Practice Test 20 - AP Biology

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

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1. A researcher studies transport of polar solute R across a membrane. With a functional transporter present, R enters cells rapidly when outside concentration exceeds inside concentration. When the transporter gene is deleted, R entry becomes negligible despite the same gradient, and ATP inhibitors have no effect in either strain. Which explanation best accounts for these results?
2. A student examines a meiotic cell in prophase I and observes a tetrad with a visible chiasma between two nonsister chromatids. The cell is heterozygous at a locus F/f located on the chromosome segment involved in the chiasma. After meiosis, some gametes contain chromatids with a mix of segments originally from different homologs. Which process best explains the chromosomal basis for these mixed-segment chromatids?
3. In a eukaryotic cell, a gene contains three exons separated by two introns. A researcher isolates nuclear RNA shortly after transcription and observes that the RNA includes sequences complementary to both introns and exons. Later, RNA isolated from the cytoplasm contains the exon sequences joined together with the intron sequences absent. No changes are detected in the DNA sequence of the gene. Which explanation best accounts for the difference between the nuclear RNA and cytoplasmic RNA?
4. A lab performs reverse transcription PCR (RT-PCR) to test whether a gene is being expressed in liver cells. Total RNA is isolated, treated to remove DNA, and incubated with reverse transcriptase and primers to synthesize complementary DNA (cDNA). The cDNA is then amplified by PCR using gene-specific primers. A strong PCR band appears for the liver sample but not for a negative control sample lacking reverse transcriptase. Which conclusion is best supported by these results?
5. A cell produces a large protein that is packaged into secretory vesicles. Over time, the protein appears in the extracellular medium. Microscopy shows vesicles moving to and fusing with the plasma membrane, releasing the protein outside. The process decreases when ATP is depleted. Which mechanism best explains how the protein crosses the plasma membrane?
6. Two populations of mammals are separated by a newly formed canyon that prevents most movement between sides. After 3,000 generations, individuals from the two sides can mate in captivity and produce healthy F1 offspring, but F1 offspring produce few viable gametes, resulting in very low fertility. Genetic analyses show substantial divergence in allele frequencies between the canyon populations. Which reproductive isolation mechanism is most directly demonstrated by the captive breeding results?
7. A researcher measures uptake rate of a polar solute in two conditions. With a specific membrane transporter present, uptake rate rises with increasing external solute but reaches a maximum. Without the transporter, uptake is near zero even at high external solute. ATP depletion does not change uptake when the transporter is present. Which mechanism best explains the transporter-mediated uptake?
8. In a plant, leaf shape is determined by a gene with multiple alleles. Allele $L^1$ and $L^2$ are codominant, producing a speckled leaf when together ($L^1L^2$). Genotypes L^1L^1 and L^2L^2 produce solid leaves of two different colors. A speckled plant ($L^1L^2$) is crossed with a solid $L^1$ plant ($L^1L^1$). Which outcome best predicts the leaf phenotypes of the offspring?
9. A plant cell wall polysaccharide is composed of glucose monomers connected by $β$-1,4 glycosidic bonds. Adjacent chains align and form many hydrogen bonds between hydroxyl groups, creating strong microfibrils. A different glucose polymer with mostly $α$-1,4 bonds forms a coiled structure with fewer stable chain-to-chain contacts. Which statement best predicts a consequence of the $β$-1,4 structure for the cell wall material?
10. In a prairie plant population, stem height is heritable. Plants of intermediate height produce the most seeds because very short plants are shaded and very tall plants break more often in wind. Over 10 generations, the mean height stays similar, but extreme heights become less common and overall variation decreases. The population remains large with no detected migration. Which pattern best illustrates the selection acting on stem height?
11. In a bacterial population, some cells carry a plasmid with an antibiotic-resistance gene, and other cells lack the plasmid. Before antibiotic exposure, plasmid-bearing cells are 5% of the population. When the antibiotic is added, plasmid-bearing cells survive and reproduce more than plasmid-free cells. After 24 hours, plasmid-bearing cells make up 80% of the population. No evidence indicates the resistance gene arose during the 24-hour period. Which statement best explains the population-level change?
12. Hydrothermal vent systems can create strong chemical gradients. In a lab setup, researchers built a microchamber with a pH gradient across a thin mineral barrier and added CO2 and simple sulfur compounds. They observed formation of small organic acids on the side with higher pH, while a control chamber without a gradient produced much lower amounts. Which conclusion is best supported about early Earth environments and prebiotic chemistry?
13. Early Earth likely experienced frequent impacts that delivered extraterrestrial material. Researchers analyzed carbon-rich meteorite samples and detected multiple amino acids with non-biological isotopic signatures, distinguishing them from modern contamination. These amino acids were present within the meteorite matrix rather than only on exposed surfaces. Which conclusion is best supported by this evidence about sources of prebiotic organic molecules?
14. A student fills dialysis tubing (permeable to water but not to starch) with 0.60 M starch solution and places it into a beaker containing 0.20 M starch solution. The initial mass of the tubing is recorded, and after 20 minutes the tubing has increased in mass. Assume temperature and pressure remain constant and no starch crosses the tubing. Which outcome is most likely responsible for the mass change?
15. In a laboratory, two identical cultures of yeast were grown with the same initial sugar concentration. Culture L started with $10^5$ cells, and Culture H started with $10^7$ cells. After 24 hours at the same temperature, Culture H had a lower per-cell division rate and higher cell death than Culture L. No antibiotics or toxins were added. Which factor most likely explains the reduced growth in Culture H as density dependent?
16. A cell membrane is a phospholipid bilayer in which hydrophilic heads contact water and hydrophobic tails form a core. Membrane proteins can be embedded with regions that are hydrophobic where they contact lipid tails and hydrophilic where they contact water or line pores. A researcher observes that a newly added transmembrane protein remains stably embedded only when it contains a long stretch of nonpolar amino acids. Which feature best explains why the nonpolar amino acid stretch stabilizes the protein in the membrane?
17. In a beetle species, striped wings (S) are completely dominant to solid wings (s). A striped beetle is crossed with a solid beetle (ss). Among 80 offspring, all have striped wings. Assume the trait is controlled by a single gene with complete dominance and that the sample size is sufficient to reflect expected ratios. Which genotype is most likely for the striped parent?
18. Cells are briefly exposed to a spindle poison that prevents microtubule polymerization. Treated cells complete S phase and enter mitosis with duplicated chromosomes, but no functional spindle forms, so kinetochores cannot attach to microtubules. The spindle checkpoint remains active when kinetochores are unattached, preventing anaphase onset. After the poison is washed out, microtubules polymerize and attach to kinetochores, allowing chromosomes to align. Which outcome is most likely immediately after proper attachment is restored?
19. A eukaryotic gene is controlled by a transcriptional repressor that binds a silencer element and recruits a corepressor complex. The corepressor includes a protein that directly interacts with the mediator complex to reduce RNA polymerase II initiation. A mutation deletes the repressor's corepressor-interaction domain, but the repressor still binds the silencer. Which effect is most likely on transcription of the gene?
20. In a captive breeding program for rabbits, only individuals with the longest ears are chosen as parents each generation. After many generations, the average ear length increases, but the range of ear lengths becomes narrower. Which outcome is most likely in the rabbit population after this selection regime?

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