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Practice Test 43 - AP Biology

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

  1. A grassland experiences a prolonged drought that reduces plant biomass by 50% and lowers seed production. In the same period, small herbivore populations decline and predator sightings decrease. Which consequence is most likely for energy flow in this ecosystem?
  2. In a large population of beetles, a locus with alleles C and c is studied. At the start of the season, allele frequencies are p(C)=0.70p(C)=0.70 and q(c)=0.30q(c)=0.30. Adults mate randomly, and no migration is detected. During gamete formation, a rare mutation converts C alleles to c alleles at a measurable rate, while the reverse mutation is negligible. No differences in survival or reproduction among genotypes are detected. Researchers predict allele frequencies among zygotes in the next generation. Which prediction is most consistent with these conditions?
  3. A student compares two cells. Cell X is in G1 with 2C DNA and one chromatid per chromosome. Cell Y is in G2 with 4C DNA and two sister chromatids per chromosome. Both cells are then induced to enter mitosis at the same time. Cell X enters mitosis without first replicating DNA, while Cell Y enters mitosis normally. After cytokinesis, Cell X produces daughter cells with fewer total DNA molecules than typical, while Cell Y produces typical daughter cells. Which conclusion best explains the difference between Cell X and Cell Y outcomes?
  4. A eukaryotic gene contains an intron with the sequence 5′-GU…AG-3′ at its ends. A mutation changes the 3′ splice site from AG to AA, while leaving the rest of the pre-mRNA unchanged. Transcription and 5′ capping occur normally. After processing, most RNA molecules still contain the intron sequence. Which explanation best accounts for intron retention in the processed RNA?
  5. A researcher isolates a biomolecule made of repeating subunits; each subunit has an amino group and a carboxyl group. During synthesis, the carboxyl group of one subunit reacts with the amino group of another, releasing a molecule of water and forming a covalent linkage. As additional subunits join, a long chain forms whose properties depend on the sequence of side chains (R groups) attached to the backbone. This macromolecule category is commonly associated with enzymatic activity and structural support in cells. Which feature best explains why this molecule is classified as a polymer?
  6. In a fish species, scale color shows codominance. Allele SGS^G produces green scales and allele SBS^B produces blue scales. Heterozygotes (SGSBS^GS^B) display both green and blue scales on the same fish. Two heterozygous fish are crossed (SGSS^GS^B × SGSS^GS^B). Which phenotype ratio is expected among offspring?
  7. A protein segment contains several amino acids with side chains ending in –NH2 groups (containing N) and several amino acids with side chains ending in –COOH groups (containing C and O). In water near neutral pH, many –NH2 groups accept H+ and become positively charged, while many –COOH groups donate H+ and become negatively charged. When the protein folds, oppositely charged side chains can attract each other, helping stabilize a specific conformation. Which interaction best describes this stabilization at the molecular level?
  8. Early Earth likely experienced wet–dry cycles in coastal environments. In an experiment, a mixture of amino acids on a mineral surface was repeatedly dried and rehydrated. After multiple cycles, researchers detected short polypeptides, whereas a continuously wet control produced few or none. Which conclusion is best supported by the evidence about prebiotic chemical evolution?
  9. A cell-free system contains purified ATP synthase embedded in a membrane separating two solutions. The pH is set to 5.0 on one side and 8.0 on the other, with ADP and inorganic phosphate present on the pH 8.0 side. ATP is produced on the pH 8.0 side without adding any electron carriers. Which process most directly provides the energy for ATP synthesis in this setup?
  10. In cultured mammalian cells, passage through the G1 checkpoint requires active cyclin D–CDK4/6 complexes that phosphorylate a checkpoint protein, reducing its inhibition of S-phase entry. A small molecule added at time 0 binds CDK4/6 and prevents its kinase activity without changing cyclin D levels. After treatment, cells accumulate with 2N DNA content and show reduced phosphorylation of the checkpoint protein compared with untreated cells. Which change would most likely restore progression past the G1 checkpoint in treated cells?
  11. A scientist studies two populations of the same plant species in adjacent fields. In Field 1, plants grow at low density (2 plants per square meter) and produce an average of 80 seeds per plant. In Field 2, plants grow at high density (12 plants per square meter) and produce an average of 25 seeds per plant. Soil nutrients, rainfall, and sunlight are similar in both fields. Which factor most likely causes the lower seed production in Field 2?
  12. A plant that is heterozygous for both traits is crossed with a plant that has yellow fruit and dwarf vines. What is the expected phenotypic ratio of the offspring?
  13. A bacterial two-component system uses membrane sensor kinase S and response regulator R. Ligand L binds S, triggering autophosphorylation on histidine and transfer of the phosphate to aspartate on R. Phosphorylated R binds enzyme E and increases its catalytic rate. S also has phosphatase activity that dephosphorylates R when L is absent. A mutant S lacks phosphatase activity but retains kinase activity. After brief L exposure, phosphorylated R remains high longer in the mutant than in wild type. Which outcome is most likely in the mutant after L removal?
  14. Researchers examined the effect of clay minerals on polymer formation in an early Earth simulation. Equal concentrations of activated nucleotides were incubated in water with either powdered clay or inert glass beads of similar size. After 48 hours, samples with clay contained many short RNA-like oligomers, while samples with glass beads contained mostly unreacted nucleotides. No proteins were added to either condition. Which conclusion is best supported by the evidence?
  15. In a freshwater lake, a population of snails has two alleles at a shell-thickness locus: T (thick) and t (thin). After an invasive crab appears, researchers track allele frequencies for 7 generations. The frequency of allele T increases from 0.40 to 0.79, and the snail population continues reproducing in the lake with no stocking. Which observation best demonstrates ongoing evolution in this snail population?
  16. A human stands up quickly after lying down for several minutes. Within seconds, their heart rate increases and they feel briefly lightheaded; within a minute, symptoms fade while heart rate remains slightly elevated. Which response best explains the mechanism that restores blood pressure during this short-term change in position?
  17. In a cell, an enzyme that generates a signaling lipid is restricted to the inner leaflet of the plasma membrane, while an enzyme that degrades the lipid is located on internal membranes. When internal membranes are fragmented and mixed with the plasma membrane, the signaling lipid level at the cell surface drops. Which feature best explains how compartmentalization influences the lipid's concentration at the plasma membrane?
  18. A cell is heterozygous for two genes, R and S, located on the same homologous chromosome pair in the arrangement RS on one homolog and rs on the other. During prophase I, a crossover occurs between the R and S loci on non-sister chromatids. After meiosis, some gametes carry Rs and rS chromatids in addition to RS and rs. No mutations occur. Which outcome illustrates how meiosis increased genetic variation in the gametes?
  19. In eukaryotic cells, small nuclear RNAs (snRNAs) within snRNPs base-pair with sequences at the 5′ splice site of an intron. A mutation changes several nucleotides at the 5′ splice site of intron 4, reducing complementarity to the snRNA, while the 3′ splice site and branch point remain unchanged. RNA analysis shows increased retention of intron 4 in the processed RNA, but other introns are removed normally. Which explanation best accounts for the increased intron 4 retention?
  20. Red blood cells (internal solute concentration 300 mOsm) are placed into a solution of 300 mOsm NaCl for 5 minutes, then transferred to a 150 mOsm NaCl solution. The membrane is permeable to water but not to NaCl. Immediately after transfer, the cells begin changing volume. Which outcome is most likely after several minutes in the 150 mOsm solution?

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