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Practice Test 62 - AP Biology

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

  1. In a bacterial population, allele S at a gene has frequency 0.00 at time 0. After exposure to UV radiation, allele S is detected at frequency 0.001 in the next generation, while the population size remains extremely large. No migration is possible in the closed culture flask, and the environment does not differentially affect reproduction among genotypes at this gene. Which process most likely explains the appearance of allele S in the population?
  2. A population of dividing cells is synchronized so that all cells start in G1 with unreplicated chromosomes. The cells are then exposed to a chemical that prevents DNA polymerase from completing replication, so replication forks stall throughout S phase. After several hours, measurements show that DNA content has increased above G1 levels but has not reached the fully doubled amount typical of G2. Microscopy shows no condensed chromosomes and an intact nuclear envelope. The cells do not enter mitosis during the observation period. Which explanation best accounts for the cells' failure to enter M phase?
  3. Genetically identical dandelion seedlings are grown in soil with either low nitrogen or high nitrogen. After 4 weeks, high-nitrogen plants have larger leaves and higher levels of nitrate-reductase mRNA in leaf cells. DNA sequencing shows no differences between groups. Which explanation best accounts for the larger leaves in high-nitrogen plants?
  4. A population of mice is tracked in a warehouse. When the population is estimated at 30 mice, the average number of offspring per female per month is 6. When the population reaches 120 mice, the average number of offspring per female per month drops to 2. Food deliveries to the warehouse remain constant each week, and no trapping occurs. Which explanation best supports density-dependent regulation of the mouse population?
  5. Two cells have identical shapes and membrane permeability. Cell X has surface area 300 µm² and volume 150 µm³; Cell Y has surface area 900 µm² and volume 1,350 µm³. Both rely on diffusion for O2 uptake, and O2 use per unit volume is the same. Which explanation best accounts for why Cell Y is more likely to experience lower internal O2?
  6. A fossil series documents changes in a horse lineage over time. Older strata contain small-bodied individuals with four toes on the front limb. Intermediate strata contain individuals with three toes, with the middle toe enlarged. Younger strata contain larger-bodied individuals with a single enlarged toe and reduced side toes. The strata are continuous and dated, and multiple specimens occur in each layer. Which conclusion is best supported by this fossil evidence?
  7. Two populations of birds were separated on different islands for 8,000 years. When storms later allow occasional migration between islands, hybrids are produced but have reduced fertility compared with offspring from within-island matings. Over subsequent generations, observations show fewer hybrid pairings as birds increasingly choose mates with local song patterns. Genetic data show continued divergence despite some contact. Which process most directly explains the decrease in hybridization after secondary contact?
  8. In a lab, chloroplasts are illuminated while suspended in a solution lacking ADP but containing NADP+ and an artificial electron donor that can replace water as an electron source. Electron flow to NADP+ continues, and NADPH accumulates. However, because ADP is absent, ATP synthase cannot phosphorylate ADP to ATP even though protons are pumped into the thylakoid lumen. CO2 is present at normal levels. Which outcome is most likely for carbon fixation under these conditions?
  9. A population of mice shows genetic variation at a coat-color locus. In a small isolated valley, a wildfire reduces the population to 12 survivors with a higher-than-average frequency of the dark-coat allele. Several generations later, the dark-coat allele remains common even though predation rates are similar across coat colors. Which statement best explains this change in allele frequency?
  10. A cell is heterozygous at two loci, A and B, located on the same pair of homologous chromosomes in arrangement AB on one homolog and ab on the other. During prophase I of meiosis, the homologous chromosomes synapse and form a tetrad. A crossover occurs between the A and B loci on nonsister chromatids. After meiosis completes, some gametes contain allele combinations that were not present on either original homolog. Which outcome illustrates the direct chromosomal result of this crossover event?
  11. In a population of snails, shell banding is controlled by alleles GG and gg. Researchers calculate allele frequencies p=0.40p=0.40 and q=0.60q=0.60. If the population meets Hardy-Weinberg conditions, what is the expected frequency of the gggg genotype in the next generation?
  12. A mutation occurs in a tRNA gene such that the anticodon is altered, but the aminoacyl-tRNA synthetase still attaches the same amino acid to that tRNA as before. The altered anticodon now base-pairs with a different codon on mRNA. Ribosomes and mRNAs are otherwise normal. Which outcome is most likely in proteins translated in cells carrying this mutation?
  13. Two groups of genetically identical fruit fly larvae were raised on diets differing only in protein content. Adults from the high-protein diet had greater average body mass than adults from the low-protein diet. Sequencing of a growth-regulating gene showed identical DNA sequences in both groups, and the mass difference decreased when low-protein larvae were switched to high-protein food early in development. Which explanation best accounts for the diet-associated mass differences?
  14. A small, isolated lizard population on a desert outcrop has allele TT at frequency 0.52 at a neutral locus. Over 20 generations, with no immigration and no detected genotype differences in survival or reproduction, p(T)p(T) fluctuates unpredictably between 0.30 and 0.70. The census population size remains around 80 adults each generation. Which factor most directly explains these allele frequency fluctuations?
  15. A single nucleotide substitution changes a G–C pair to an A–T pair in double-stranded DNA. Which change best describes the immediate molecular effect on local stability?
  16. Two cells of the same shape differ only in size. Both rely on diffusion across the plasma membrane for O2_2 entry, and both consume O2_2 at the same rate per unit volume. In identical O2_2 environments, the larger cell develops a lower internal O2_2 concentration than the smaller cell. Which explanation best accounts for this pattern at the cellular level?
  17. A cell expresses a transporter that allows fructose to cross the membrane. When extracellular fructose is 25 mM and intracellular fructose is 2 mM, fructose enters. When intracellular fructose is experimentally raised to 25 mM, net fructose movement becomes zero, although the transporter remains functional. Which explanation best accounts for the change in net movement?
  18. In a eukaryotic cell, researchers isolate nuclear RNA from a gene containing three exons separated by two introns. A newly synthesized transcript is detected that hybridizes to probes complementary to exon 1, intron 1, exon 2, intron 2, and exon 3. After 10 minutes, the predominant nuclear RNA hybridizes only to exon probes, and sequencing shows exon 1 directly joined to exon 2 and exon 2 joined to exon 3. No changes are detected in the DNA sequence of the gene. Which explanation best accounts for the change in hybridization pattern over time?
  19. Two cube-shaped cells are identical except for size. Cell A has side length 4 µm; Cell B has side length 12 µm. Both release a signaling molecule by simple diffusion across the plasma membrane at the same permeability. Over equal time, the concentration of the molecule outside Cell A rises more per unit cell volume than outside Cell B. Which explanation best accounts for this observation?
  20. A eukaryotic primary transcript includes a 5′ untranslated region, a coding region, and a 3′ untranslated region. An enzyme complex cleaves the transcript at a specific polyadenylation signal and then adds a poly(A) tail. A mutation deletes the polyadenylation signal sequence but leaves the promoter and splice sites intact. In mutant cells, transcripts extend far beyond the normal 3′ end, and poly(A) tails are rarely detected on these RNAs. Which explanation best accounts for these observations?

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