Practice Test 71 - AP Biology

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

A double-stranded DNA molecule is exposed to increasing salt concentration. The DNA strands become less likely to separate at a given temperature. Which statement best explains the effect of salt on DNA stability at the molecular level?
Phospholipids in a membrane can have one saturated tail and one cis-unsaturated tail. The unsaturated tail introduces a bend that reduces how closely adjacent phospholipids can pack. In a cell exposed to a sudden temperature decrease, membranes risk becoming less fluid as molecules move less. Which change in phospholipid tail composition would best counteract reduced membrane fluidity at lower temperature?
A protease is tested in two buffered solutions with identical substrate concentration and temperature. In pH 8 buffer, the rate is 30 nmol/min; in pH 10 buffer, the rate is 9 nmol/min. The protease active site includes amino acids that must be in specific protonation states to form transient hydrogen bonds with the substrate during catalysis. Changing pH alters the protonation of those side chains, which can reduce catalytic efficiency without changing enzyme amount. Which outcome is most likely at pH 10 compared with pH 8?
Two spherical cells have identical membrane permeability to water and are placed into a hypotonic solution. Cell 1 has radius $4\,\mu m$ and Cell 2 has radius $12\,\mu m$ . Water enters by osmosis, and neither cell has a cell wall. Over the first 30 seconds, Cell 1's volume increases by a larger percentage than Cell 2's volume. Which explanation best accounts for the difference in percent volume change?
In a cat species, fur pattern is X-linked. Allele $X^O$ produces orange fur and allele $X^o$ produces black fur. Heterozygous females ( $X^O$ $X^o$ ) show patches of orange and black. A patchy female is crossed with a black male ( $X^o$ Y). Assume no other genes affect color. Which outcome best predicts the phenotypes of male offspring?
A gene's coding sequence contains the DNA codon 5′-GAG-3′, which corresponds to glutamic acid in the protein. A mutation changes this codon to 5′-GTG-3′. Transcription and translation occur normally. Which outcome is most likely for the protein produced from the mutated gene?
A population of field mice includes two hemoglobin alleles, H1 and H2. In 2000, $f(H2)=0.10$ in a lowland population. A drought from 2001–2004 reduced available water, and the same population was sampled each year: 2002 $f(H2)=0.16$ , 2004 $f(H2)=0.23$ , 2006 $f(H2)=0.24$ . H2 is associated with improved oxygen delivery during dehydration stress. Which observation best demonstrates evolution occurring in this mouse population?
In an RNA-world test, a lab-evolved RNA molecule catalyzed the joining of RNA nucleotides onto a short RNA template, producing a complementary strand in vitro. The reaction occurred without any proteins but required Mg2+ ions. Which conclusion is best supported regarding early life origins?
A small mammal population was tracked for one year in a meadow. During spring, births greatly exceeded deaths and the population rose from 200 to 420. During summer, births and deaths were approximately equal and the population stayed near 430. During autumn, deaths exceeded births and the population fell to 300. No immigration or emigration was detected. Which explanation best accounts for the population remaining near 430 in summer?
A protein binds a small ligand through a pocket formed by tertiary structure. The pocket is stabilized by hydrophobic interactions among nonpolar R groups and a single hydrogen bond between a polar side chain and the ligand. A mutation changes that polar amino acid to a nonpolar amino acid of similar size. The protein remains folded but shows lower ligand affinity. Which feature best explains the reduced binding?
Woodlice move faster and turn more in dry air but do not orient toward moisture. What is the best conclusion?
A population has alleles M and m. Observed genotype frequencies are MM: 0.64, Mm: 0.32, mm: 0.04. Assume Hardy-Weinberg conditions are met. Which allele frequency is most consistent with these data?
Two polysaccharides are compared. Both contain only C, H, and O, but one has frequent branching due to additional covalent linkages between carbon atoms at branch points. In water, enzymes that bind to the ends of polysaccharide chains can attach to more sites on the branched polymer than on the unbranched polymer of the same total length. The increased number of chain ends results from the polymer's covalent structure, not from changes in monomer composition. Which statement best predicts a consequence of the branched structure at the molecular level?
During S phase, a DNA double helix is unwound, and each original strand serves as a template. DNA polymerase adds nucleotides to the 3′ end of a growing strand using complementary base pairing (A–T, C–G). Because the two template strands are antiparallel, one new strand is synthesized continuously toward the replication fork, while the other is synthesized in short segments that are later joined. A student observes that after a brief pulse of labeled nucleotides, labeling appears in multiple short patches on one newly made strand but as one long region on the other. Which outcome is most likely to occur if the joining step between short segments is inhibited?
In bacteria, a gene's promoter is followed immediately by a leader region that can form either a terminator hairpin (stopping transcription early) or an anti-terminator (allowing transcription to continue). When an amino acid is abundant, a ribosome rapidly translates the leader peptide, favoring terminator formation and reducing full-length mRNA. A mutation slows ribosome movement through the leader peptide without changing the promoter. Which outcome is most likely when the amino acid is abundant?
A breeder selects for a rare trait—extra-large seeds—in a bean population by planting only seeds from the largest 5% of plants each generation. Seed size is influenced by alleles S (larger) and s (smaller). After several generations, extra-large seeds become more common, and the smallest seeds are seldom produced. No new alleles enter the population. Which outcome is most likely in the bean population over time?
A cytosolic enzyme is encoded by a gene whose coding sequence includes codons 45–47: 5'-GAA CCT TTT-3', producing mRNA 5'-GAA CCU UUU-3' and amino acids Glu–Pro–Phe. In a mutant, one nucleotide is deleted from the DNA within codon 45, changing the sequence to 5'-GAC CTT TT…-3' from that point onward. No other changes occur in the gene. The enzyme's active site depends on amino acids encoded downstream of codon 45. Which outcome is most likely from this mutation?
A student models cells as cubes with equal membrane permeability. Cube A has side length 1 unit; Cube B has side length 3 units. Both cubes are placed in a solution containing a dye that enters only across the surface. After the same time interval, Cube A becomes uniformly colored sooner than Cube B. Which explanation best accounts for the observed difference in dye uptake efficiency?
A chloroplast preparation is illuminated with two different wavelengths that are absorbed differently by photosystems. In both cases, absorbed light excites electrons that move through thylakoid carriers, pumping protons and enabling ATP synthesis, while electrons ultimately reduce NADP $^+$ . The researcher observes that one wavelength produces a lower rate of NADPH formation but similar ATP formation compared with the other. Which explanation best accounts for this pattern?
A protist contains two different energy-related organelles. Organelle 1 has a double membrane, circular DNA, and ribosomes inhibited by streptomycin. Organelle 2 has a single membrane, no detectable DNA, and is continuous with a network of membranes connected to the nuclear envelope. Which explanation best accounts for the origin of organelle 2?

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