Skip to main content
Sign in Create a free account
Varsity Tutors
Sign in Sign up

Practice Test 22 - AP Biology

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

  1. A researcher compares DNA replication in two cell extracts. In both, helicase separates strands and DNA polymerase extends new DNA by adding nucleotides to the 3′ end, guided by base complementarity. In extract 1, the polymerase has normal proofreading ability that removes mismatched nucleotides soon after insertion. In extract 2, the polymerase lacks this mismatch-removal activity but still polymerizes at the same rate. After one round of replication, both samples are sequenced. Which explanation best accounts for a higher frequency of point mutations in extract 2?
  2. A bacterial population uses quorum sensing by releasing small molecule A into the environment. At low cell density, A remains below a threshold and cells show no change in membrane activity. At high density, A accumulates and binds a cytosolic receptor only after A enters cells by diffusion. If the medium is continuously replaced to prevent A buildup, cells remain unresponsive even at high density. Which outcome is most likely when medium replacement stops?
  3. A checkpoint monitors completion of DNA replication before mitosis by regulating CDK activation. When replication forks are stalled, signaling increases an inhibitory phosphate on the mitotic CDK, preventing activation even if cyclin is bound. A replication-stalling agent is applied, and cells arrest in G2. A second drug is added that blocks the kinase responsible for adding the inhibitory phosphate, while leaving cyclin levels unchanged. Which outcome is most likely after adding the second drug?
  4. Two populations of a grass species occur in adjacent fields with no physical barrier. Population M grows on soil contaminated with heavy metals; population N grows on nearby uncontaminated soil. Pollen moves freely between fields, and flowering times overlap. Crosses produce viable seeds, but in reciprocal transplant experiments, seedlings with mixed ancestry have much lower survival on metal-contaminated soil than seedlings from population M, while survival on uncontaminated soil is similar across types. Genetic data show reduced gene flow near loci associated with metal tolerance. Which process most directly explains the increasing reproductive isolation?
  5. A cultured animal cell is exposed to signaling molecule X, which cannot cross the plasma membrane. Within seconds, intracellular cAMP levels increase. When researchers add a competitive antagonist that binds the extracellular side of receptor R, the cAMP increase is eliminated. However, when cells are treated with a membrane-permeable cAMP analog, the intracellular response occurs even in the presence of the antagonist. Receptor R spans the membrane and is present at similar levels in all treatments. Which of the following best explains how molecule X initiates the intracellular response?
  6. In a cell line, APC/C activation requires phosphorylation by an upstream mitotic kinase. Once active, APC/C ubiquitinates securin and M-cyclin. A drug inhibits the upstream mitotic kinase but does not affect APC/C abundance or substrate binding. Cells form spindles and align chromosomes but do not initiate chromatid separation. Which change would most likely trigger chromatid separation despite the drug?
  7. A membrane contains many identical transport proteins for solute S. When extracellular S is increased from 1 mM to 10 mM, the net influx rate increases. Increasing extracellular S further from 10 mM to 100 mM produces little additional increase in influx. ATP inhibitors do not reduce influx at any concentration. Which explanation best accounts for the observed rate pattern?
  8. A researcher isolates neurons and skin fibroblasts from the same mouse. Genomic analysis indicates both cell types contain the same set of genes. When the researcher measures RNA levels, neurons have high mRNA for a neurotransmitter receptor gene, while fibroblasts have high mRNA for a collagen gene. The cells are kept at identical temperature and nutrient conditions. Which explanation best accounts for the observed differences in mRNA abundance between the two cell types?
  9. A tundra ecosystem has producers (mosses), primary consumers (lemmings), and secondary consumers (foxes). Researchers found that mosses store 4,000 kJ/m2^2/yr as new biomass, lemmings store 200 kJ/m2^2/yr, and foxes store 8 kJ/m2^2/yr. Which conclusion is best supported about energy transfer efficiencies between these trophic levels?
  10. A lake food chain shows 12,000 kJ stored in phytoplankton biomass, 1,200 kJ in zooplankton biomass, and 120 kJ in small fish biomass over the same interval. Which outcome is most likely if the phytoplankton energy storage drops to 6,000 kJ, with similar transfer efficiencies?
