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Practice Test 1 - AP Biology

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

  1. DNA replication relies on complementary base pairing and the requirement that DNA polymerase extend from a 3′ end. At one replication fork, a student labels the template strand running 3′→5′ toward the fork as Template X and the opposite template running 5′→3′ toward the fork as Template Y. DNA polymerase synthesizes one strand continuously and the other discontinuously in fragments that are later joined. Which statement best accounts for which template produces the discontinuous (lagging) strand at this fork?
  2. A membrane transporter contains several α-helices; these secondary structures are stabilized by hydrogen bonds between backbone atoms, while the helices' side chains interact with the lipid bilayer. A point mutation substitutes proline for alanine within one transmembrane α-helix. Transport rate decreases, though the protein is still inserted in the membrane. Which feature best explains the decreased transport?
  3. A scientist observes a macromolecule with a backbone of repeating sugar and phosphate groups. Adjacent units are connected by covalent bonds formed during dehydration reactions. The variable part of each unit is a nitrogen-containing ring that can differ among monomers, creating different sequences. This structure supports storage of information at the molecular level. Which feature best explains how this polymer can encode different information?
  4. In a plant, leaf shape shows codominance: allele L produces lobed leaves, allele R produces round leaves, and heterozygotes (LR) have both lobed and round sections on the same leaf. A lobed plant (LL) is crossed with a plant showing both sections (LR). Which offspring phenotypes are expected?
  5. A scientist compares epithelial cells from the intestine with epithelial cells from the trachea of the same rat. Both cell types show identical DNA markers at multiple loci. Microarray data indicate high expression of a mucin gene in tracheal cells and high expression of a digestive enzyme gene in intestinal cells. The scientist asks what causes the difference in gene expression. Which explanation best accounts for the results?
  6. A polysaccharide in the extracellular matrix contains repeating disaccharide units, one of which is an amino sugar that is frequently sulfated. Sulfate groups carry negative charges at physiological pH, increasing the overall charge density of the polymer. The matrix surrounding cells becomes highly hydrated and resists compression when this polysaccharide is abundant. Which feature best explains the resistance to compression at the molecular level?
  7. A cytosolic enzyme requires a disulfide bond between two cysteine side chains to maintain the shape of its active site. The enzyme's primary structure includes two cysteines that become close during folding, contributing to tertiary structure stability. A reducing agent is added to the cytosol, converting disulfide bonds to sulfhydryl groups without cutting peptide bonds. After treatment, enzyme activity decreases while the amino acid sequence remains unchanged. Which feature best explains the activity decrease at the molecular level?
  8. An ecologist compared two coral reef sites of equal area. Reef A had 25 fish species; the five most common species together made up 40% of individuals. Reef B had 25 fish species; the five most common species together made up 85% of individuals. Sampling effort and season were identical. Which conclusion is best supported about biodiversity at the two reefs?
  9. In a skeletal muscle cell, a neurotransmitter activates a GPCR that stimulates PLC to produce IP3_3, causing Ca2+^{2+} release from the sarcoplasmic reticulum. Ca2+^{2+} binds to a regulatory protein that directly inhibits PLC activity at the membrane. With constant neurotransmitter, IP3_3 production rises briefly and then decreases while receptor activation remains constant. Which change would most likely increase the duration of IP3_3 production by altering the feedback described?
  10. A diploid animal cell is observed just after completing DNA replication in S phase; each chromosome now consists of two sister chromatids. Before mitosis begins, the cell experiences extensive DNA damage from radiation. Proteins that monitor DNA integrity at the G2 checkpoint detect the damage, and the cell does not proceed into mitosis during the observation period. Spindle fibers are not formed, and chromosomes do not condense further. DNA content remains at the post-S phase level. Which outcome is most likely while the G2 checkpoint prevents progression?
  11. In a population of beetles, shell color is controlled by two alleles. Before a drought, 60% are light and 40% are dark. During the drought, birds more easily detect light beetles on dry, dark soil. After four generations, 20% are light and 80% are dark, while total population size remains similar. Both color types are still present in each generation, and offspring resemble parents in shell color. Which statement best explains how variation in shell color affected evolution in this population?
  12. A population of field mice shows fur color controlled by alleles D (dark) and d (light). On a dark lava flow, predators capture mice they see most easily. Over 10 generations, the proportion of dark-furred mice rises from 30% to 85%, while average litter size for dark mice remains higher than for light mice each generation. No new predators appear, and migration is not detected. Which explanation best accounts for the change in fur-color frequency over generations?
  13. In a certain fish, scale color is controlled by multiple alleles at one locus. Allele G produces gold scales, allele S produces silver scales, and allele g produces no reflective pigment. G and S are codominant to each other, and both are dominant over g. A fish with genotype Gg is crossed with a fish with genotype Sg. Which offspring phenotype is expected to occur at a frequency of 25%?
  14. In a human cell line, transcription of gene G decreases when a specific microRNA (miR-1) is introduced. miR-1 has partial complementarity to a sequence in the 3' UTR of gene G mRNA and recruits proteins that reduce translation and can promote mRNA degradation. RNA polymerase II binding at the gene G promoter is unchanged after miR-1 introduction. Which statement best accounts for why gene G transcription is not directly reduced by miR-1?
  15. A population of snails in a pond was tracked monthly. For six months, the population increased quickly, but when snail density exceeded 200 per square meter, the average number of eggs laid per adult decreased and juvenile survival dropped. The pond's temperature and water level stayed within normal ranges. Which factor best explains the reduced population growth rate at high density?
  16. Two populations of mammals are separated by a newly formed canyon that prevents most movement between sides. After 3,000 generations, individuals from the two sides can mate in captivity and produce healthy F1 offspring, but F1 offspring produce few viable gametes, resulting in very low fertility. Genetic analyses show substantial divergence in allele frequencies between the canyon populations. Which reproductive isolation mechanism is most directly demonstrated by the captive breeding results?
  17. A neuron releases neurotransmitter Y into a synaptic cleft. Postsynaptic Cell P responds, but only when Y is released locally at the synapse. When Y is injected into the bloodstream at the same concentration, Cell P does not respond. Which of the following best explains why synaptic release is effective but bloodstream injection is not?
  18. A signaling pathway is initiated when ligand Z binds a receptor that is itself an ion channel. Upon binding, the channel opens and allows Ca2+^{2+} influx. Ca2+^{2+} activates a Ca2+^{2+}-dependent protein kinase (CDPK), which phosphorylates multiple copies of enzyme F, increasing its catalytic activity; enzyme F then produces many molecules of product P per second, amplifying the response. A cytosolic Ca2+^{2+}-binding buffer protein reduces free Ca2+^{2+} concentration by sequestering Ca2+^{2+}. Cells overexpressing the buffer show reduced phosphorylation of enzyme F after Z addition. Which change would most likely increase product P formation in buffer-overexpressing cells?
  19. A gene in yeast is normally silenced when a repressor binds its promoter and recruits a histone deacetylase (HDAC), decreasing histone acetylation and reducing transcription. A mutant strain has the same repressor and promoter sequence, but gene mRNA is high even when the repressor is bound. Protein assays show the HDAC is absent. Which change would most likely return transcription to low levels?
  20. Two spherical cells have equal volumes of cytoplasm but different shapes: Cell 1 is a smooth sphere, and Cell 2 is a sphere with many thin membrane folds that increase total membrane area without changing cytoplasmic volume. Both are placed in the same dilute solution and lose water by osmosis at the same water potential difference. Cell 2 loses water faster than Cell 1. Which explanation best accounts for the difference in water loss rates?

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