Practice Test 16 - AP Biology

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

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1. A lab compares meiosis in two cells from the same individual. In cell 1, no chiasmata are observed for chromosome pair 3. In cell 2, a chiasma is observed between the homologs of chromosome pair 3 before metaphase I. After meiosis II, cell 2 produces some gametes with chromatids containing a segment originally on the other homolog, while cell 1 does not. Assume all other chromosomes behave normally. Which process best explains why cell 2 produces additional chromosomal combinations in its gametes?
2. In a lab model of early cells, repeated osmotic swelling causes the plasma membrane to invaginate and pinch off, forming internal vesicles. After many cycles, microscopy shows a stable network of internal membranes that remains continuous with the plasma membrane in some regions, and the internal compartments contain the same lipid composition as the cell surface. No internal compartment contains its own DNA, and all ribosomes observed are free in the cytosol. Which explanation best accounts for the origin of these internal membranes?
3. A scientist isolates cells that have just completed cytokinesis. In these cells, chromosomes have decondensed, nuclear envelopes are intact, and each cell contains a single copy of each chromosome (one chromatid per chromosome). The cells have not begun DNA replication, and total DNA content is 2C. The scientist then provides nutrients that allow cells to continue cycling. Assuming normal progression, which change will occur during the next S phase in each cell?
4. A kinase cascade activated by cell stress increases activity of a Cdk inhibitor (CKI) by phosphorylating it, which strengthens CKI binding to cyclin–Cdk complexes. In treated cells, cyclin levels are unchanged, but overall Cdk activity decreases. The stress signal is then removed, and a phosphatase normally dephosphorylates CKI to weaken binding. Which change would most likely prolong cell-cycle arrest after stress removal?
5. A lab compares two proteins with identical amino acid composition but different sequences. In water, Protein 1 folds into a compact globular shape with a specific active site; Protein 2 folds into a different shape and shows no catalytic activity. Both proteins have the same numbers of polar, nonpolar, acidic, and basic side chains, but their order differs. Which statement best explains why only Protein 1 is catalytically active?
6. A cell maintains high cytosolic K+ and low cytosolic Na+ using a membrane pump that hydrolyzes ATP each cycle. When ATP levels drop sharply, the Na+ gradient across the membrane decreases over time. Which explanation best accounts for the change in gradient as energy flow changes?
7. A plant cell's vacuolar membrane contains an H+ pump that moves H+ from cytosol (pH 7.2) into the vacuole (pH 5.5). When ATP is supplied, vacuolar pH decreases further; when ATP is removed, the pH gradient dissipates over time through existing leak pathways. Adding an inhibitor of ATP hydrolysis stops further acidification immediately. Which transport mechanism is directly responsible for creating the H+ gradient?
8. A signaling molecule S is hydrophobic and is added to animal cells. Within 30 seconds, a cytosolic enzyme becomes phosphorylated. When the plasma membrane proteins are removed with a mild protease, S still enters cells and the enzyme is phosphorylated. However, when a kinase inhibitor is added, S enters but phosphorylation does not occur. Which prediction is most consistent with how S is received and early transduction begins?
9. During the spindle assembly checkpoint, unattached kinetochores generate a signal that inhibits the anaphase-promoting complex/cyclosome (APC/C), preventing ubiquitination of securin. When all kinetochores are attached, APC/C becomes active, securin is ubiquitinated, and separase is freed to cleave cohesin. In an experiment, a peptide is introduced that directly inhibits APC/C even when kinetochores are properly attached. Which outcome is most likely in treated cells?
10. A protist species contains chloroplast-like organelles with two membranes. Inside the organelle are thylakoid membranes, circular DNA, and ribosomes similar to bacterial ribosomes. When the protist is treated with antibiotics that inhibit bacterial ribosomes, organelle protein synthesis decreases while cytosolic protein synthesis is largely unaffected. The organelles replicate by a fission process. Which explanation best accounts for the origin of these chloroplast-like organelles?
