundled fibers?
Two populations of the same frog species occupy neighboring valleys separated by a low ridge. Adults can cross the ridge, but they rarely do. In valley A, males call at 900 Hz; in valley B, males call at 1,200 Hz. Females show strong preference for the local call frequency, and cross-valley matings are uncommon even when adults are placed together at a breeding pond. Genetic markers show increasing divergence between valleys. Which isolating mechanism most directly reduces gene flow between the populations?
In a study of cell specialization, researchers examine two cell types from the same frog: a lens cell and a stomach cell. Both contain identical DNA. The lens cell has high levels of mRNA for crystallin proteins, while the stomach cell has high levels of mRNA for a digestive enzyme; the other transcript is nearly absent in each cell type. Which explanation best accounts for this pattern?
In cultured liver cells, epinephrine binds a GPCR, activating Gs and adenylyl cyclase to raise cAMP. cAMP activates protein kinase A (PKA). Active PKA phosphorylates a scaffolded phosphodiesterase (PDE), increasing PDE catalytic activity and accelerating cAMP hydrolysis to AMP. When PDE is inhibited, cAMP remains elevated longer after the same epinephrine pulse. Which outcome best illustrates the role of the PKA→PDE feedback in this pathway?
A population of fish is introduced to a new, enclosed lake with abundant food and no predators. For the first two years, the population increases rapidly. By year five, population size fluctuates slightly around 12,000 individuals, and measurements show reduced average food availability per fish compared with year one. No fish enter or leave the lake. Which explanation best accounts for the stabilization around 12,000 fish?
In a bacterial culture, a researcher adds a compound that specifically inhibits ATP synthase. Shortly after, the proton gradient across the plasma membrane becomes larger, and the rate of ATP synthesis by chemiosmosis decreases. Oxygen consumption continues for a short time before slowing. The inhibitor does not directly block electron carriers, and the membrane remains otherwise intact. Which statement best explains why ATP production by chemiosmosis decreases?
A strain of bacteria with identical genomes was grown in either lactose-containing medium or glucose-containing medium. After 30 minutes, cells in lactose medium produced high levels of β-galactosidase, while cells in glucose medium produced very low levels. DNA sequencing of the β-galactosidase gene was identical in both cultures. When glucose-grown cells were transferred to lactose medium, β-galactosidase levels increased. Which explanation best accounts for the enzyme level differences between media?
In a cell, a ligand activates a receptor that stimulates production of ROS (reactive oxygen species) near the membrane. ROS transiently inactivates a protein tyrosine phosphatase (PTP) by oxidizing its active-site cysteine, increasing receptor phosphorylation and downstream kinase activation. As downstream kinase activity rises, it activates an antioxidant enzyme that reduces ROS back to baseline. Inhibiting the antioxidant enzyme increases the duration of receptor phosphorylation. Which outcome best illustrates how feedback alters pathway behavior?
A mitotic cell is observed with chromosomes aligned at the metaphase plate. All kinetochores appear attached, but one chromosome is attached to microtubules from both poles on the same kinetochore (merotelic attachment), reducing proper tension. The spindle checkpoint responds to attachment and tension before anaphase. In cells where this checkpoint fails to detect the problem, anaphase proceeds on schedule. Which outcome is most likely for the affected chromosome during segregation?
Two identical aquariums are stocked with guppies and supplied with the same amount of food each day. Tank A starts with 15 guppies and Tank B starts with 90 guppies. After 4 weeks, Tank A has 40 guppies, while Tank B has 95 guppies. Water temperature, pH, and light are kept constant, and no fish are removed. Observations show more fin damage and aggressive chasing in Tank B than Tank A. Which factor is most likely density dependent in Tank B?
A marine crab is moved from seawater (high salinity) to brackish water (lower salinity) for 30 minutes. The crab begins producing larger volumes of more dilute urine while maintaining activity level. Which response best explains how the crab maintains internal ion and water balance during this short-term exposure?
An epithelial tissue sample is stained for cell–cell junction proteins. The apical region shows continuous "belt-like" junctions that prevent dye in the lumen from passing between cells. Which feature best explains how these junctions limit leakage?

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