Practice Test 79 - AP Biology

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

Separase is kept inactive by binding to securin until APC/C ubiquitinates securin, leading to securin degradation. A mutant securin cannot be ubiquitinated but still binds separase normally. CDK–cyclin activity and spindle attachment are otherwise normal, and the spindle checkpoint signal is off. Cells enter metaphase but fail to separate sister chromatids. Which signaling-based change would most likely restore chromatid separation in the mutant?
A lab compares two treatments in synchronized G2 cells. Treatment 1 increases M-cyclin concentration; treatment 2 increases activity of a Cdk1-inhibitory kinase that adds an inhibitory phosphate to Cdk1. In both cases, M-cyclin can bind Cdk1. Nuclear envelope breakdown occurs only when active M-cyclin–Cdk1 exceeds a threshold. Which treatment is more likely to delay nuclear envelope breakdown?
A membrane contains an antiporter that moves H $^+$ into the cell while moving Na $^+$ out. Na $^+$ export continues even when extracellular Na $^+$ is higher than intracellular Na $^+$ , but it stops if the H $^+$ gradient is eliminated. The antiporter does not bind ATP. Another protein uses ATP to pump H $^+$ out, maintaining higher extracellular H $^+$ . Which process best explains Na $^+$ export by the antiporter?
Early Earth's atmosphere likely lacked free oxygen and contained gases such as $\mathrm{CH_4}$ , $\mathrm{NH_3}$ , $\mathrm{H_2}$ , and water vapor. In a laboratory simulation, researchers circulated these gases with water in a closed apparatus and applied electrical sparks to model lightning. After one week, chemical analysis of the water phase detected several amino acids and other small organic molecules that were absent at the start. A control setup identical in all ways except without electrical sparks showed no detectable amino acids. Which conclusion is best supported by these results about the origin of organic molecules on early Earth?
In yeast cells, pheromone F binds a GPCR and causes dissociation of the G protein. The freed Gβγ subunits activate a scaffold protein that brings together three kinases in sequence: K1 phosphorylates K2, and K2 phosphorylates K3. Active K3 phosphorylates a cytosolic protein that changes its binding to the cytoskeleton, altering cell shape within minutes. A mutation prevents the scaffold from binding K2, but binding of K1 and K3 to the scaffold is unaffected; receptor activation by F is normal. Which prediction is best supported for mutant cells exposed to pheromone F?
The plasma membrane is composed of a phospholipid bilayer with embedded proteins. Hydrophilic heads face the aqueous environments, while hydrophobic tails form an interior barrier. Some transport proteins act as pumps that move ions across the membrane in a specific direction, even when that direction is opposite the ion's concentration gradient. In an experiment, a membrane protein moves $\mathrm{Ca^{2+}}$ from low concentration outside to higher concentration inside. Which feature best supports that the protein is a pump rather than a channel?
A researcher uses CRISPR-Cas9 with a guide RNA targeting a specific exon in a eukaryotic gene. After Cas9 creates a double-strand break, the cell repairs the break by nonhomologous end joining (NHEJ) without a repair template. Sequencing of the target region shows small insertions and deletions in many cells. Which outcome is most likely at the molecular level?
A ligand triggers rapid activation of a membrane-associated G protein. A toxin that ADP-ribosylates the G protein alpha subunit prevents GTP hydrolysis but does not affect GDP-to-GTP exchange. After ligand addition, downstream second messenger levels rise and remain elevated long after ligand removal. Which of the following best explains the early transduction effect of the toxin?
A laboratory maintains a population of fruit flies and allows only flies with curly wings to reproduce each generation. Wing shape is affected by alleles C (curly) and c (straight). At generation 0, $f(c)=0.70$ . After many generations, most flies have curly wings, and straight-winged flies are rarely observed. Which outcome is most likely for allele frequencies in the population?
A student models three spherical cells with radii 2 µm, 4 µm, and 8 µm. All have identical membrane permeability and are in the same environment. A nutrient enters by diffusion and is used at the same rate per unit volume. The student predicts the 8 µm-radius cell will show the lowest nutrient uptake per unit volume. Which explanation best supports this prediction?
A phospholipid bilayer separates extracellular fluid from cytosol. The membrane interior is hydrophobic, so diffusion across it favors small, nonpolar molecules. Polar molecules cross slowly, and charged molecules cross extremely slowly because charge is energetically unfavorable in the hydrophobic core. Compare alanine in its zwitterionic form (has both positive and negative charges at physiological pH) and alanine methyl ester (neutral, less polar). No transport proteins are present. Which molecule would most likely be more permeable across the bilayer?
Two populations of spherical cells differ only in size. In Population A, radius is 5 µm; in Population B, radius is 15 µm. Both exchange ions by diffusion through open channels, with the same channel density per µm² and the same ion production rate per unit volume. Population B shows higher intracellular ion concentration. Which explanation best accounts for the higher concentration in Population B?
A fish farm maintains a breeding population of 1,000 trout. Each generation, managers choose eggs only from females that mature at the youngest ages and fertilize them with sperm from the youngest-maturing males. After 12 generations, the average age at maturity is lower than in the original population. Which statement best describes what has changed in the population?
A meiotic cell is experimentally treated so that spindle fibers fail to attach to the kinetochores of one homologous chromosome pair during meiosis I. The other homologous pairs attach and segregate normally. Cytokinesis still occurs. Meiosis II proceeds with normal spindle attachment to any chromosomes present in each cell. Which outcome is most likely for the distribution of that affected homologous pair among the gametes?
Researchers compared two kelp forest communities after a storm. Community R had 11 invertebrate species; Community S had 7 species. In both communities, individuals were similarly distributed among species (no strong dominance). Sampling area and effort were identical. Which conclusion is best supported about biodiversity differences?
A cell has a membrane channel selective for a neutral sugar. Outside sugar is 12 mM, inside is 1 mM. When the channel is closed, sugar uptake is near zero; when opened, uptake increases without ATP use. Which explanation best accounts for the dependence of uptake on channel state?
A biologist studies two bird species that feed on nectar. Both have long, curved beaks. Skeletal comparisons show one species' beak bones match those of finch relatives, while the other's beak bones match those of honeyeater relatives. DNA sequences cluster the two species with their respective relatives rather than with each other. Which conclusion is best supported by the anatomical and molecular evidence?
A cell builds a polymer by repeatedly joining monomers that each contain multiple hydroxyl groups and a carbonyl group. Each time a new monomer is added, a water molecule is released, and a covalent bond forms between the two monomers. The resulting polymer has many polar hydroxyl groups that can form hydrogen bonds with water. Which classification best fits both the monomers and the polymer described?
Two groups of genetically identical bean plants are grown for 3 weeks. Group 1 receives high light (full spectrum, 14 h/day); Group 2 receives low light (4 h/day). Leaves from high-light plants have more chlorophyll per gram and higher expression of chlorophyll-synthesis enzymes, but DNA sequencing shows the same alleles in both groups. Which explanation best accounts for the chlorophyll differences?
In a plant leaf, water molecules move upward through narrow xylem tubes as a continuous column. Each water molecule is polar because oxygen is more electronegative than hydrogen, giving oxygen a partial negative charge and hydrogens partial positive charges. Neighboring water molecules form hydrogen bonds when the partially positive hydrogen of one molecule is attracted to the partially negative oxygen of another. In a narrow tube, these transient hydrogen bonds can link many water molecules together, allowing a pulling force at the top of the column to be transmitted through the connected molecules. Which feature best explains how hydrogen bonding supports upward water movement in xylem?

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