Practice Test 39 - AP Biology

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

To test whether simple compartments could maintain internal chemistry, scientists formed fatty-acid vesicles in water and added a fluorescent dye that increases signal when bound to RNA. When RNA strands were added outside the vesicles, fluorescence increased inside some vesicles over time, consistent with RNA entering or being retained within compartments. A control lacking vesicles showed uniform fluorescence with no localized increase. Which conclusion is best supported about protocells?
In a developing embryo, a cell releases morphogen M that diffuses away from the source. Cells close to the source respond, while distant cells do not. When a protein that binds and sequesters M is added uniformly, the responding region becomes smaller. Which of the following best explains this change?
A ligand L binds receptor R and activates kinase K, leading to phosphorylation of protein P. Phosphorylated P opens a chloride channel, changing membrane potential. A second protein, U, binds phosphorylated P and targets it for dephosphorylation by recruiting phosphatase D. In cells with a mutation that prevents U from binding P, channel opening persists longer after L removal. Which change would most likely reverse the persistence in the mutant cells?
In a cell-free extract, adding purified M-cyclin rapidly increases Cdk1 kinase activity and drives phosphorylation of mitotic substrates. Adding a Cdk inhibitor protein (CKI) binds the cyclin–Cdk complex and blocks its active site. When the CKI is phosphorylated by an upstream kinase, it dissociates from the complex. A researcher introduces a nonphosphorylatable CKI variant into the extract. Which effect would most likely be observed with the nonphosphorylatable CKI?
A population of prairie flowers varies in stem height. In years with frequent strong winds, very tall stems break more often, and very short stems are shaded and produce fewer seeds. Plants with intermediate stem height produce the most seeds across multiple seasons. After 20 generations, the variance in stem height decreases, while the mean height remains approximately the same. Alleles associated with extreme heights decline in frequency. Which selection pattern best accounts for these population-level trends?
A biologist adds a drug that specifically inhibits helicase in rapidly dividing cells. In untreated cells, helicase separates the parental DNA strands at replication origins, allowing DNA polymerase to copy each template strand using complementary base pairing. After drug treatment, the biologist measures incorporation of labeled deoxyribonucleotides into DNA and finds a large decrease. Which explanation best accounts for the decreased nucleotide incorporation in treated cells?
Many cell-surface proteins are glycoproteins with short, branched oligosaccharides attached. These carbohydrate chains contain numerous hydroxyl groups that can form hydrogen bonds with water and with complementary binding sites on other molecules. Compared with a purely hydrocarbon chain of similar size, an oligosaccharide is typically more hydrophilic in an aqueous environment. Which feature best explains why cell-surface oligosaccharides interact readily with water?
A large plant cell has a central vacuole occupying most of its volume. For cell size, this arrangement mainly...
A student models DNA and notes that uniform helix diameter depends on pairing rules. Which statement best explains how base structure maintains a consistent double-helix width?
A gene's coding region contains the sequence 5'-…ATC GAA TTT…-3' on the coding strand, corresponding to mRNA 5'-…AUC GAA UUU…-3' and amino acids Ile–Glu–Phe. In a mutant, a single base substitution changes the middle codon from 5'-GAA-3' to 5'-GTA-3', producing mRNA 5'-GUA-3'. The protein's length remains the same, and translation reaches the normal stop codon. Which outcome is most likely from this mutation?
A lipid bilayer's hydrophobic core reduces permeability of molecules that are strongly polar or charged. Consider two solutes: water (H2O), which is very small but polar, and butane (C4H10), which is larger but nonpolar. No aquaporins or other proteins are present. Both solutes are uncharged. Which prediction is most consistent with membrane permeability rules for simple diffusion?
A researcher compares two ligands that bind different receptors on the same cell type. Ligand A causes a rapid increase in cytosolic cAMP, while ligand B causes a rapid increase in cytosolic Ca $^{2+}$ . When cells are treated with a drug that inhibits adenylyl cyclase, the response to ligand A is blocked but the response to ligand B remains. When cells are treated with a PLC inhibitor, the response to ligand B is blocked but the response to ligand A remains. Both ligands are unable to cross the plasma membrane. Which of the following best explains the most likely difference in early transduction between the two pathways?
Researchers add an uncoupling protein to the inner mitochondrial membrane of actively respiring cells. The protein provides a pathway for protons to move from the intermembrane space back into the matrix without passing through ATP synthase. Electron transport complexes still transfer electrons from NADH to oxygen and continue pumping protons outward, but the proton gradient is reduced because protons leak back across the membrane. Oxygen remains available and ADP + Pi are present. Which outcome is most likely after the uncoupling protein is active?
A cell with genotype H/h and I/i has the H/h alleles on one chromosome pair and I/i alleles on a separate chromosome pair. Investigators prevent crossing over by inhibiting synapsis, but meiosis proceeds normally. Even without crossing over, gametes still show multiple combinations of H or h with I or i. Which process best accounts for genetic variation still observed among these gametes?
In humans, ABO blood type is determined 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 blood ( $I^AI^B$ ) has a child with a person with type O blood (ii). Which set lists all possible child blood types from this cross?
A nuclear gene in a vertebrate cell is transcribed in the nucleus, and its mRNA is translated by cytosolic ribosomes using the standard genetic code. A point mutation in the coding region, upstream of the normal stop codon, changes one codon in the mRNA from UGG to UGA. All other codons remain unchanged and in frame. Which outcome is most likely for the polypeptide translated from this mutant mRNA?
A cell is placed in a solution containing a small nonpolar molecule X at 50 mM; the cytosol initially contains 0 mM X. The membrane has no known transporters for X. When the external concentration is doubled, the initial rate of X entry doubles. ATP depletion has no effect on X entry, and no saturation is observed. Which mechanism best explains X movement into the cell?
A freshwater protist is placed into a solution with a much higher solute concentration than its cytoplasm. Over several minutes, the cell's volume decreases and the plasma membrane pulls slightly away from the cell wall–like outer layer. Microscopy shows no change in the number of mitochondria, but the contractile vacuole becomes inactive. Which outcome is most likely due to the structure and properties of the plasma membrane?
An enzyme is incubated for 5 minutes at 55°C, then cooled to 25°C before measuring activity at constant pH and substrate. The post-cooling rate is 3 units versus 20 units for an enzyme kept at 25°C. Which explanation best accounts for the reduced rate after cooling?
In skeletal muscle cells, Ca2+ is stored at high concentration inside the sarcoplasmic reticulum (SR), while cytosolic Ca2+ remains low at rest. Upon stimulation, Ca2+ is released from the SR into the cytosol, briefly increasing cytosolic Ca2+. If the SR membrane becomes leaky, resting cytosolic Ca2+ rises and contraction becomes less precisely controlled. Which feature best explains how compartmentalization supports control of muscle contraction?

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