Practice Test 36 - AP Biology

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

In an experiment, a mitotic cell is treated with a drug that prevents microtubules from polymerizing. The cell has already completed DNA replication in S phase and entered M phase. Chromosomes condense, but kinetochores cannot attach to spindle microtubules, so the spindle checkpoint remains active. Which outcome is most likely for the treated cell during M phase?
A bacterial gene F is regulated by an alternative sigma factor that recognizes a specific -35 and -10 promoter sequence. Under heat shock, the alternative sigma factor increases in concentration and associates with RNA polymerase, leading to increased transcription of gene F. A mutant strain produces normal RNA polymerase and normal sigma factor levels but shows low gene F mRNA during heat shock. Which change would most likely explain the reduced transcription in the mutant?
A diploid cell with $2n=4$ contains two homologous pairs. During metaphase I, each tetrad can orient with either homolog facing either pole. Which outcome is most likely if the two tetrads orient independently before anaphase I begins?
A ligand L binds receptor R and activates phospholipase A2 (PLA2), releasing arachidonic acid (AA) from membrane lipids. AA is converted by enzyme COX to prostaglandin P, which binds a second membrane receptor R2 on the same cell, increasing cAMP and activating PKA. PKA phosphorylates ion channel C, increasing conductance. When COX is inhibited, L still activates PLA2 but channel conductance does not increase. Which inference is best supported?
Clonal cuttings from one houseplant are grown for 6 weeks under either high light or low light. High-light plants develop smaller, thicker leaves and show increased expression of genes encoding photosynthetic proteins in leaf cells; sequencing of these genes shows identical DNA in both groups. When low-light plants are moved to high light, new leaves resemble those of high-light plants. Which explanation best accounts for the difference in leaf thickness between the two treatments?
A neurotransmitter T is released into a synaptic cleft and binds a receptor embedded in the postsynaptic membrane. In an experiment, T binding is normal, but a toxin that prevents the receptor from changing shape is added. Under these conditions, no increase in intracellular Ca $^{2+}$ is detected during the first 10 seconds after T exposure. Which prediction is most consistent with these results about early signal transduction?
The divergence of house sparrow populations in North America demonstrates that evolution is an ongoing process by showing...
A single cell maintains ion balance by passive diffusion through membrane channels. When the cell's linear dimensions double, channel density per unit membrane area stays constant, and cytoplasmic ion requirement scales with volume. Which explanation best accounts for the expected change in the cell's ability to maintain ion balance using passive diffusion alone?
In humans, red-green color blindness is caused by an X-linked recessive allele ( $X^c$ ), while normal color vision is dominant ( $X^C$ ). A woman with normal vision whose father was color-blind has children with a man who has normal vision ( $X^CY$ ). Which prediction best describes the probability that a son will be color-blind?
A coastal fish population experienced a sharp decline from 10,000 to 120 individuals after a toxic spill, then rebounded to 9,500 within four years. Before the spill, allele $m$ at a neutral microsatellite locus had frequency 0.48; after the rebound, $p(m)=0.11$ . No consistent differences in survival or reproduction among genotypes at this locus are observed. Which factor most directly explains the allele frequency shift?
A model membrane is composed of a phospholipid bilayer with a hydrophobic interior. Molecules that are small and nonpolar tend to partition into the lipid core and diffuse across, whereas polar molecules interact strongly with water and are less soluble in the membrane interior. Charged molecules are surrounded by hydration shells and experience a large energetic barrier to entering the hydrophobic region. Consider two uncharged molecules of similar size: ethanol (contains a hydroxyl group) and propane (a hydrocarbon). No channels or carriers are present. Which explanation best accounts for propane crossing the membrane more readily than ethanol?
Researchers culture myoblasts and differentiate them into muscle fibers. DNA sequencing shows no change in the actin gene sequence during differentiation. Yet actin mRNA increases greatly in muscle fibers compared with myoblasts. A DNA-binding protein is detected in muscle fiber nuclei that binds a regulatory region upstream of the actin gene, but it is absent in myoblast nuclei. Which explanation best accounts for the increased actin mRNA in muscle fibers?
In a cell line, ligand U triggers a rapid increase in IP $_3$ and a subsequent rise in cytosolic Ca $^{2+}$ . When IP $_3$ receptors on the endoplasmic reticulum (ER) are blocked, ligand binding and IP $_3$ production are unchanged, but cytosolic Ca $^{2+}$ does not rise. Which of the following best explains the earliest role of the ER IP $_3$ receptor in transduction?
Cells are labeled with a dye that binds DNA, and fluorescence intensity reflects DNA content. A cell in G1 shows baseline intensity (2C). As it progresses through S phase, intensity rises until it reaches double baseline (4C) in G2. The cell then enters mitosis and completes anaphase and telophase, followed by cytokinesis. If fluorescence is measured in each daughter cell immediately after cytokinesis, which intensity is most likely relative to the original G1 cell?
In immune cells, a soluble ligand binds a receptor tyrosine kinase (RTK) on the plasma membrane. Within 1 minute, the receptor becomes phosphorylated on cytosolic tyrosines, and a cytosolic adaptor protein binds only to the phosphorylated receptor. A mutation prevents receptor dimerization but does not affect ligand binding. Which outcome is most consistent with early transduction in the mutant cells?
During a sudden loud sound, a rabbit freezes for several seconds and its breathing rate increases. When the environment becomes quiet again, the rabbit resumes movement and breathing slows. Which response best explains the short-term mechanism causing the rabbit's immediate changes?
A protein-free phospholipid bilayer is exposed to equal concentrations of sucrose (342 Da, polar uncharged) and glycerol (92 Da, polar uncharged). Which molecule would most likely diffuse across the bilayer faster?
A biologist monitors a rabbit population in two fenced grassland plots of equal size. In Plot A, rabbit density increases from 10 to 40 rabbits per hectare over two months, and the percentage of rabbits with visible skin lesions rises from 2% to 28%. In Plot B, density remains near 10 rabbits per hectare and lesions stay below 3%. No changes in weather or food availability are recorded during the study. Which outcome is most consistent with density-dependent regulation in Plot A?
In eukaryotic cells, many enzymes that add phosphate groups to proteins are located on the cytosolic side of specific membranes, while enzymes that remove phosphate groups may be enriched in different regions of the cytosol. If a mutation causes these enzymes to become uniformly dispersed throughout the cytosol, the same total amount of each enzyme remains present, but the rate of phosphorylation of membrane-associated target proteins decreases. Which feature best explains how localized environments within cells influence reaction efficiency?
A bacterial cell is placed in pure water. The cell does not burst, while a similar cell that has had its cell wall enzymatically removed swells and lyses. Which feature best explains the difference?

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