AP Biology Practice Test: Practice Test 69

A researcher stains cells with a fluorescent dye proportional to DNA content and counts fluorescence intensity. One group of cells shows a single peak at 2C, and microscopy shows intact nuclei and no visible condensed chromosomes. Another group shows a single peak at 4C, and microscopy shows condensed chromosomes with sister chromatids still joined. The 4C group does not show separated chromatids or two nuclei. Based on these observations, the 4C group is most likely paused at a checkpoint during mitosis. Which checkpoint is most consistent with the data?

A culture of aerobic cells is shifted from normal oxygen conditions to very low oxygen. Glycolysis in the cytosol converts glucose to pyruvate and produces a small amount of ATP by substrate-level phosphorylation while reducing NAD+ to NADH. Under normal oxygen, NADH is oxidized by the mitochondrial electron transport chain, allowing continued glycolysis. When oxygen becomes limiting, electrons cannot be passed efficiently to the final electron acceptor, and NAD+ regeneration by the electron transport chain slows. Which outcome is most likely in the cells shortly after oxygen drops?

In a dividing cell, DNA replication has completed and the cell enters M phase. During prometaphase, spindle microtubules attach to kinetochores, and by metaphase all chromosomes are properly bi-oriented. The spindle checkpoint is satisfied, allowing progression into anaphase. Which event is most likely to occur next in this cell?

In two identical ponds, ecologists introduced 50 and 200 juvenile sunfish, respectively. After 8 weeks, both ponds had similar water temperature and dissolved oxygen, but the 200-fish pond showed lower average mass gain and more fin damage from aggressive interactions. No new predators were observed, and no storms occurred during the study period. Which outcome is most likely caused by a density-dependent factor regulating the sunfish population?​​

A student compares DNA replication and transcription. In replication, both DNA strands are copied to produce two double-stranded DNA molecules, and DNA polymerase uses DNA nucleotides (A, T, C, G) added to a 3′ end. In transcription, one DNA strand serves as a template to build an RNA molecule using ribonucleotides (A, U, C, G). A cell is treated with a drug that prevents incorporation of uracil-containing nucleotides. Which process is most directly inhibited by the drug?

During eukaryotic mRNA maturation, the 3′ end of a pre-mRNA is cleaved downstream of a specific sequence in the transcript, and then a poly(A) tail is added by poly(A) polymerase using ATP as the adenine source. A mutant cell line has a defective cleavage factor, so cleavage at the 3′ end occurs inefficiently, while transcription proceeds normally through the gene. Compared with wild type, which change is most likely observed in RNA molecules produced from this gene?

A plant is placed near a window so light comes from one side. Over the next 24 hours, the stem bends toward the light source while the plant remains rooted in place. When the pot is rotated 180°, the stem gradually bends in the new direction of the light. No new leaves form during the observation period. Which response best explains the stem bending toward the light?

A peptide hormone H binds receptor RTK on liver cells, causing receptor dimerization and autophosphorylation of tyrosines. An adaptor protein binds the phosphotyrosines and activates Ras (GDP→GTP). Ras-GTP activates kinase K1, which phosphorylates K2, which phosphorylates K3. K3 phosphorylates enzyme E, increasing glycogen breakdown within minutes. A phosphatase P removes phosphate groups from K3 and E. In cells with a loss-of-function mutation in P, H produces a larger and longer-lasting increase in glycogen breakdown than in wild type. Which explanation is best supported by these results?

A double-stranded DNA molecule contains a region rich in adenine and thymine. In this region, adjacent base pairs stack similarly to other regions, but A=T pairs form two hydrogen bonds per base pair, whereas G≡C pairs form three. A helicase-like protein applies the same mechanical force to separate strands in two regions of equal length: one A/T-rich and one G/C-rich. No covalent bonds in the backbone are broken during separation. Which statement best predicts which region separates more easily and why?

A researcher compares two receptors, R1 and R2, on the same cell. Ligand L binds R1 with high affinity and R2 with low affinity. At low L concentration, only the R1-mediated response occurs; at high L concentration, both occur. Which of the following best explains this pattern?

A student models translation using an mRNA with a start codon followed by several sense codons and then a stop codon. They note that tRNAs bind codons through complementary base pairing between the codon (mRNA) and anticodon (tRNA), and the ribosome catalyzes peptide bond formation as each amino acid is added. At the stop codon, no tRNA anticodon pairs; instead, a release factor binds and hydrolyzes the bond between the polypeptide and the tRNA in the ribosome. Which explanation best accounts for how translation terminates at a stop codon?

Enzyme W catalyzes S → P. At [S] = 1 mM, adding inhibitor K reduces the initial rate from 50 to 25 µM/min. When enzyme concentration is doubled while keeping [S] = 1 mM and inhibitor K constant, the initial rate increases from 25 to 50 µM/min. Temperature and pH are constant. Which explanation best accounts for the restored rate when enzyme concentration is increased?

