Practice Test 99 - AP Biology

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

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1. A cell in G1 has 2C DNA and unreplicated chromosomes. It passes the G1 checkpoint and enters S phase, completing DNA replication so that each chromosome now has two sister chromatids. The cell enters G2 with 4C DNA and then proceeds into M phase. During mitosis, sister chromatids separate at anaphase and move to opposite poles. Cytokinesis then divides the cytoplasm. Which description best matches chromosome composition in each daughter cell immediately after mitosis and cytokinesis finish?
2. In a plant, cells from a leaf and cells from a root are collected from the same individual. DNA analysis indicates both tissues contain the same genes. RNA sequencing shows high levels of chlorophyll-binding protein mRNA in leaf cells but not in root cells, while root cells show high levels of a membrane transporter mRNA. The student notes both cell types have mitochondria and are alive at sampling. Which explanation best accounts for these tissue-specific RNA patterns?
3. After a nonnative predatory fish is introduced into a lake, surveys show zooplankton decline sharply and phytoplankton density increases within two months; dissolved oxygen near the bottom drops. Which consequence is most likely for the lake ecosystem over the next season?
4. Two frog populations live in the same wetland complex and breed in the same months. Males from population 1 produce a low-pitched call; males from population 2 produce a high-pitched call. Playback experiments show females approach only the call type from their own population. When researchers manually fertilize eggs using sperm from the other population, embryos develop normally. Which isolating mechanism most directly limits gene flow between the frog populations in nature?
5. A bacterial cell is exposed to an antibiotic that weakens its cell wall. When placed in a hypotonic solution, many treated cells swell and burst, while untreated cells remain intact. In bacteria, the cell wall is a rigid layer outside the plasma membrane that resists internal pressure generated by water entering the cell. Which feature best explains why treated cells are more likely to lyse in hypotonic conditions?
6. A researcher uses a table to summarize observations from three cell populations. Population 1 has 2C DNA and uncondensed chromatin. Population 2 has DNA content between 2C and 4C and uncondensed chromatin. Population 3 has 4C DNA and condensed chromosomes aligned at the metaphase plate. The researcher wants to identify which population is actively replicating DNA. Which population is most likely in S phase based on these observations?
7. A eukaryotic pre-mRNA is transcribed and then processed at its 3′ end. In a wild-type cell, a cleavage and polyadenylation specificity factor (CPSF) recognizes the AAUAAA sequence in the pre-mRNA and promotes cleavage downstream, followed by addition of a poly(A) tail. In mutant cells, the AAUAAA sequence is changed to AACAAA, but transcription through the region continues. Compared with wild type, the predominant RNA from mutant cells is longer and lacks a poly(A) tail. Which explanation best accounts for these observations?
8. A biologist compares a 12–amino acid protein segment among five species. Species W and X differ at 1 position; W and Y differ at 4; W and Z differ at 5; W and V differ at 6. X and Y differ at 4; X and Z differ at 5; X and V differ at 6. Y and Z differ at 2; Y and V differ at 5; Z and V differ at 5. Assuming substitutions accumulate over time and no reversals, fewer differences indicate a more recent common ancestor. Which pair of species is most closely related?
9. In a population of flowering plants, petal pigment is influenced by alleles R and r at one locus. Researchers estimate allele frequencies as $p(R)=0.80$ and $q(r)=0.20$. The population is large, and no differences in survival or fecundity among genotypes are detected. However, pollen-transfer observations show that pollinators move primarily between nearby plants, and plants occur in clustered patches separated by open ground. Genotype frequencies in the combined population show a deficit of heterozygotes compared with $2pq$. Which Hardy-Weinberg condition is most directly violated?
10. A membrane protein contains a transmembrane region enriched in amino acids with nonpolar side chains composed mostly of C and H. The lipid bilayer interior is also largely nonpolar because it is formed by fatty acid tails with many C–H bonds. In contrast, amino acids with polar or charged side chains (often containing O, N, or S) are more common in regions exposed to the aqueous cytosol. The protein remains stably embedded in the membrane over time. Which statement best explains the stability of the transmembrane region within the bilayer?
11. A plant population has two alleles, $R$ and $r$. Initial allele frequencies are $p(R)=0.7$ and $q(r)=0.3$. After one generation, the allele frequencies remain $0.7$ and $0.3$, but genotype frequencies are $RR=0.60$, $Rr=0.20$, $rr=0.20$. Which conclusion is best supported?
12. A ligand R binds a receptor on the plasma membrane of muscle cells. In cells expressing a receptor variant lacking its cytosolic tail, R binding is normal but no rapid increase in cytosolic cAMP occurs. In cells expressing the full receptor, cAMP rises within 8 seconds of R addition. Which of the following best explains why the cytosolic tail is required for early signaling?
13. In a forest understory, a shrub species grows poorly beneath a canopy tree that releases leaf litter containing inhibitory chemicals. In plots where the tree's litter is removed weekly but shade remains, shrub growth increases compared with unmanipulated plots. No evidence of herbivory differences is observed between treatments. Which interaction best explains the reduced shrub growth in unmanipulated plots?
14. A proposed "RNA world" model suggests RNA could store information and catalyze reactions. In a lab, an RNA molecule (ribozyme) was provided with activated RNA nucleotides; over time, it produced complementary RNA strands, increasing total RNA length and amount compared with a control lacking the ribozyme. No proteins were present in either condition. Which claim is best supported by these results about early genetic systems?
15. APC/C activity is regulated by association with an activator protein present in late mitosis. When the activator binds APC/C, APC/C targets M-cyclin for ubiquitination, reducing Cdk1 activity. A mutation prevents the activator from binding APC/C, but APC/C and substrates are otherwise normal. Which outcome is most likely in mutant cells?
16. A cell is observed at two time points. At time 1, the cell has uncondensed chromatin, an intact nuclear envelope, and a DNA content of 4C. At time 2, the cell has two nuclei forming at opposite poles and chromosomes are beginning to decondense; the cell has not completed cytokinesis yet. No additional DNA replication occurs between these time points. Which statement best describes the DNA content in each forming nucleus at time 2?
17. During meiosis in a heterozygous individual, a tetrad forms for a homologous chromosome pair. A crossover occurs near the middle of the chromosome between non-sister chromatids, producing two chromatids with exchanged segments and two chromatids unchanged. The cell then completes meiosis I and II with normal segregation. At the chromosomal level, this event changes which alleles are linked on a single chromatid without changing chromosome number. Which process best explains the source of the new allele combinations in some gametes?
18. A molecule X is uncharged but polar. In cells lacking a specific membrane protein, X does not enter even when extracellular X is high. In cells expressing the protein, X enters until inside and outside concentrations match. Uptake shows a maximum rate at high X concentrations and does not require ATP. Which mechanism best explains transport of X?
19. A student compares diffusion of ethanol and Na+ across a pure phospholipid bilayer lacking proteins. Ethanol crosses slowly but measurably, while Na+ shows almost no movement. The bilayer has polar phosphate heads facing water and a hydrophobic interior formed by fatty acid tails. The student then adds a membrane with embedded ion channels and observes rapid Na+ movement. Which membrane feature best explains why Na+ requires channels for efficient transport?
20. Cells are placed in a solution containing a high concentration of glucose. The plasma membrane is a phospholipid bilayer with a hydrophobic interior and embedded transport proteins. Glucose is polar and does not readily pass through the hydrophobic core by simple diffusion. In one group, a membrane protein that spans the bilayer and has a glucose-binding site is abundant; in another group, that protein is absent. Which feature best explains why glucose enters cells more rapidly when the protein is present?

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