  11. Amylase activity is measured by the time required to reduce a starch-iodine color to half intensity. At 37°C the time is 20 s; at 5°C the time is 120 s. Amylase is a protein enzyme whose catalysis depends on frequent collisions between enzyme and substrate and proper alignment at the active site. Lower temperature decreases kinetic energy and molecular movement but does not directly break covalent bonds. Substrate concentration and pH are constant. Which explanation best accounts for the slower reaction at 5°C?
  12. In yeast cells, a pheromone binds a specific membrane receptor and triggers rapid activation of a cytosolic MAP kinase within 2 minutes. A mutant receptor binds pheromone normally but lacks most of its cytosolic tail. In the mutant, MAP kinase activation is greatly reduced. Which of the following best explains why the cytosolic tail is important for early transduction?
  13. Two cube-shaped cells have the same membrane composition and are in identical conditions. Cell S has side length 3 μm3\,\mu m; Cell T has side length 9 μm9\,\mu m. A toxin enters only through the plasma membrane by simple diffusion and is not broken down inside the cell. After a fixed time, Cell S reaches a higher intracellular toxin concentration than Cell T. Which explanation best accounts for the difference in intracellular toxin concentration?
  14. Researchers examine the arrangement of bones in the middle ear of mammals and the jaw of certain reptiles. Mammals have three middle ear bones (malleus, incus, stapes). In reptiles, two jaw bones (articular and quadrate) occupy positions and have shapes that correspond to the malleus and incus in mammals. Developmental studies show that mammalian malleus and incus arise from embryonic tissues that also form the reptilian articular and quadrate. Which inference is best supported by these observations?
  15. A double-stranded DNA molecule is analyzed after replication errors are induced. Sequencing reveals that at one position, a guanine on one strand is opposite a thymine on the other strand. This mismatch alters local hydrogen bonding compared with normal base pairing and slightly distorts the helix. In subsequent replication cycles, DNA polymerase copies each strand as a template, using complementary base pairing rules. Which statement best explains how the structure of DNA base pairing can lead this mismatch to become a permanent mutation?
  16. In an experiment, isolated mitochondria are supplied with NADH and ADP + Pi. When oxygen is removed from the chamber, electrons can no longer be transferred to the final electron acceptor in the electron transport chain. As a result, proton pumping across the inner mitochondrial membrane decreases, reducing the proton-motive force that drives ATP synthase. The citric acid cycle enzymes remain present, but their ability to regenerate NAD+ depends on electron flow through the chain. No inhibitors of ATP synthase are added. Which outcome is most likely when oxygen is absent?
  17. In a species of beetle, long antennae (L) are completely dominant to short antennae (l). Two beetles with long antennae are crossed. One parent is known to be homozygous dominant (LL), while the other is heterozygous (Ll). Assume a single autosomal gene with complete dominance. Which genotype ratio is expected among the offspring?
  18. A scientist measures ATP production in chloroplasts under two conditions. Condition 1: intact thylakoid membranes in light produce ATP. Condition 2: thylakoid membranes are punctured so protons can freely equilibrate across the membrane; light-driven electron flow still occurs, but ATP production is near zero. The punctures prevent a proton gradient from forming across the thylakoid membrane. ATP synthase requires a proton-motive force to catalyze phosphorylation of ADP. Which explanation best accounts for the loss of ATP production in Condition 2?
  19. A marine ecologist studies a mussel bed on a rocky shore. When mussel density is low, most individuals remain attached and survival over a month is high. When density is high, many mussels are dislodged, and survival decreases. The ecologist notes that at high density, mussels are stacked in multiple layers and waves frequently knock loose individuals from the upper layers. Water temperature and salinity remain stable during the study. Which factor most directly links density to decreased survival?
  20. In E. coli, gene X is preceded by a promoter and an operator that overlaps the promoter. A repressor protein binds the operator and blocks RNA polymerase binding. When lactose is added, it is converted to allolactose, which binds the repressor and reduces its affinity for the operator. In cells grown without lactose, gene X mRNA levels are low; after lactose addition, gene X mRNA increases within minutes. Which change would most likely prevent the lactose-dependent increase in transcription of gene X?

Practice tests