Practice Test 14 - AP Biology

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

In a eukaryotic nucleus, RNA polymerase II transcribes a gene into a 2,400-nucleotide primary transcript containing four exons (total 900 nt) and three introns (total 1,500 nt). After transcription, the transcript is capped at the 5′ end and polyadenylated at the 3′ end. A mutation disrupts the 5′ splice site of intron 2, preventing spliceosome recognition of that intron's boundaries, while all other splice sites remain functional. The cell still performs cleavage and poly(A) addition at the normal site. Which explanation best accounts for the nucleotide composition of the processed RNA produced from this mutant transcript?
Two enzymes in a pathway produce a reactive intermediate that can damage proteins if it diffuses widely. In one cell type, both enzymes are located inside the same membrane-bound compartment; in another, the first enzyme is cytosolic and the second is in an organelle. The first cell type shows less protein damage at similar pathway flux. Which feature best explains the difference?
In a moth population, allele $D$ produces darker wings than allele $d$ . In an industrial area, birds capture more light moths than dark moths. Across eight generations, the frequency of allele $D$ rises from 0.30 to 0.78. Which explanation best accounts for the allele-frequency change in this population?
Chloroplasts are illuminated in a solution containing CO2, ADP + Pi, and NADP+. A researcher adds a compound that selectively inhibits ATP synthase without directly affecting photosystems or electron carriers. Protons continue to be pumped into the thylakoid lumen, increasing the H+ gradient, but ATP production drops sharply. NADPH can still be produced initially as electrons reach NADP+. Over several minutes, the electron transport rate declines as the gradient becomes very steep. Which explanation best accounts for the later decline in electron transport?
In a rabbit population, fur thickness varies continuously. A colder winter increases mortality among rabbits with thinner fur, while thicker-furred rabbits survive and reproduce more. Over time, the population's average fur thickness increases. Which statement best explains the role of variation in this evolutionary process?
The chromosomal basis for Mendel's law of independent assortment is that...
A toxin locks heterotrimeric Gα in the GTP-bound state by preventing GTP hydrolysis. In cells where ligand L normally causes a transient rise in cAMP through Gα activation of adenylyl cyclase, toxin-treated cells show a sustained high cAMP level even after L is washed out. Receptor binding properties are unchanged. Which prediction is most consistent with the toxin's effect?
A diploid organism has three homologous chromosome pairs (2n = 6). A student models meiosis assuming no crossing over and normal segregation. The model shows each homologous pair aligning independently of the others at metaphase I, with either the maternal or paternal homolog facing a given pole. After meiosis II, each gamete contains one chromosome from each pair. The student compares gametes from different meioses and finds multiple distinct combinations of whole chromosomes. Which meiotic feature best explains the production of these different gamete chromosome combinations?
During mitosis in a somatic cell, sister chromatids are held together at centromeres until anaphase. A biologist observes a cell with condensed chromosomes aligned at the metaphase plate, and microtubules are attached to kinetochores. The spindle checkpoint is satisfied. Immediately after, the centromeres split and chromatids begin moving toward opposite poles. Based on these observations, which phase transition has just occurred?
In a plant species, leaf shape shows incomplete dominance. Allele L produces long leaves, and allele l produces round leaves. LL plants have long leaves, ll plants have round leaves, and Ll plants have intermediate leaves. Two intermediate-leaf plants are crossed. Assuming a single autosomal gene, which phenotypic ratio is expected among the offspring?
Scientists compared two early-atmosphere simulations. Setup 1 used CH4, NH3, H2, and H2O with electrical sparks; Setup 2 used CO2, N2, and H2O with identical sparks. After one week, Setup 1 produced a higher diversity and concentration of amino acids than Setup 2, though both produced some organics. Which conclusion is best supported by these results?
During S phase, a cell copies DNA by separating the two parental strands; each exposed base serves as a template for adding complementary nucleotides (A–T, C–G). DNA polymerase extends new DNA only by adding nucleotides to a free 3′ OH, so synthesis proceeds 5′→3′. At a replication fork, one new strand is synthesized continuously in the same direction as fork movement, while the other is synthesized discontinuously as short fragments that are later joined. If a base is mismatched, the shape of the helix is distorted, increasing the likelihood it is removed and replaced. Which outcome is most likely if DNA polymerase cannot add nucleotides to a 3′ end?
In a signaling assay, cells are exposed to hormone M and show a rapid rise in cytosolic Ca $^{2+}$ . M binds a cell-surface receptor. When an IP $_3$ receptor blocker is applied, M still binds the receptor but Ca $^{2+}$ does not rise. Which of the following best links receptor activation to the early Ca $^{2+}$ response?
During transcription in a eukaryotic nucleus, RNA polymerase II produces an RNA strand complementary to the DNA template strand. A student claims the RNA sequence will match the template strand except that uracil replaces thymine. Another student claims the RNA sequence will match the non-template (coding) strand except that uracil replaces thymine. No RNA processing steps are considered. Which explanation best accounts for the relationship between the RNA and the DNA strands?
A growth factor binds an RTK, leading to receptor dimerization and phosphorylation. The phosphorylated RTK recruits PI3K, which converts PIP $_2$ to PIP $_3$ in the inner leaflet of the plasma membrane. PIP $_3$ recruits kinase Akt to the membrane, where Akt becomes activated and phosphorylates cytosolic target S, increasing S activity. PTEN is a lipid phosphatase that converts PIP $_3$ back to PIP $_2$ . In cells with reduced PTEN activity, growth factor exposure produces a larger and longer-lasting increase in S activity than in control cells. Which change best explains the altered signaling output?
Two cells have the same shape and membrane permeability and are placed in identical solutions. Cell M has 4× the volume of Cell N. The rate of solute entry across the membrane is proportional to surface area, and solute requirement is proportional to volume. Which explanation best accounts for the relative solute entry per unit requirement?
A coral reef experiences repeated heat waves that cause bleaching events in successive summers. Over time, live coral cover declines and macroalgae cover increases. Which consequence is most likely for reef ecosystem stability?
A cell is heterozygous for two genes on different chromosome pairs: genotype Aa on chromosome 1 and Bb on chromosome 2. No crossing over occurs. During metaphase I, each homologous pair aligns independently at the metaphase plate, and homologs segregate in anaphase I. The resulting gametes from many meioses include AB, Ab, aB, and ab in roughly equal proportions. Which outcome best illustrates how meiosis generates these four gamete types?
Two polysaccharides are compared for structural support in plants: Polymer P is composed of glucose units linked by $β(1\to4)$ bonds; Polymer Q is composed of glucose units linked by $α(1\to4)$ bonds with occasional $α(1\to6)$ branches. When subjected to the same pulling force, P resists elongation more than Q. Which statement best accounts for the difference at the molecular level?
In a rabbit population, fur thickness varies. During unusually cold winters, rabbits with intermediate fur thickness leave the most surviving offspring; very thin and very thick fur rabbits leave fewer. After six generations, the population mean remains near the original value, but the proportion of extreme phenotypes declines. Which explanation best accounts for the observed change in fur thickness distribution?

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