Practice Test 67 - AP Biology

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

In an aquatic plant, a storage carbohydrate is described as a polysaccharide made of many glucose monomers linked primarily by $α$ (1;4) glycosidic bonds, producing a helical chain. The helical structure allows many hydroxyl groups to remain exposed on the outside of the coil, enabling extensive hydrogen bonding with surrounding water molecules. In cells, this polymer is packaged in granules that can rapidly exchange water with the cytosol without forming rigid fibers. Which feature best explains why this carbohydrate remains compact yet interacts readily with water?
To determine the genotype of the long-winged male fly, the researcher should perform a test cross. This involves crossing the male fly with a female of which of the following genotypes?
A cell is treated with a drug that makes the inner mitochondrial membrane highly permeable to protons. Electron transport chain complexes still transfer electrons to oxygen, and the citric acid cycle continues producing reduced electron carriers from acetyl-CoA. However, because protons leak back into the matrix, the proton gradient across the inner membrane remains small. ATP synthase is present but depends on the proton-motive force to phosphorylate ADP. The drug does not affect glycolysis enzymes in the cytosol. Which outcome is most likely in treated cells?
Genetically identical bacterial cells are grown with or without lactose. Only cells grown with lactose produce high levels of β-galactosidase enzyme and show increased lac operon mRNA; DNA sequencing shows the lac genes are identical in both conditions. When lactose is removed, lac mRNA and enzyme levels decrease. Which explanation best accounts for the lactose-dependent enzyme production?
A scientist designs two short nucleic acid probes to bind a target single-stranded DNA by complementary base pairing. Probe 1 is DNA and contains thymine; Probe 2 is RNA and contains uracil. Each probe is the same length and perfectly complementary to the target sequence, differing only by T versus U and the sugar (deoxyribose versus ribose). Binding depends on hydrogen bonding between bases and proper geometric fit. Which statement best describes how substituting U for T affects base pairing with adenine in the target DNA?
In a plant, tall (T) is completely dominant to short (t), and round seeds (R) are completely dominant to wrinkled seeds (r). Two plants that are heterozygous for both genes (TtRr) are crossed. Assume the genes assort independently and each allele segregates normally into gametes. Which proportion of offspring is expected to be short with wrinkled seeds?
A membrane protein opens in response to a voltage change and allows Na $^+$ to cross. When open, Na $^+$ moves rapidly into the cell from high extracellular concentration to lower cytosolic concentration. The movement stops when the channel is blocked and is unaffected by ATP depletion. Which mechanism best explains Na $^+$ movement through this protein?
According to Mendel's law of independent assortment, which of the following is true for a dihybrid cross involving two unlinked genes?
A researcher measures kinase activity across the cell cycle and finds that Cdk2 activity peaks at the G1/S transition, while Cdk1 activity peaks at G2/M. A small molecule selectively stabilizes an S-phase cyclin so it is not efficiently removed later in the cycle. Cdk2 remains active beyond S phase, while Cdk1 regulation is unchanged. Which effect is most likely from prolonged S-phase cyclin–Cdk2 signaling?
In humans, red-green color blindness is caused by a recessive allele $c$ on the X chromosome; allele $C$ results in normal color vision. A woman with normal vision whose father was color-blind has a child with a man with normal vision ( $X^CY$ ). Which outcome best predicts the probability that a son will be color-blind?
A lizard species occupies a chain of desert oases. Adjacent oases exchange migrants each generation, but the two oases at the ends of the chain rarely exchange migrants directly. Genetic distance increases steadily with geographic distance along the chain. When lizards from neighboring oases are paired, they mate and produce fertile offspring, but lizards from the two end oases show low mating success and produce few viable offspring. No single physical barrier separates the end oases; instead, gene flow occurs mainly between neighboring sites. Which pattern most directly accounts for the divergence observed?
A researcher tests a Cdk1 variant that cannot be phosphorylated at the inhibitory site but still binds M-cyclin and can be activated at the activating site. In cells with this variant, checkpoint signaling that normally inhibits Cdc25-like phosphatase remains intact. After mild DNA damage, cells with wild-type Cdk1 delay mitotic entry. Which effect is most likely in cells expressing the noninhibitable Cdk1 variant?
In a population of corn, kernels can be red or yellow. A farmer saves seeds only from plants with the reddest kernels to plant the next year. Kernel color is influenced by alleles R (red) and r (yellow). Over four planting cycles, the fraction of red-kernel plants rises from 40% to 85%. Which change in the population is most likely responsible for this shift?
A growth factor activates a receptor kinase that triggers a cytosolic phosphorylation cascade, increasing activity of G1 cyclin–Cdk complexes. In a mutant cell line, the receptor kinase can bind growth factor but cannot phosphorylate downstream targets. Internal cyclin–Cdk complexes and checkpoints are otherwise normal. Which change would most likely allow these mutant cells to pass the G1 checkpoint?
In a GPCR cascade, where does signal amplification first occur?
A pure phospholipid bilayer is used to compare diffusion of two steroids: cortisol (has several polar hydroxyl groups) and cholesterol (fewer polar groups, largely nonpolar). Both are large and uncharged, but cortisol is more polar. The bilayer interior is hydrophobic, so increased polarity reduces solubility in the membrane core. No transport proteins are present. Which molecule would most likely have higher permeability through the bilayer?
A bacterial gene is translated beginning at an AUG start codon. In the wild-type sequence, the coding region includes 5′-ATG GAA CCT TGC...-3′. A mutation deletes the second nucleotide of the coding region, producing 5′-ATG AAC CTT GC...-3′. Translation still initiates at the original start codon. Which outcome is most likely for the resulting protein?
A cell-cycle checkpoint uses a phosphatase to activate a CDK by removing an inhibitory phosphate. A mutation increases the phosphatase's activity, causing faster removal of the inhibitory phosphate on CDK1. Cyclin B levels are unchanged. Which change is most likely in the mutant cells' cell-cycle timing?
Two double-stranded nucleic acid molecules have identical base sequences but differ in backbone sugar: one uses ribose and the other uses deoxyribose. Both have phosphodiester bonds linking nucleotides. Under mildly basic conditions, the ribose-containing molecule shows more frequent strand scission over time. Which structural feature best explains the increased backbone cleavage in the ribose-containing molecule?
A nonpolar gas (O2) is at higher concentration outside a cell than inside. The cell membrane contains no specific O2 transport proteins. When extracellular O2 is increased, intracellular O2 rises proportionally, and the process is unaffected by ATP depletion. Which mechanism best explains O2 entry into the cell?

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