Practice Test 94 - AP Biology

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

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1. In adipocytes, hormone H binds GPCR R and activates Gα$_s$, increasing cAMP and activating PKA. PKA phosphorylates enzyme L, increasing lipolysis. A separate protein, arrestin, binds phosphorylated R and prevents further G protein activation while promoting receptor internalization. In cells lacking arrestin, cAMP remains elevated longer after H addition. Which change would most likely restore normal timing of cAMP decline in arrestin-deficient cells?
2. A researcher infers relationships among five bacterial strains (R–V) using DNA differences in a 500 bp region. Pairwise differences are: R–S 3, R–T 3, R–U 10, R–V 11; S–T 2, S–U 10, S–V 11; T–U 9, T–V 10; U–V 1. Assuming mutation accumulation reflects time since divergence, which pair is most closely related?
3. A student builds a model of RNA and DNA nucleotides and notes that RNA nucleotides include a hydroxyl group on the 2′ carbon of the sugar, while DNA nucleotides have a hydrogen at that position. Under alkaline conditions, RNA strands break more readily than DNA strands. Which feature best explains this difference at the molecular level?
4. A lab group compares two macromolecules. Molecule X is a chain built from monomers that each contain an amino group and carboxyl group, and monomers join by dehydration. Molecule Y is built from monomers that each contain a phosphate, a sugar, and a nitrogenous base, and monomers join by dehydration. Both are polymers, but they differ in the chemical groups that form the backbone. Which feature best distinguishes the monomers of molecule Y from those of molecule X?
5. In a plant, leaf pattern is codominant. Allele $L^S$ produces solid green leaves and allele $L^M$ produces mottled leaves. Homozygotes ($L^SL^S$) are solid, ($L^ML^M$) are mottled, and heterozygotes ($L^SL^M$) show both patterns on the same leaf. Two heterozygous plants are crossed. Which phenotype ratio is expected among offspring?
6. Researchers isolate an organelle from a protist and find it has two membranes. The organelle contains circular DNA and 70S ribosomes, and it divides by a process resembling binary fission independent of the cell cycle. The organelle's inner membrane contains electron transport proteins similar to those in certain bacteria. Which evidence best supports a specific mechanism for the origin of this organelle?
7. In a culture dish, Cell Type A releases peptide P into the surrounding fluid. When peptide P is added to a mixed culture, only Cell Type B rapidly shows increased membrane ion flow, while Cell Type C shows no immediate change. A competitive inhibitor that binds the same extracellular site as peptide P prevents the response in Cell Type B even though peptide P is still present. No direct cell-to-cell contacts are observed among cells. Which of the following best explains why only Cell Type B responds to peptide P?
8. Two groups of genetically identical fruit fly larvae are reared at either 18°C or 29°C. Adult flies from 29°C have darker abdominal pigmentation and higher mRNA levels for a pigment-synthesis enzyme in epidermal cells. Genome sequencing shows no DNA sequence differences. Which explanation best accounts for the pigmentation difference?
9. In a cell, many small solutes with charged or polar regions disperse throughout the cytosol. Water molecules are polar, with oxygen partially negative and hydrogens partially positive. Hydrogen bonds can form when water's partially positive hydrogen is attracted to an electronegative atom or negative region on another molecule. When a polar solute is added to water, water molecules orient so that the appropriate partial charges face the solute, forming a hydration shell that separates solute particles from one another. This reduces direct solute-solute attraction and increases dispersion. Which statement best describes how hydrogen bonding contributes to solute dissolution in water?
10. A secreted protein is stabilized by many hydrogen bonds that support β-pleated sheets in its secondary structure. The primary structure includes alternating polar and nonpolar amino acids that allow sheet formation and further folding into a compact tertiary structure. When the protein is heated, its activity in binding a specific ligand decreases, but peptide bonds are not hydrolyzed. Which statement best describes the molecular basis for the reduced ligand binding after heating?
11. A researcher measures DNA content in individual cells. One cell begins in G1 with 2C DNA. It completes S phase and enters G2 with 4C DNA, then proceeds through mitosis. During metaphase, all chromosomes align at the metaphase plate and each kinetochore is attached to spindle microtubules from opposite poles. The spindle checkpoint is satisfied, and anaphase begins. Cytokinesis completes, producing two daughter cells. Which DNA content is most likely in each daughter cell immediately after cytokinesis?
12. During meiosis in a diploid organism, homologous chromosomes pair and form tetrads. A researcher labels maternal chromatids with one fluorescent marker and paternal chromatids with another. After meiosis, some gamete chromosomes show both markers along the same chromatid, indicating that segments of maternal and paternal chromatids are present on a single chromosome. The chromosome number in each gamete remains haploid. Which process best explains how a single chromatid can contain both maternal and paternal segments?
13. Genetically identical mouse pups are raised from birth in two environments: enriched cages with running wheels and toys, or standard cages. At 8 weeks, neurons from the hippocampus are analyzed. Mice from enriched cages show higher levels of BDNF mRNA and increased dendritic branching, while DNA sequencing of the BDNF gene shows no differences between groups. When enriched-cage mice are moved to standard cages for 4 weeks, BDNF mRNA levels decrease toward the standard-cage level. Which explanation best accounts for the observed phenotype differences between groups?
14. A population of mice has heritable variation in coat pattern: spotted or solid. In a habitat where snow cover decreases, the background becomes mostly dark soil, and predators capture spotted mice more often than solid mice. Solid-pattern mice survive and reproduce at higher rates, though both patterns still appear among pups. Which outcome is most likely over time?
15. Cells in a tissue release cytokine C, and nearby cells respond within seconds. When a neutralizing antibody that binds C is added to the extracellular fluid, the response decreases sharply. However, when the same antibody is injected into the cytoplasm of responding cells, the response is unchanged. Which of the following best explains where C is detected by responding cells?
16. A cell membrane contains phospholipids and a steroid with four fused rings and a small polar hydroxyl group. The rings are rigid and interact with neighboring fatty acid tails, while the hydroxyl group can align near phospholipid head groups. When the steroid concentration increases, the movement of adjacent hydrocarbon tails becomes more constrained in the local region. Which statement best describes the molecular-level consequence for the membrane's physical behavior in that region?
17. A coding sequence contains the segment 5'-...GAA CTC GGT...-3', translated in-frame. A point mutation changes the middle codon from CTC to CTA, producing 5'-...GAA CTA GGT...-3'. The gene's expression level and mRNA processing are unchanged. Using the standard genetic code, which outcome is most likely for the translated protein?
18. In a dehydration–rehydration cycle experiment modeling tidal pools, a mixture of nucleotides was repeatedly dried and rewetted on hot rock. After multiple cycles, analysis showed short RNA-like polymers in the dried-phase residues, while a continuously wet control produced mostly monomers. Which conclusion is best supported about conditions that may have promoted polymer formation on early Earth?
19. A gene's coding region includes the sequence 5′-ATG CCG GAA-3′ (coding strand). A mutation inserts three nucleotides, 5′-TTA-3′, between ATG and CCG, producing 5′-ATG TTA CCG GAA-3′. Translation starts at the same AUG and continues in the same frame. Which outcome is most likely for the protein?
20. In a temperate forest, a fungal pathogen infects a dominant tree species, causing widespread canopy loss. Within two years, understory wildflowers and shade-intolerant shrubs increase in cover, and bird species that forage in dense understory become more common. Soil moisture and temperature remain within typical seasonal ranges. The pathogen is not observed infecting other plant species. Which outcome is most likely responsible for the increase in understory plants and associated birds?

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