Practice Test 57 - AP Biology

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

A cell experiences DNA damage that converts some cytosines on one parental strand into uracils before S phase. During replication, DNA polymerase uses the damaged strand as a template and incorporates bases by complementarity, adding nucleotides only to the 3′ end. The replication machinery does not detect the uracil during synthesis. After replication and the next round of replication, which change is most likely at a site where a U was present in the original template?
In a hydrothermal-vent analogue, researchers circulated warm, mineral-rich water through porous rock while cooler water flowed outside, creating stable pH and temperature gradients across thin mineral walls. Simple organic molecules accumulated to higher concentrations inside pores than in the surrounding water. Which conclusion is best supported about early life origins?
A phospholipid bilayer membrane is tested for permeability to different solutes. The hydrophobic interior favors diffusion of solutes that are nonpolar and small. Polar solutes pass more slowly, and charged solutes pass least readily. Compare two uncharged molecules: diethyl ether (relatively nonpolar) and sucrose (large and highly polar with many hydroxyl groups). No transport proteins are present. Which molecule would most likely have greater permeability across the bilayer?
Two trials measure initial rate at the same substrate concentration. Trial 1 uses Enzyme A alone. Trial 2 uses the same amount of Enzyme A plus a second molecule that binds tightly to a site distinct from the active site. In Trial 2, fewer enzyme–substrate complexes form per unit time, and increasing substrate concentration does not restore the original rate. Which change would most likely restore the initial rate in Trial 2?
In eukaryotes, spliceosomes recognize conserved sequences at intron boundaries and a branch-point adenosine within the intron. A mutation changes the branch-point adenosine of intron 3 to cytosine, without altering the 5′ or 3′ splice sites. In mutant cells, intron 3 is frequently retained in the RNA, while other introns are removed normally. Which explanation best accounts for how the branch-point mutation alters RNA processing?
An ion pump in the plasma membrane exports Ca $^{2+}$ from the cytosol to the extracellular space. Cytosolic Ca $^{2+}$ is kept lower than extracellular Ca $^{2+}$ . When ATP is removed, Ca $^{2+}$ export stops and cytosolic Ca $^{2+}$ rises. The pump protein binds ATP and Ca $^{2+}$ during transport. Which mechanism best explains Ca $^{2+}$ export?
Eukaryotic pre-mRNA from a gene is processed in the nucleus. In one cell line, a mutation changes the 3′ splice site at the end of an intron from AG to AC, while the rest of the gene remains unchanged. When RNA from this cell line is analyzed, many transcripts retain the intron sequence between the two flanking exons. Transcription initiation and elongation rates are unchanged compared to wild type. Which explanation best accounts for the increased intron retention in the RNA?
A lab compares two lipid hormones: Molecule S is a steroid with four fused hydrocarbon rings and a small polar functional group; Molecule P is a short peptide with many polar amino acids. The steroid's structure is mostly nonpolar and relatively rigid, while the peptide has many polar groups that interact strongly with water. When each molecule is placed near a phospholipid bilayer, one crosses the hydrophobic interior more readily without needing a transport protein. Which statement best explains the difference in bilayer crossing?
A fish population in a lake is sampled monthly. When density is 5 fish per cubic meter, average juvenile survival to adulthood is 60%. When density rises to 20 fish per cubic meter, juvenile survival drops to 25%. Water temperature, dissolved oxygen, and pH remain stable across months. The lake contains the same number of nesting sites throughout the year. Which factor best explains the observed change in juvenile survival as density increases?
In a lab, liver cells and skeletal muscle cells taken from the same mouse are cultured. DNA sequencing shows the two cell types have identical nuclear DNA. When mRNA levels are measured, liver cells contain abundant mRNA for albumin but little mRNA for myosin heavy chain, while muscle cells show the opposite pattern. Which explanation best accounts for these differences in mRNA abundance between the two cell types?
A scientist compares two signaling molecules: peptide P and steroid S. P causes a response only when added outside intact cells, while S causes a response even when added to cell-free cytosolic extracts containing receptor proteins. Which of the following best explains the difference in how P and S are detected?
A plant cell is transferred from an isotonic solution to a hypertonic solution. Within minutes, the plasma membrane pulls away from the cell wall and the central vacuole volume decreases substantially. The cell wall remains rigid and unchanged in shape. Which feature best explains why the overall cell shape changes less than the membrane position?
A student examines replication of a linear eukaryotic chromosome end. DNA polymerase requires a primer and can extend only from an existing 3′ end, synthesizing DNA 5′→3′. After the final RNA primer on the lagging strand is removed, there is no upstream 3′ OH available for DNA polymerase to fill in the remaining gap at the extreme 5′ end of the new strand. Over many cell divisions, this leads to progressive shortening of chromosome ends. Which explanation best accounts for the shortening based on replication mechanism?
Red blood cells contain aquaporin proteins in the plasma membrane. When cells are placed in a hypotonic solution, water enters quickly; when aquaporins are blocked, water entry is much slower. No ATP is consumed during either condition. Which explanation best accounts for the faster water movement when aquaporins are present?
In a bacterial cell, translation can begin on an mRNA while the mRNA is still being synthesized. A mutation changes the start codon AUG to AUC in an mRNA, but the rest of the coding region remains unchanged and contains no other start codons nearby. Assume the ribosome requires AUG to initiate efficiently at that site. Which outcome is most likely for translation of the mutated mRNA?
A cell from a diploid species (2n = 8) enters meiosis. During prophase I, homologous chromosomes synapse and form tetrads; later, homologs separate in anaphase I, and sister chromatids separate in anaphase II. No nondisjunction occurs. At the end of meiosis II, four cells form. Which outcome is most likely regarding chromosome number and chromatid state in each final cell?
A scientist hydrolyzes two carbohydrates to identify their monomers. Sample M yields only glucose. Sample N yields glucose and fructose. Both samples were initially small, water-soluble molecules rather than long polymers. Which statement best describes the most likely identity of Sample N based on its monomers?
A ligand binds a membrane receptor and triggers rapid production of IP $_3$ . When an IP $_3$ receptor on the endoplasmic reticulum (ER) is blocked, cytosolic Ca $^{2+}$ does not increase, but IP $_3$ production is unchanged. Which of the following best explains the role of IP $_3$ in early signal transduction?
A meiotic cell (2n = 4) contains two homologous pairs. No crossing over occurs. Homologs segregate independently at anaphase I, and sister chromatids separate at anaphase II. Which outcome is most likely regarding the chromosome combinations in the gametes?
A leaf is illuminated while a drug specifically blocks ATP synthase in the thylakoid membrane. Light still excites electrons in photosystems, and electron carriers can pass electrons to NADP $^+$ , but protons cannot flow back through ATP synthase. Protons continue to be pumped into the thylakoid lumen by electron transport. Which outcome is most likely shortly after the drug is added?

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