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Practice Test 85 - AP Biology

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

  1. A phospholipid consists of a glycerol backbone (C, H, O), two fatty acid tails (mostly C and H), and a phosphate-containing head group (P and O) that often also includes N. The phosphate group has multiple electronegative oxygens, giving the head a strong partial negative charge and enabling interactions with water. The fatty acid tails are largely nonpolar because C–H bonds share electrons nearly equally. When placed in water, many phospholipids spontaneously form a bilayer with heads facing outward and tails facing inward. Which statement best describes the molecular property that drives this bilayer arrangement?
  2. Cells are exposed to ionizing radiation during G2, producing chromosome breaks after DNA replication has finished. DNA content remains at the G2 level, and sister chromatids are present. In untreated cells, chromosomes condense and mitosis begins within an hour. In irradiated cells, chromosomes remain uncondensed and the nuclear envelope stays intact for several hours. Which outcome is most likely if the checkpoint controlling this transition is functional?
  3. In a coastal grass population, leaf salt-tolerance is controlled by two alleles: T and t. After a storm surge, soil salinity stays high for 12 years (about 6 generations). In year 1, the T allele frequency is 0.40. Plants with genotype TT produce an average of 18 viable seeds per season, Tt produce 12, and tt produce 5. By year 12, the T allele frequency is 0.78 in the same marsh, with similar population size each year and no evidence of migration. Which pattern best illustrates the selection acting on salt tolerance in this population?
  4. A gene's coding sequence normally includes the mRNA codon UGG, which codes for tryptophan. A point mutation changes the corresponding DNA so that the mRNA codon becomes UGA. Transcription and mRNA processing occur normally, and translation begins at the usual start codon. Which outcome is most likely for the translated polypeptide?
  5. A researcher studies how nucleic acids interact with positively charged proteins. Both DNA and RNA have sugar-phosphate backbones in which phosphate groups carry negative charges at physiological pH. The researcher observes that a basic protein binds strongly to a nucleic acid even when the bases are chemically modified so they cannot form hydrogen bonds with complementary bases. The binding remains strong despite disrupted base pairing. Which molecular feature best explains why binding can remain strong without base pairing?
  6. A population of mice has variation in body mass. In a stable environment with limited food and high predation, researchers find that mice of intermediate mass survive to reproduce more often than very small or very large mice. Over 25 generations, the distribution of body mass narrows: fewer individuals occur at the extremes, while the average body mass remains similar. Allele frequencies at multiple loci associated with extreme body mass decline. Which pattern best describes the selection acting on body mass?
  7. A growth factor activates a receptor tyrosine kinase (RTK), causing autophosphorylation and recruitment of a MAPK cascade that activates ERK. Active ERK phosphorylates a docking site on the RTK's cytosolic tail, reducing adaptor binding and decreasing further MAPK activation while ligand remains present. Cells expressing an RTK mutant lacking the ERK phosphorylation site show prolonged ERK activity after a brief ligand pulse. Which outcome best explains the effect of this feedback on pathway behavior?
  8. On a rocky intertidal shore, two barnacle species occupy similar substrate. When species A is removed from small plots, species B expands downward into the lower zone within one month. When species A is present, species B remains mostly in the upper zone even though temperature and wave exposure are similar across zones. Predators were not observed during the study period. Which interaction best explains why species B expands only when species A is removed?
  9. A mitotic cell is observed with condensed chromosomes and a fully formed spindle. All kinetochores are attached, but a chemical is added that prevents separase from cleaving cohesin. The chromosomes align at the metaphase plate and remain there, and DNA content stays at the G2/M level. The cell does not show chromatid movement toward poles. Which event is most directly blocked by the chemical treatment?
  10. In a diploid cell with genotype A/a and B/b on two different chromosome pairs, the cell undergoes meiosis. During metaphase I, each homologous pair aligns independently, and no crossing over occurs. Four gamete types (AB, Ab, aB, ab) are produced in roughly equal frequencies. Which meiotic process best explains the equal production of these four allele combinations among gametes?
