USABO Open Exam 2025/2026 — Questions and
Answers (Complete Solutions)
Question 1: In a eukaryotic cell line treated with a drug that selectively inhibits the TIM23
complex of the mitochondrial inner membrane, which of the following consequences would be
observed first?
A. Accumulation of nuclear-encoded mitochondrial proteins in the cytosol
B. Immediate cessation of the TCA cycle
C. Loss of mitochondrial DNA replication
D. Rapid depolarization of the plasma membrane
Answer: B. Accumulation of nuclear-encoded mitochondrial proteins in the cytosol
Rationale: The TIM23 complex mediates translocation of pre-proteins from the cytosol into the
mitochondrial matrix. Its inhibition blocks import, causing precursor accumulation in the
cytosol. The TCA cycle would slow only after depletion of matrix enzymes, a later event.
Mitochondrial DNA replication uses nucleus-encoded polymerases imported via TIM23, but its
shutdown is secondary to the immediate import block. Plasma-membrane potential is not
directly dependent on mitochondrial protein import.
Question 2: A newly discovered plant transcription factor contains a bZIP domain. Which
nucleotide sequence is most likely to constitute its optimal binding site?
A. 5′-TGACGTCA-3′
B. 5′-TATAAAA-3′
C. 5′-CCAATCT-3′
D. 5′-GGGCGGG-3′
Answer: B. 5′-TGACGTCA-3′
Rationale: bZIP proteins recognize palindromic ACGT cores; the perfect palindrome TGACGTCA
is the canonical G-box. TATAAAA is a TATA-box bound by TBP, CCAATCT by CBF/NF-Y, and
GGGCGGG by Sp1-like zinc-finger factors.
Question 3: The graph below shows oxygen dissociation curves for hemoglobin at two different
CO₂ partial pressures (Pco₂ = 30 mmHg and Pco₂ = 50 mmHg). Which statement correctly
interprets the data?
A. The curve at 50 mmHg CO₂ is shifted left, indicating decreased O₂ affinity.
B. The curve at 30 mmHg CO₂ is shifted left, indicating increased O₂ affinity.
C. Higher CO₂ has no effect on hemoglobin saturation.
D. The P₅₀ value is lower at 50 mmHg CO₂.
Answer: B. The curve at 30 mmHg CO₂ is shifted left, indicating increased O₂ affinity.
Rationale: The Bohr effect causes CO₂ and H⁺ to lower O₂ affinity (right shift). Thus 30 mmHg
CO₂ produces a left shift and higher affinity (lower P₅₀). Option A reverses the direction, C
denies the Bohr effect, and D incorrectly assigns a lower P₅₀ to the right-shifted curve.
Question 4: In C₄ photosynthesis, the primary carbon fixation catalyzed by PEP carboxylase
occurs in which cell type?
A. Bundle-sheath cells
B. Mesophyll cells
C. Guard cells
D. Xylem parenchyma cells
Answer: B. Mesophyll cells
Rationale: PEP carboxylase fixes HCO₃⁻ to PEP in mesophyll cells, generating oxaloacetate.
pg. 1
, Bundle-sheath cells perform the Calvin cycle, guard cells regulate stomata, and xylem
parenchyma is involved in transport, not initial C₄ fixation.
Question 5: A frameshift mutation at codon 120 of a protein-coding gene introduces a
premature stop codon at position 140. The wild-type protein is 400 amino acids. Which term
best describes this mutant allele?
A. Hypomorph
B. Amorph
C. Hypermorph
D. Neomorph
Answer: B. Amorph
Rationale: The allele produces a severely truncated, likely non-functional product, representing
a loss-of-function amorph (null). A hypomorph would reduce activity slightly, a hypermorph
increase activity, and a neomorph confer novel function.
Question 6: During fast axonal transport, which motor protein carries vesicles away from the
cell body along microtubules?
A. Cytoplasmic dynein
B. Kinesin-1
C. Myosin Va
D. Dynamin
Answer: B. Kinesin-1
Rationale: Kinesins move cargo toward the plus end of microtubules (anterograde). Dynein
drives retrograde transport; myosin Va operates on actin in dendrites; dynamin mediates
membrane scission, not long-range transport.
Question 7: In a population in Hardy-Weinberg equilibrium, the frequency of an autosomal
recessive allele is 0.2. What percentage of individuals are expected to show the recessive
phenotype in the next generation?
A. 4 %
B. 16 %
C. 20 %
D. 40 %
Answer: B. 4 %
Rationale: q = 0.2, so q² = 0.04 → 4 % recessive phenotype. 16 % would be the carrier frequency
2pq, 20 % equals q itself, and 40 % has no HW basis.
Question 8: Which enzyme adds the 5′ cap to eukaryotic pre-mRNA?
A. RNA polymerase II CTD kinase
B. Guanylyltransferase
C. Poly(A) polymerase
D. RNase P
Answer: B. Guanylyltransferase
Rationale: Guanylyltransferase catalyzes the 5′-5′ linkage of GMP to the first transcribed
nucleotide. CTD kinase phosphorylates Pol II, poly(A) polymerase adds the 3′ tail, and RNase P
cleaves tRNA precursors.
Question 9: A photosynthetic alga is exposed to intermittent light–dark cycles. The graph of O₂
evolution shows repeated peaks that gradually decline in amplitude. Which process is most
likely limiting?
