Practice Test 76 - AP Biology

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

DNA replication begins when the two strands separate, exposing bases that guide addition of complementary nucleotides. DNA polymerase can extend DNA only from a free 3′ OH, synthesizing 5′→3′. On one template, synthesis is continuous; on the antiparallel template, synthesis occurs in short fragments that must be joined to form a continuous strand. In an experiment, joining of fragments is blocked, but base pairing and polymerase activity remain normal. Which outcome is most likely after replication completes?
A population of wild sunflowers grows along roadsides where de-icing salt is applied each winter. Salt-tolerance is associated with allele T at a transporter gene. Researchers sampled plants from the same 2 km stretch of road: in 2012, $f(T)=0.14$ ; in 2015, $f(T)=0.26$ ; in 2018, $f(T)=0.37$ ; in 2021, $f(T)=0.40$ . Plants with allele T produce more seeds in salty soils than plants without T. Which observation best demonstrates evolution in this sunflower population?
In a large population of wildflowers, genotype fitnesses at a locus are $W_{AA}=0.70$ , $W_{Aa}=1.00$ , and $W_{aa}=0.65$ across multiple years. Starting with allele $A$ frequency 0.60, the population shifts toward an intermediate allele frequency and then remains stable. Migration is negligible and population size is large. Which evolutionary mechanism most directly explains the stable intermediate allele frequency?
A snail population shows shell thickness variation that is heritable. A crab predator crushes shells, and snails with very thin shells and very thick shells each have higher reproductive success than snails with intermediate thickness, which are crushed more often. After 20 generations, intermediate-thickness shells become rarer, while both extremes increase in frequency. The population remains large and isolated with no migration. Which pattern best illustrates the selection acting on shell thickness?
A double-stranded DNA region has a high proportion of G–C base pairs compared with another region of equal length that has more A–T base pairs. When both regions are heated, one requires a higher temperature to separate the strands. At the molecular level, strand separation depends on disrupting hydrogen bonds between paired bases. Which feature best explains the higher melting temperature of the G–C–rich region?
A snail species is found in two nearby valleys connected by a low pass, allowing occasional migration. In valley 1, most adults have right-coiling shells; in valley 2, most adults have left-coiling shells. When snails from different valleys are brought together, they frequently attempt to mate but rarely transfer sperm successfully because shell coiling causes misalignment of reproductive openings. Genetic markers show divergence in allele frequencies between valleys despite occasional migration. Which isolating mechanism most directly reduces gene flow between the populations?
A student measures catalase activity by collecting O $_2$ produced from H $_2$ O $_2$ for 2 minutes. At pH 7, 12 mL O $_2$ is produced; at pH 3, 2 mL is produced, with equal enzyme and substrate concentrations and constant temperature. Which explanation best accounts for the lower activity at pH 3?
In a freshwater pond, small fish feed heavily on zooplankton. When fish are added to experimental enclosures, zooplankton abundance decreases and algae concentration increases. When fish are removed from enclosures, zooplankton increases and algae concentration decreases. Nutrient inputs and light levels are controlled and equal across treatments. Which community-level interaction pattern best explains the observed changes across trophic levels?
Cells are observed after completing S phase; each chromosome consists of two sister chromatids. Before mitosis begins, a chemical is applied that prevents completion of DNA repair, leaving many replication-associated lesions. DNA content remains at the G2 level, and chromosomes do not condense. Over time, the cells do not enter mitosis, and nuclear envelopes remain intact. Which explanation best accounts for the observed lack of mitotic entry?
Trypsin activity is tested at 37°C in two buffered solutions containing equal enzyme and substrate concentrations. In buffer at pH 8.0, 70% of substrate is digested in 5 minutes; in buffer at pH 3.0, only 5% is digested. The enzyme is not degraded and is transferred directly between buffers. Which outcome is most likely when trypsin is returned from pH 3.0 to pH 8.0? Changes in pH can alter amino acid side-chain charges that stabilize tertiary structure and the active site.
