Practice Test 50 - AP Biology

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

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1. A gene produces a pre-mRNA with exons 1–4. In tissue X, the mature mRNA contains exons 1, 2, and 4. In tissue Y, the mature mRNA contains exons 1, 3, and 4. DNA sequencing shows the gene is identical in both tissues, and transcription produces the same pre-mRNA sequence. The difference arises during nuclear RNA processing. Which explanation best accounts for the two different mature mRNAs?
2. A eukaryotic gene is transcribed to produce a pre-mRNA that includes a 5′ end, multiple exons, and a 3′ end. In one experimental condition, sequencing of nuclear RNA shows transcripts that begin at the normal transcription start site but terminate at variable positions downstream of the coding region. In a second condition, transcripts terminate at a consistent position, and a stretch of ~200 adenosines is present at the 3′ end that is not encoded in the DNA. All other processing factors are unchanged. Which explanation best accounts for the consistent 3′ ends in the second condition?
3. A farmer selects only wheat plants with the highest protein content in seeds to produce the next generation. Each year, the farmer saves seed only from selected plants and discards seed from the rest. After multiple generations, the average seed protein content increases. Which outcome is most likely regarding alleles affecting seed protein content in this wheat population?
4. Which of the following best explains how a tumor can become resistant to chemotherapy over time, demonstrating continuing evolution on a cellular level?
5. A cell is heterozygous at three loci on three different chromosome pairs: A/a, B/b, and C/c. Crossing over does not occur. During meiosis I, each homologous pair orients independently relative to the poles. As meiosis completes, gametes show multiple combinations of whole chromosomes carrying either A or a, B or b, and C or c. Which process best explains the variety of chromosome combinations among the gametes?
6. A researcher labels maternal and paternal homologs of a chromosome pair and observes meiosis in a heterozygous cell. During prophase I, homologs synapse, and a chiasma forms at a site between two loci on the chromosome. Later, the researcher detects chromatids in gametes that contain a maternal segment adjacent to a paternal segment on the same chromatid. The number of chromosomes per gamete is normal, indicating no nondisjunction. Which meiotic mechanism best accounts for the mixed-origin chromatid segments?
7. A muscle cell is treated with a compound that collapses the pH difference between the cytosol and the lumen of lysosomes, while lysosomal membranes remain intact. After treatment, macromolecule digestion inside lysosomes slows markedly, and cytosolic proteins remain mostly undamaged. Which conclusion is best supported about how compartmentalization affects digestion?
8. A diploid cell has $2n=4$ with two homologous pairs (pair 1 and pair 2). At metaphase I, the orientation of each homologous pair toward opposite poles is independent of the other pair. The cell completes meiosis I and II without errors and without considering crossing over. Which outcome is most likely regarding the chromosome combinations in the resulting gametes from many such cells?
9. In cultured liver cells, hormone H binds a membrane GPCR (R) and increases intracellular cAMP within 5 s. cAMP activates protein kinase A (PKA), which phosphorylates enzyme E, increasing its catalytic rate. When a phosphodiesterase (PDE) inhibitor is added, cAMP remains elevated longer and E stays phosphorylated longer. A mutant cell line expresses a receptor that binds H normally but fails to activate the G protein; in these cells, H does not change cAMP or E phosphorylation. Which change would most likely restore E phosphorylation after H addition in the mutant cells?
10. A toxin inhibits the enzyme that oxidizes NADH at the first major complex of the mitochondrial electron transport chain. Cells are provided glucose, and glycolysis continues to produce pyruvate and NADH in the cytosol. In the mitochondria, electrons from NADH normally enter the chain and ultimately reduce oxygen, while proton pumping helps generate ATP. After toxin exposure, mitochondrial NADH oxidation slows greatly. Which outcome is most likely to occur as a direct consequence in the mitochondria?
11. An enzyme's active site depends on tertiary structure created by R-group interactions. At low pH, excess $\text{H}^+$ can change the protonation state of acidic and basic side chains, altering their charges. A student measures enzyme activity across pH and finds activity drops sharply below pH 3, but the amino acid sequence (primary structure) remains unchanged. Which statement best describes how low pH reduces enzyme function at the molecular level?
12. In a grass population, plants vary in height due to a heritable gene with alleles H (tall) and h (short). A herd of grazing mammals feeds by clipping vegetation at a consistent height, removing a larger fraction of tall plants before they produce seeds. Short plants are less likely to be clipped and more often set seed. All plants release pollen and seeds within the same field, and no new alleles enter the population. Which outcome is most likely after many generations of grazing?
13. An animal cell is exposed to a chemical that prevents lysosomal acidification. Soon after, the cell accumulates undigested material inside vesicles. Which feature best explains why digestion decreases when lysosomes cannot acidify?
14. A membrane protein binds lactose on the extracellular side and releases it into the cytosol. Lactose is 1 mM outside and 10 mM inside, yet lactose still accumulates inside. Transport stops when ATP is removed, and lactose accumulation continues even when the concentration difference increases. Which mechanism best explains lactose uptake?
15. What is the probability that an offspring from this cross will have the genotype $bbRR$?
16. A cell has 2 mM K+ outside and 140 mM K+ inside. When a selective K+ channel opens, K+ leaves the cell, and the net movement slows as the concentration difference decreases. Blocking ATP production does not change the initial rate of K+ movement through the open channel. Which mechanism best explains K+ movement when the channel is open?
17. A membrane receptor activates a G protein that stimulates production of second messenger SM. Each activated receptor produces ~100 SM molecules per second; each SM activates one kinase molecule, and each kinase phosphorylates ~50 substrate molecules per second. When receptor activation is halved, the rate of substrate phosphorylation decreases substantially. Which feature of the pathway best explains how a small change at the receptor can cause a large change in substrate phosphorylation rate?
18. In an in vitro translation reaction, tRNAs are charged with amino acids by aminoacyl-tRNA synthetases before entering the ribosome. A mutation reduces the ability of one synthetase to attach its amino acid to its specific tRNA, but the tRNA can still base-pair normally with its matching codon in the ribosome. Ribosomes, mRNA, and all other charged tRNAs are abundant. Which outcome is most likely for polypeptides produced from mRNAs containing that codon?
19. To create a recombinant plasmid, a researcher cuts a plasmid vector and a DNA fragment containing a gene with the same restriction enzyme, generating complementary sticky ends. The mixture is incubated with DNA ligase and then used to transform bacteria. Only transformed cells are plated on medium containing ampicillin, which the plasmid encodes resistance to. Colonies grow on the ampicillin plate. Which molecular event most directly allows some colonies to contain the inserted gene?
20. A student models cells as cubes. Cube Cell A has side length 1 unit; Cube Cell B has side length 3 units. Both have the same membrane permeability to glucose and rely on passive transport to import glucose. Each cell uses glucose at the same rate per unit volume. When placed in the same glucose solution, Cell B's internal glucose concentration rises more slowly than Cell A's. Which explanation best accounts for the slower increase in Cell B?

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