A. Cyclic photophosphorylation only
B. Calvin cycle turnover
pg. 2
Answers (Complete Solutions)
Question 1: In a eukaryotic cell line treated with a drug that selectively inhibits the TIM23
complex of the mitochondrial inner membrane, which of the following consequences would be
observed first?
A. Accumulation of nuclear-encoded mitochondrial proteins in the cytosol
B. Immediate cessation of the TCA cycle
C. Loss of mitochondrial DNA replication
D. Rapid depolarization of the plasma membrane
Answer: B. Accumulation of nuclear-encoded mitochondrial proteins in the cytosol
Rationale: The TIM23 complex mediates translocation of pre-proteins from the cytosol into the
mitochondrial matrix. Its inhibition blocks import, causing precursor accumulation in the
cytosol. The TCA cycle would slow only after depletion of matrix enzymes, a later event.
Mitochondrial DNA replication uses nucleus-encoded polymerases imported via TIM23, but its
shutdown is secondary to the immediate import block. Plasma-membrane potential is not
directly dependent on mitochondrial protein import.
Question 2: A newly discovered plant transcription factor contains a bZIP domain. Which
nucleotide sequence is most likely to constitute its optimal binding site?
A. 5′-TGACGTCA-3′
B. 5′-TATAAAA-3′
C. 5′-CCAATCT-3′
D. 5′-GGGCGGG-3′
Answer: B. 5′-TGACGTCA-3′
Rationale: bZIP proteins recognize palindromic ACGT cores; the perfect palindrome TGACGTCA
is the canonical G-box. TATAAAA is a TATA-box bound by TBP, CCAATCT by CBF/NF-Y, and
GGGCGGG by Sp1-like zinc-finger factors.
Question 3: The graph below shows oxygen dissociation curves for hemoglobin at two different
CO₂ partial pressures (Pco₂ = 30 mmHg and Pco₂ = 50 mmHg). Which statement correctly
interprets the data?
A. The curve at 50 mmHg CO₂ is shifted left, indicating decreased O₂ affinity.
B. The curve at 30 mmHg CO₂ is shifted left, indicating increased O₂ affinity.
C. Higher CO₂ has no effect on hemoglobin saturation.
D. The P₅₀ value is lower at 50 mmHg CO₂.
Answer: B. The curve at 30 mmHg CO₂ is shifted left, indicating increased O₂ affinity.
Rationale: The Bohr effect causes CO₂ and H⁺ to lower O₂ affinity (right shift). Thus 30 mmHg
CO₂ produces a left shift and higher affinity (lower P₅₀). Option A reverses the direction, C
denies the Bohr effect, and D incorrectly assigns a lower P₅₀ to the right-shifted curve.
Question 4: In C₄ photosynthesis, the primary carbon fixation catalyzed by PEP carboxylase
occurs in which cell type?
A. Bundle-sheath cells
B. Mesophyll cells
C. Guard cells
D. Xylem parenchyma cells
Answer: B. Mesophyll cells
Rationale: PEP carboxylase fixes HCO₃⁻ to PEP in mesophyll cells, generating oxaloacetate.
pg. 1
, Bundle-sheath cells perform the Calvin cycle, guard cells regulate stomata, and xylem
parenchyma is involved in transport, not initial C₄ fixation.
Question 5: A frameshift mutation at codon 120 of a protein-coding gene introduces a
premature stop codon at position 140. The wild-type protein is 400 amino acids. Which term
best describes this mutant allele?
A. Hypomorph
B. Amorph
C. Hypermorph
D. Neomorph
Answer: B. Amorph
Rationale: The allele produces a severely truncated, likely non-functional product, representing
a loss-of-function amorph (null). A hypomorph would reduce activity slightly, a hypermorph
increase activity, and a neomorph confer novel function.
Question 6: During fast axonal transport, which motor protein carries vesicles away from the
cell body along microtubules?
A. Cytoplasmic dynein
B. Kinesin-1
C. Myosin Va
D. Dynamin
Answer: B. Kinesin-1
Rationale: Kinesins move cargo toward the plus end of microtubules (anterograde). Dynein
drives retrograde transport; myosin Va operates on actin in dendrites; dynamin mediates
membrane scission, not long-range transport.
Question 7: In a population in Hardy-Weinberg equilibrium, the frequency of an autosomal
recessive allele is 0.2. What percentage of individuals are expected to show the recessive
phenotype in the next generation?
A. 4 %
B. 16 %
C. 20 %
D. 40 %
Answer: B. 4 %
Rationale: q = 0.2, so q² = 0.04 → 4 % recessive phenotype. 16 % would be the carrier frequency
2pq, 20 % equals q itself, and 40 % has no HW basis.
Question 8: Which enzyme adds the 5′ cap to eukaryotic pre-mRNA?
A. RNA polymerase II CTD kinase
B. Guanylyltransferase
C. Poly(A) polymerase
D. RNase P
Answer: B. Guanylyltransferase
Rationale: Guanylyltransferase catalyzes the 5′-5′ linkage of GMP to the first transcribed
nucleotide. CTD kinase phosphorylates Pol II, poly(A) polymerase adds the 3′ tail, and RNase P
cleaves tRNA precursors.
Question 9: A photosynthetic alga is exposed to intermittent light–dark cycles. The graph of O₂
evolution shows repeated peaks that gradually decline in amplitude. Which process is most
likely limiting?
A. Cyclic photophosphorylation only
B. Calvin cycle turnover
pg. 2