11. A beetle population includes green and brown individuals. Color is heritable and controlled by alleles at a single locus. After a wildfire, the ground becomes darker, and birds more easily spot green beetles than brown beetles, reducing the number of green beetles that reproduce. Which explanation best accounts for how allele frequencies will change over generations following the fire?
12. A synthetic vesicle is made only of phospholipids, creating a hydrophobic membrane core. Molecules that are small and nonpolar cross more readily than molecules that are large, polar, or charged. Consider glycerol (small but polar due to three hydroxyl groups) and methane (very small and nonpolar). Neither molecule carries a net charge. No transport proteins are present, and temperature is constant. Which molecule would most likely have the higher permeability across the vesicle membrane?
13. A diploid cell has two alleles of a gene encoding a membrane channel. One allele acquires a single-base substitution in the coding region so that one mRNA codon becomes UAG instead of CAG. The other allele remains unchanged. The mutant allele is transcribed, and its mRNA is translated using the standard genetic code, with translation beginning normally. Which outcome is most likely for the mutant allele's protein product?
14. An enzyme is incubated for 5 minutes in three conditions before adding substrate: (1) pH 7 at 25°C, (2) pH 7 at 80°C, and (3) pH 7 at 25°C with 2 M urea. After incubation, all samples are cooled to 25°C and placed into identical pH 7 reaction mixtures with saturating substrate. Urea disrupts hydrogen bonding and hydrophobic interactions that stabilize tertiary structure. Which sample will most likely show the lowest activity after transfer to the reaction mixture?
15. The plasma membrane is a phospholipid bilayer with embedded proteins. The arrangement of hydrophilic heads and hydrophobic tails creates a selective barrier: small nonpolar molecules diffuse through the bilayer more readily than polar molecules. A researcher increases the membrane's cholesterol content while keeping phospholipid types and transport protein numbers constant. Afterward, the rate of simple diffusion of small nonpolar molecules decreases at the same temperature. Which change in membrane structure most likely caused the decreased diffusion?
16. Two enzymes, E1 and E2, catalyze the same reaction using the same substrate. At identical enzyme concentrations and assay conditions, E1 reaches a maximum initial rate of 100 units/min, while E2 reaches a maximum initial rate of 50 units/min. At low substrate concentrations, E2's rate increases more steeply than E1's. No inhibitors are present, and both enzymes remain stable. Which inference is most consistent with these observations about the enzymes' kinetic properties?
17. In pancreatic cells, ligand Z binds a GPCR that activates Gαq, stimulating PLC to generate IP3. IP3 opens ER Ca2+ channels, increasing cytosolic Ca2+. Ca2+ binds calmodulin, and the Ca2+-calmodulin complex activates kinase Cmk, which phosphorylates cytosolic enzyme V. A drug that chelates cytosolic Ca2+ is added just before Z. After Z addition, IP3 levels rise normally, but enzyme V phosphorylation is greatly reduced. Which step is most directly blocked by the Ca2+ chelator?
18. A biologist observes a population of frogs breeding in ponds. Pond A has 40 adult frogs; Pond B has 400 adult frogs. Both ponds have similar water chemistry and experience the same precipitation. Tadpole counts show that a smaller fraction of eggs in Pond B survive to metamorphosis, and the biologist records frequent tadpole-to-tadpole contact and more cloudy water in Pond B. A parasite that spreads through waterborne contact is detected more often in Pond B. Which explanation best accounts for the lower survival in Pond B?
19. A researcher compares two polysaccharides made only of C, H, and O. One polymer has glucose subunits connected mainly by β-1,4 glycosidic bonds, producing long, straight chains that align side-by-side. The other polymer has many α-1,6 branch points in addition to α-1,4 linkages, producing a highly branched structure. Both polymers contain numerous hydroxyl (–OH) groups capable of hydrogen bonding with water and with neighboring chains. In a plant cell wall, the straight-chain polymer forms tightly packed microfibrils that resist stretching because many hydrogen bonds form between adjacent chains. Which feature best explains why the straight-chain polymer increases tensile strength in cell walls?
20. A pesticide runoff event reduces aquatic insect larvae abundance in a stream by 70%. In the following month, algae on rocks increases and dissolved oxygen shows larger daily swings. Which consequence is most likely?

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