A student investigates whether protocell-like compartments could support internal reactions. Fatty-acid vesicles are loaded with a pH-sensitive dye and then placed into a solution containing a weak acid. Over time, dye color changes inside vesicles, indicating internal pH shifts, while vesicles remain intact. Which conclusion is best supported about early protocells?

A human gene’s coding strand normally includes the sequence 5’-ATG GAA TTT CCG TAA-3’, producing a short polypeptide. A mutation changes the third codon from TTT to TTA, yielding 5’-ATG GAA TTA CCG TAA-3’. Transcription and translation occur normally, and the reading frame is unchanged. The gene is expressed at the same level as before. (Assume the standard genetic code and that the sequence shown is in-frame beginning at ATG.) Which outcome is most likely from this mutation?

A peptide ligand L binds a receptor tyrosine kinase (RTK) on the plasma membrane, causing receptor dimerization and autophosphorylation on cytosolic tyrosines. An adaptor protein binds the phosphotyrosines and recruits a small GTPase (Ras), promoting exchange of GDP for GTP. Ras-GTP activates a kinase cascade (K1→K2→K3) in which each kinase phosphorylates multiple copies of the next kinase, amplifying the signal. A GTPase-activating protein (GAP) increases Ras GTP hydrolysis, returning Ras to the GDP-bound state. Cells expressing a hyperactive GAP show reduced K3 phosphorylation after L addition. Which manipulation would most likely restore K3 phosphorylation in cells with hyperactive GAP?

A cell-surface receptor activates a signaling pathway that increases cGMP within 20 seconds of ligand addition. When cells are treated with a membrane-permeable inhibitor of guanylyl cyclase, ligand binding occurs normally but cGMP does not increase. No changes in intracellular Ca$^{2+}$ are detected in either condition. Which of the following best explains the early transduction step blocked by the inhibitor?

A marine plankton population varies in spine length. Over 25 generations, predatory fish consumed plankton with very short spines and very long spines more often than those with intermediate spines. Intermediate-spined individuals averaged 2.3 surviving offspring, while short- and long-spined individuals averaged 1.0 and 1.1, respectively. The population’s mean spine length remained near the original value, and variance decreased. Which type of selection is most consistent with these data?

A scientist compares two versions of the same enzyme pathway: one where all enzymes float freely in the cytosol, and another where the enzymes are tethered to the inner surface of a small membrane-bound vesicle. The tethered system produces product faster at the same substrate concentration. Which feature best explains how compartmentalization increases pathway efficiency in the tethered system?

A population of 1,200 frogs has allele frequencies $p=0.6$ and $q=0.4$. Observed genotype frequencies are $AA=0.36$, $Aa=0.48$, and $aa=0.16$. No migration or mutation is detected. Which conclusion is best supported?

A DNA-binding protein recognizes a specific base sequence using an α-helix that fits into the major groove; binding depends on the helix’s shape and the positions of charged and polar R groups. Heating the protein increases molecular motion and disrupts many noncovalent interactions, but peptide bonds remain intact. After heating, the protein no longer binds DNA. Which statement best describes why binding is lost?

A cytosolic enzyme is composed of two identical polypeptide subunits; each subunit’s primary structure folds into a tertiary structure, and the two subunits associate via noncovalent interactions to form a functional quaternary structure. A mutation replaces a surface leucine at the subunit interface with lysine, introducing a positively charged R group. The enzyme’s monomers still fold normally, but activity drops sharply. Which feature best explains the loss of activity?

A phospholipid bilayer forms a barrier with a hydrophobic interior. Molecules cross by simple diffusion most readily when they are small and nonpolar. Polar molecules diffuse more slowly because they do not dissolve well in the lipid core, and ions diffuse extremely slowly due to charge and hydration shells. Two molecules are compared: carbon dioxide (CO2), which is small and nonpolar overall, and urea, which is small but polar due to multiple electronegative atoms. No membrane proteins are present.

Which molecule would most likely diffuse across the bilayer faster?

A marine ecologist compares barnacle survival on two stretches of rocky shoreline. In both areas, wave action is similar. On Site A, barnacle density averages 50 per square decimeter and annual survival is 40%. On Site B, density averages 10 per square decimeter and annual survival is 75%. The ecologist observes that a parasitic worm spreads through direct contact between neighboring barnacles. Which factor is most likely density dependent in this system?​

A steroid-like ligand X is membrane-impermeable and binds a cell-surface receptor that activates a heterotrimeric G protein. The G
subunit activates adenylyl cyclase, increasing cAMP. cAMP activates a cyclic nucleotide–gated channel, allowing Ca$^{2+}$ influx; Ca$^{2+}$ activates a cytosolic protease that cleaves protein M, changing its activity within 90 s. When adenylyl cyclase is inhibited, Ca$^{2+}$ influx and M cleavage are not detected. Which change would most likely bypass the need for adenylyl cyclase in this pathway?

A student touches a hot metal surface briefly and immediately withdraws their hand before consciously describing the sensation. The withdrawal occurs even when the student is distracted. Which response best explains the mechanism producing this rapid behavior in response to the stimulus?