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Practice Test 86 - AP Biology

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

  1. A comparative anatomist examined the middle-ear region of a mouse, dog, and lizard. Both mammals have three small middle-ear bones (malleus, incus, stapes) with the malleus and incus articulating in the same distinctive joint shape, while the lizard has a single middle-ear bone and lacks that joint. The mammal and lizard skulls otherwise share many vertebrate features. Which inference is best supported by the middle-ear evidence?
  2. In constant darkness a mouse is active every 24.1 h; under 12 h light/12 h dark, only at night. Best supported?
  3. A polysaccharide used for structural support in an aquatic organism must remain insoluble and form stable fibers in water. The polymer is composed of repeating monosaccharides with many hydroxyl groups and has unbranched chains that can align closely. Which linkage type between monomers would best support fiber formation under these conditions?
  4. A small population of 40 mice colonizes a new meadow from a much larger source population where allele CC has frequency 0.60. In the first generation in the meadow, allele CC frequency is 0.15, and subsequent generations remain near 0.15 with no evidence of genotype fitness differences. No additional migrants arrive. Which process most likely explains the initial low allele CC frequency in the meadow population?
  5. A short RNA strand folds back on itself to form a hairpin. In the stem region, bases pair by hydrogen bonding; in the loop, bases remain unpaired. The RNA contains several G and C nucleotides in the stem and fewer A and U. G-C pairs form three hydrogen bonds, while A-U pairs form two. The phosphate-sugar backbone remains intact throughout folding. A mutation replaces several G nucleotides in the stem with A nucleotides without changing strand length. Which feature best explains the most likely effect on hairpin stability at the molecular level?
  6. During development, two cell types arise from the same zygote. Scientists find both cell types contain the same DNA sequence for a receptor gene, but only one cell type produces receptor mRNA. The promoter region of the receptor gene is heavily methylated in the nonexpressing cell type and minimally methylated in the expressing cell type. Which explanation best accounts for the receptor gene being silent in the methylated cell type?
  7. In a replication bubble, the two parental DNA strands are antiparallel and serve as templates. DNA polymerase adds nucleotides only to the 3′ end, so each new strand grows 5′→3′. As a result, at each replication fork one daughter strand is synthesized continuously, while the other is synthesized discontinuously as short fragments later joined. A student labels the "leading strand" as the one synthesized away from the moving fork. Which explanation best accounts for why this label is incorrect?
  8. In G2, M-cyclin accumulates and binds Cdk1, but the complex is kept inactive by an inhibitory kinase that phosphorylates Cdk1. A separate activating kinase phosphorylates Cdk1 at a different site required for full activity once the inhibitory phosphate is removed. A drug selectively inhibits the activating kinase while leaving the inhibitory kinase and phosphatase unaffected. Which result is most likely after drug treatment?
  9. Two spherical cells have the same membrane composition and transport proteins. Cell X has radius 5 µm; Cell Y has radius 10 µm. Both are placed in identical solutions containing a small polar solute that crosses only by facilitated diffusion. After 2 minutes, Cell X shows a greater increase in internal solute concentration than Cell Y. Which explanation best accounts for the difference in transport efficiency between the two cells?
  10. A deer population on an island (no immigration or emigration) was 300 in year 1. A new predator arrived in year 2, and the deer population changed to 260, 220, 190, and 165 by years 3–6. Plant biomass increased during the same period. Which explanation best accounts for the deer population trend from years 2–6?
  11. A paleontologist compared skulls of a modern horse and a fossil mammal found in older rock layers. Both skulls share the same distinctive arrangement of molars: three cusps in a specific triangular pattern and an identical groove between the second and third cusp. Several other mammals in the same deposits lack this cusp pattern. The fossil skull has smaller teeth and a shorter face, but the cusp pattern and groove occur in the same relative positions. Which inference is best supported by these observations?
  12. In a eukaryotic alga, mitochondria have double membranes and contain circular DNA and 70S ribosomes. Proteins encoded by nuclear genes are imported into the mitochondria through membrane transport complexes. Which evidence best supports an endosymbiotic origin for mitochondria?
  13. Researchers compare two populations of eukaryotic pre-mRNA from the same gene. Population 1 contains sequences from exon 1, exon 2, and exon 3 joined in order. Population 2 contains sequences from exon 1 joined directly to exon 3, with exon 2 absent; intron sequences are absent from both populations. The DNA sequence of the gene is identical in both samples, and transcription initiates at the same start site. No evidence indicates RNA degradation differences between the samples. Which explanation best accounts for the difference between the two mature mRNA populations?
  14. A cell with 0.10 M nonpenetrating solute inside is placed into a 0.20 M nonpenetrating solute solution. The membrane is permeable to water but not to the solute. After 8 minutes, the cell's volume decreases. Which outcome is most likely?
  15. In an animal tissue, a hormone is released into the bloodstream and reaches many cell types. Only liver cells respond rapidly, and the response is eliminated when a competitive antagonist that binds the hormone-binding site is added. Muscle cells show no response with or without antagonist. Which outcome is most likely when the antagonist is present in the blood?
  16. A eukaryotic cell's mitochondria and chloroplasts each have double membranes and their own circular DNA. The cell's nucleus has a double membrane but contains only the cell's linear chromosomes and no separate genome. Which conclusion is best supported by these observations about compartment origins?
  17. A student builds artificial membranes using phospholipids. Each phospholipid has a glycerol backbone, two fatty acid tails, and a phosphate-containing head group. In water, the phosphate head is polar and interacts with water, while the hydrocarbon tails are nonpolar and avoid water. When phospholipids are shaken in water, they spontaneously form bilayers with heads facing outward and tails facing inward, creating a hydrophobic interior. Which statement best explains why a small nonpolar molecule crosses this bilayer faster than a charged ion?
  18. A researcher observes that centromeres do not divide during anaphase I, but they do divide during anaphase II. Which outcome is most likely for chromosome separation across the two meiotic divisions?
  19. Researchers stocked three identical aquaria with guppies at different starting densities: 10, 30, and 60 fish. All tanks received the same food mass per day and had the same water quality. After 6 weeks, the 60-fish tank had the highest incidence of a contagious fungal infection and the lowest juvenile survival, while the 10-fish tank had few infections and high juvenile survival. No chemicals were added, and temperatures remained constant. Which factor best explains the observed pattern as density dependent?
  20. A cell's plasma membrane contains many cholesterol molecules. When temperature decreases moderately, the membrane remains less rigid than a similar membrane lacking cholesterol. Which feature best explains cholesterol's effect?

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