Practice Test 59 - AP Biology

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

An mRNA is translated starting at the first AUG encountered by the ribosome, establishing the reading frame. A mutation changes the mRNA sequence upstream of the coding region, but the AUG start codon and all codons after it are unchanged. The ribosome binding and initiation still occur at the same AUG, and elongation proceeds codon by codon until the same stop codon. No RNA editing occurs. Which outcome is most likely for the polypeptide produced from the mutant mRNA compared with the original?
Enzyme T catalyzes S → P. At constant [S] = 2 mM and [E] = 1 µM, the initial rate is 70 µM/min at pH 7.0. At pH 5.0, the initial rate is 10 µM/min, and returning the mixture to pH 7.0 restores the rate to 70 µM/min. Substrate concentration is unchanged throughout. Which explanation best accounts for the reversible decrease in activity at pH 5.0?
In a eukaryotic nucleus, a gene contains three exons separated by two introns. After transcription, the primary RNA transcript is analyzed and found to include all five regions in the same 5′→3′ order as the DNA template. A second RNA sample from the same gene is shorter and lacks the sequences that correspond to the introns, while the exon sequences remain contiguous. No changes are detected in the DNA sequence. Which explanation best accounts for the difference between the two RNA samples?
In a culture of animal cells, a drug is added during M phase that prevents microtubules from shortening but still allows them to attach to kinetochores. After 30 minutes, microscopy shows condensed chromosomes aligned at the metaphase plate, and each sister chromatid pair has kinetochore microtubules attached from both spindle poles. DNA replication was completed earlier in S phase, and the nuclear envelope has already broken down. Despite alignment, the cells do not begin anaphase and remain in this state. Which outcome is most likely in these cells due to checkpoint behavior?
Chitin is a structural polysaccharide found in arthropod exoskeletons and fungal cell walls. It consists of repeating N-acetylglucosamine monomers joined by β-1,4 glycosidic bonds, producing straight chains. The chains align closely, and hydrogen bonds form between hydroxyl groups and acetamide-containing groups on neighboring chains, creating tough fibers. Which feature best explains chitin's ability to form strong structural fibers?
In a small island population of 50 rabbits, allele F has frequency $p=0.50$ and allele f has frequency $q=0.50$ . There is no selection, migration, or mutation, and mating is random. After several generations, allele frequencies differ among replicate island populations founded the same way. Which evolutionary force best explains the differences?
A receptor activates enzyme E1, which converts substrate S to second messenger M. M binds protein P, changing P's conformation and allowing P to activate enzyme E2. E2 phosphorylates target T. In cells with a point mutation in P that prevents M binding, receptor activation produces normal amounts of M but does not increase T phosphorylation. Which intervention would most likely restore T phosphorylation in the mutant cells?
After spring warming, a fish's heat tolerance increases. What is this change?
In a large population of prairie mice, coat color is determined by a single gene with alleles B and b. Researchers sample 1,000 mice and observe genotype frequencies of 0.36 BB, 0.48 Bb, and 0.16 bb. The population occupies a continuous grassland with no barriers to movement, and mating is observed to be random with respect to coat color. No differences in survival or reproductive success are detected among the three genotypes during the study period, and mutation at this locus is considered negligible. Based on these data, the allele frequencies are $p(B)=0.60$ and $q(b)=0.40$ . Which conclusion is best supported about Hardy-Weinberg equilibrium in this population?
A researcher measures uptake of a polar solute X that cannot cross the lipid bilayer. Cells express a specific carrier protein for X. When extracellular X is increased, uptake rate rises and then reaches a maximum even though the gradient continues to increase; ATP inhibitors do not reduce uptake. Which explanation best accounts for the uptake pattern of solute X?
In cattle, coat color shows codominance. Allele $C^R$ produces red hairs and allele $C^W$ produces white hairs. $C^R$ $C^R$ cattle are red, $C^W$ $C^W$ cattle are white, and $C^R$ $C^W$ cattle are roan (both red and white hairs visible). A roan bull is crossed with a red cow. Assume a single autosomal gene. Which offspring phenotype ratio is expected from this cross?
A bacterial chemotaxis receptor in the membrane activates a cytosolic kinase when attractant is absent. The kinase phosphorylates response regulator RR, and RR-P binds flagellar motor proteins to increase tumbling frequency. When attractant binds the receptor, kinase activity decreases, RR phosphorylation drops, and tumbling decreases within seconds. A mutant strain has normal receptor–attractant binding but maintains high RR phosphorylation even with attractant present. Which defect most likely explains the mutant phenotype?
A scientist labels newly synthesized DNA during S phase with a fluorescent nucleotide. After one full cell cycle, the scientist examines daughter cells in early G1 and finds fluorescent label in their nuclei. The scientist then follows those same daughter cells into the next S phase and observes an increase in total fluorescence as replication proceeds. No fluorescence is added outside S phase. Which inference best explains why fluorescence increases only during S phase?
A continental bird species expanded its range to include a nearby island. Over time, the island population evolved a shorter beak and different song pattern. When island birds and mainland birds are brought together in a large aviary, mating occurs and viable offspring hatch. However, male offspring from these crosses produce no functional sperm as adults, while female offspring are fertile. Allele frequencies differ consistently between island and mainland populations. Which reproductive barrier is most directly demonstrated by the cross results?
In cattle, coat color alleles show codominance: allele R produces red hairs, allele W produces white hairs, and heterozygotes (RW) have both red and white hairs (roan). Two roan cattle are crossed (RW × RW). Which offspring distribution best matches codominance?
An epithelial cell imports glucose from the extracellular fluid even when extracellular glucose is 1 mM and cytosolic glucose is 10 mM. The transporter requires extracellular Na+; removing Na+ stops glucose uptake. ATP depletion stops uptake over time, and a separate Na+/K+ ATPase is present in the same membrane. No direct ATP binding to the glucose transporter is detected. Which mechanism best explains glucose uptake?
A researcher compares two populations of the same unicellular organism grown under identical conditions. Population 1 cells average 6 µm in diameter; Population 2 cells average 12 µm in diameter. Both rely on diffusion across the membrane for ammonia excretion. Measurements show higher intracellular ammonia in Population 2. Which explanation best accounts for the higher ammonia level in the larger cells?
A researcher tracks a cell that has just completed cytokinesis. The cell enters G1 with unreplicated chromosomes and begins to grow. Later, the cell enters S phase and duplicates its DNA, forming sister chromatids for each chromosome. The cell then enters G2, where DNA replication is complete but chromosomes remain uncondensed. Immediately before mitosis, the researcher detects that one chromosome has a double-strand break and remains unrepaired. The cell does not proceed into mitosis during the observation window. Which checkpoint most likely prevented entry into M phase?
In a rabbit population, coat color is controlled by multiple alleles at one locus with a dominance hierarchy: C (full color) > c^ch (chinchilla) > $c^h$ (Himalayan) > c (albino). A chinchilla rabbit with genotype c^ch c is crossed with a Himalayan rabbit with genotype $c^h$ c. Assuming simple dominance by the hierarchy, which offspring phenotype is expected to be most common?
A scientist alters a membrane by removing most transmembrane proteins while leaving the phospholipid bilayer intact. The bilayer has hydrophilic heads facing outward and a hydrophobic interior formed by fatty acid tails. After removal, the scientist measures transport rates of CO2 (small, nonpolar) and Ca2+ (charged). CO2 transport changes little, but Ca2+ transport decreases sharply. Which feature best explains the different effects on CO2 versus Ca2+ movement?

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