  11. At a replication fork, DNA polymerase extends a primer by adding deoxyribonucleotides to the 3′ end, creating a complementary strand antiparallel to the template. In a mutant cell, the polymerase can bind DNA but can add nucleotides only to a 5′ end. Which explanation best accounts for the effect on replication?
  12. A student compares two beakers containing identical cells (internal solute 0.20 M). Beaker 1 contains 0.05 M nonpenetrating solute; Beaker 2 contains 0.35 M nonpenetrating solute. Water can cross the membrane in both cases. Which outcome is most likely for the cells in Beaker 2 after several minutes?
  13. In a temperate grassland, ecologists measured annual energy stored as new biomass at each trophic level. Producers (grasses) stored 12,000 kJ/m2^2/yr. Primary consumers (grasshoppers) stored 1,200 kJ/m2^2/yr. Secondary consumers (frogs) stored 120 kJ/m2^2/yr. Tertiary consumers (snakes) stored 12 kJ/m2^2/yr. Assume these values represent energy available to the next trophic level through consumption. Which conclusion is best supported about trophic transfer efficiency in this ecosystem?
  14. A transporter exchanges one Ca2+^{2+} from the cytosol to the extracellular space while moving three Na+^+ into the cytosol. Ca2+^{2+} export continues when cytosolic Ca2+^{2+} is already lower than extracellular Ca2+^{2+}, but it stops if the Na+^+ gradient is eliminated. The exchanger does not hydrolyze ATP, but an ATP-dependent Na+^+/K+^+ pump maintains the Na+^+ gradient. Which process best explains Ca2+^{2+} export?
  15. Cyclin levels can change rapidly when cyclins are tagged for degradation by ubiquitin ligases. In a population of dividing cells, cyclin B normally drops sharply at the metaphase-to-anaphase transition due to APC/C-mediated ubiquitination. A mutation reduces APC/C ubiquitin-ligase activity but does not affect microtubule attachment to kinetochores. Which effect is most likely observed in the mutant cells?
  16. In a lab, equal numbers of flour beetles are placed into jars containing the same amount of flour. Jar 1 starts with 10 beetles, Jar 2 with 50, and Jar 3 with 200. All jars are kept at the same temperature and humidity. After 6 weeks, Jar 1 has increased to 60 beetles, Jar 2 to 140, and Jar 3 to 210. No predators are present, and no beetles are added or removed during the experiment. Which factor is most likely limiting population growth in the highest-density jar?
  17. In a sealed chamber, cells are supplied with glucose and a limited amount of oxygen. As oxygen is consumed, the electron transport chain gradually slows because electrons cannot be transferred to the final electron acceptor. The oxidation of NADH to NAD+ in mitochondria decreases, affecting earlier steps that require NAD+. Assume the cells can use fermentation pathways to regenerate NAD+ in the cytosol when needed. Which change is most likely as oxygen becomes depleted?
  18. In a cell signaling experiment, Ligand D causes a rapid increase in cytosolic Ca2+^{2+} only when extracellular Ca2+^{2+} is present. Blocking voltage-gated Ca2+^{2+} channels prevents the Ca2+^{2+} increase, but ligand binding to the receptor is unchanged. The response occurs within milliseconds. Which of the following best predicts the receptor's early signaling mechanism?
  19. A population of human somatic cells is synchronized in G1 and then released to progress through the cell cycle. Shortly after entering S phase, the cells are treated with a chemical that stops DNA polymerase, so replication forks stall and most chromosomes remain only partially replicated. Several hours later, the cells are examined: chromosomes are not condensed, and no mitotic spindle is present. The cells do not enter M phase, even though time has passed for untreated cells to reach mitosis. Which explanation best accounts for the observed cell-cycle arrest?
  20. In a cultured animal cell, DNA replication completes during S phase, and the cell enters G2 with doubled DNA content. Microscopy shows that just before mitosis, the nuclear envelope remains intact and chromosomes are not yet condensed. The cell then enters mitosis, and by metaphase each duplicated chromosome is aligned at the cell equator. However, one chromosome's kinetochore is attached to microtubules from only one spindle pole, while all other kinetochores are attached to opposite poles. The cell remains in metaphase for an extended time without separating sister chromatids. Which outcome is most likely if the spindle checkpoint is satisfied?

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