DNA replication uses each parental strand as a template, with new nucleotides added by DNA polymerase according to base-pairing rules (A–T, C–G). DNA polymerase extends from a free 3′ OH, synthesizing 5′→3′. A proofreading activity can remove an incorrectly paired nucleotide when a mismatch distorts the helix, allowing the correct nucleotide to be inserted. In a mutant cell line, proofreading is absent but polymerase still adds nucleotides normally. Which outcome is most likely in the replicated DNA of the mutant cells?
A protein's primary structure includes a stretch of amino acids with alternating polar and nonpolar side chains. When folded, this region forms a β-sheet in which side chains project above and below the sheet, enabling one face to be more hydrophobic. This hydrophobic face contributes to tertiary structure by packing against other nonpolar regions, helping shape a ligand-binding pocket. A mutation swaps several nonpolar residues in this β-sheet for polar residues. Binding to the ligand decreases. Which feature best explains the decreased binding?
In a sedimentary rock sequence, a lineage of marine snails shows a gradual increase in average shell thickness from older (deeper) to younger (shallower) layers. In the same layers, drill holes made by predatory crabs become more frequent over time. Shell-thickness measurements overlap among layers, but the population mean increases steadily across 2 million years. No abrupt gaps or replacement by a different shell form are observed. Which conclusion is best supported by these data about the snail lineage?
A pure phospholipid bilayer separates two aqueous solutions. Molecules can cross only by simple diffusion through the hydrophobic core (no proteins present). Consider these molecules at similar concentrations: O $_2$ (nonpolar, small), glucose (large, polar), Na $^+$ (small, charged), and ethanol (small, polar uncharged). Because the bilayer interior is nonpolar, molecules that are nonpolar and small dissolve into the core more readily than polar or charged molecules. Charged ions remain strongly associated with water and are energetically unfavorable within the hydrophobic region. Larger molecules, even if uncharged, have more difficulty fitting between phospholipids than smaller molecules.
In a small island population of 120 lizards, allele $G$ at a color locus had frequency 0.50 in 2010. A hurricane in 2011 randomly killed most individuals, leaving 18 survivors that founded the next generations. By 2013, the allele $G$ frequency in the population was 0.78, and no consistent difference in survival or reproduction was detected among color phenotypes. Which process is most likely responsible for the observed change in allele frequency?
A student places animal cells with an internal solute concentration of 0.30 M into a beaker containing 0.10 M nonpenetrating solute. The plasma membrane is permeable to water but not to the solute. After 10 minutes, the cells appear swollen compared with their initial size. Which outcome is most likely for net water movement across the membrane during the 10 minutes?
In a population of field mice, fur color is controlled by two alleles. Before a wildfire, 52% of mice are light brown (genotypes BB or Bb) and 48% are dark brown (bb). After the wildfire, the habitat is mostly blackened soil. Over the next 6 generations, surveys show dark-brown mice increase to 76% while total population size remains similar. No new alleles are detected, and mating remains random within the population. Which statement best explains the observed change in the population over generations?
A lab constructs a short double-stranded DNA with one strand 5′-G C A T-3′. During analysis, the strands are shown to run in opposite directions and pair by complementary base interactions. The investigator labels the opposite strand's ends and bases accordingly. Which feature best describes the relationship between base pairing and the antiparallel orientation in this DNA segment?
Scientists produced fatty-acid vesicles in water and then added a solution of RNA fragments. After gentle agitation, some vesicles contained RNA fragments inside, whereas vesicles formed in a separate trial after RNA was removed contained none. No proteins were included in either trial. Which conclusion is best supported about a step in the origin of life?
A cell with $2n=6$ is observed in prophase I of meiosis. Homologous chromosomes have paired as bivalents, and chiasmata are visible between non-sister chromatids. Later, at anaphase I, homologous chromosomes separate to opposite poles. Which explanation best accounts for why sister chromatids of a chromosome can differ after meiosis I begins?

Practice tests