USABO OPEN EXAM NEWEST (2026/2027)
UPDATED VERSION ACTUAL EXAM 150
ACCURATE QUESTIONS & ANSWERS
(VERIFIED ANSWERS) AND DEEP EXPERT
RATIONALES | 100% GUARANTEED PASS
{JUST RELEASED}
Question 1
Both amino acids and nucleotides form complex chains. Select all the answers below that are
accurate.
• (A) The chains formed by both have polar phosphate groups.
• (B) Both contain peptide bonds.
• (C) Both contain nitrogen.
• (D) The chains formed by both can be lysed by trypsin.
• (E) The sequence of chains both form from preexisting templates.
Answer: C and E
,Expert Rationale: Amino acids contain an amino group ($-NH_2$) and nucleotides contain
nitrogenous bases (purines/pyrimidines), making nitrogen a fundamental element in both
proteins and nucleic acids (Option C). Additionally, during replication/transcription,
nucleotide chains are synthesized directly from a DNA template; during translation,
polypeptide (amino acid) chains are synthesized based on an mRNA template (Option E).
Option A is incorrect because only nucleic acids possess a sugar-phosphate backbone;
proteins do not have phosphate groups in their standard backbones. Option B is incorrect
because peptide bonds are exclusive to proteins, while nucleic acids utilize phosphodiester
bonds. Option D is incorrect because trypsin is a pancreatic protease that specifically
cleaves peptide bonds at the carboxyl side of lysine or arginine; it has no catalytic activity
on nucleic acid phosphodiester bonds.
Question 2
When a population is in Hardy-Weinberg equilibrium, which of the following is true?
• (A) All of its recessive lethal genes are wiped out, leaving only healthy dominant genes.
• (B) The population growth rate $\frac{dN}{dt} = 0$, where $N$ represents population
size.
• (C) Mutations occur at five times the normal rate to balance the loss of genes by natural
selection.
• (D) The frequencies of the alleles in the population change over time.
• (E) The frequencies of genotypes in the population are stable over time.
Answer: E
,Expert Rationale: Hardy-Weinberg equilibrium describes a hypothetical, non-evolving
population where both allele frequencies ($p, q$) and genotype frequencies ($p^2, 2pq,
q^2$) remain constant and stable from generation to generation (Option E). Option A is
incorrect because recessive lethal alleles are masked in heterozygotes ($2pq$) and are
protected from negative natural selection, meaning they are not wiped out. Option B
describes zero population growth (ecological carrying capacity), which is independent of
genetic equilibrium. Option C is incorrect because Hardy-Weinberg assumes zero mutation
and zero natural selection. Option D directly contradicts the definition of genetic
equilibrium.
Question 3
Which of the following is NOT an accurate description of protein synthesis and folding?
• (A) All protein synthesis is initiated in the cytoplasm.
• (B) Many copies of a protein can be synthesized simultaneously from the same mRNA.
• (C) The 3D shape of a protein is determined largely by its amino acid sequence.
• (D) The entire chain of amino acids exits the ribosome before it begins folding into its 3D
shape.
• (E) The first encoded amino acid in every protein is methionine.
Answer: D
Expert Rationale: Protein folding is co-translational, meaning that the nascent polypeptide
chain begins to adopt secondary and tertiary structures dynamically as it exits the
ribosome tunnel, well before translation is complete. Therefore, waiting for the entire chain
to exit before folding starts is biologically inaccurate (Option D). Option A is correct; all
translation begins on free ribosomes in the cytosol before some are targeted to the rough
endoplasmic reticulum via a signal recognition particle (SRP). Option B is correct; multiple
ribosomes bind a single mRNA simultaneously to form a polyribosome (polysome). Option
C represents Anfinsen's dogma, where primary sequence dictates tertiary structure.
Option E is correct; the start codon (AUG) universally codes for methionine (or formyl-
methionine in prokaryotes).
, Question 4
Neurotransmitter released at a chemical synapse binds to classes of receptors. Select the correct
functional pairing.
• (A) Ionotropic receptors form an ion pore and open within milliseconds in response to a
ligand.
• (B) Metabotropic receptors form an ion pore that is gated by a secondary messenger
cAMP cascade.
• (C) Receptor tyrosine kinases act as ion channels that alter neuron voltage within
milliseconds.
• (D) Ionotropic receptors alter neuron function solely by changing gene transcription.
• (E) Steroid hormone receptors alter the membrane potential within milliseconds of
binding at the synaptic cleft.
Answer: A
Expert Rationale: Ionotropic receptors are ligand-gated ion channels. When a
neurotransmitter binds to them, they undergo an immediate conformational change to
open an intrinsic pore, allowing specific ions to flow down their electrochemical gradients
within milliseconds (Option A). Option B is incorrect because metabotropic receptors are
G-protein coupled receptors (GPCRs) that do not contain an intrinsic ion pore; instead,
they trigger downstream intracellular cascades. Option C is incorrect because receptor
tyrosine kinases drive enzyme-linked intracellular pathways, not rapid voltage changes.
Option D is incorrect because ionotropic receptors primarily drive rapid postsynaptic
potentials ($EPSPs$ or $IPSPs$). Option E is incorrect because steroid receptors are
primarily intracellular transcription factors operating on slower timescales (hours to days).
Question 5
Many proteins are transported around cells in vesicles. Which of the following molecules
dictates the specific matching and fusion of a transport vesicle with its intended target
membrane?
UPDATED VERSION ACTUAL EXAM 150
ACCURATE QUESTIONS & ANSWERS
(VERIFIED ANSWERS) AND DEEP EXPERT
RATIONALES | 100% GUARANTEED PASS
{JUST RELEASED}
Question 1
Both amino acids and nucleotides form complex chains. Select all the answers below that are
accurate.
• (A) The chains formed by both have polar phosphate groups.
• (B) Both contain peptide bonds.
• (C) Both contain nitrogen.
• (D) The chains formed by both can be lysed by trypsin.
• (E) The sequence of chains both form from preexisting templates.
Answer: C and E
,Expert Rationale: Amino acids contain an amino group ($-NH_2$) and nucleotides contain
nitrogenous bases (purines/pyrimidines), making nitrogen a fundamental element in both
proteins and nucleic acids (Option C). Additionally, during replication/transcription,
nucleotide chains are synthesized directly from a DNA template; during translation,
polypeptide (amino acid) chains are synthesized based on an mRNA template (Option E).
Option A is incorrect because only nucleic acids possess a sugar-phosphate backbone;
proteins do not have phosphate groups in their standard backbones. Option B is incorrect
because peptide bonds are exclusive to proteins, while nucleic acids utilize phosphodiester
bonds. Option D is incorrect because trypsin is a pancreatic protease that specifically
cleaves peptide bonds at the carboxyl side of lysine or arginine; it has no catalytic activity
on nucleic acid phosphodiester bonds.
Question 2
When a population is in Hardy-Weinberg equilibrium, which of the following is true?
• (A) All of its recessive lethal genes are wiped out, leaving only healthy dominant genes.
• (B) The population growth rate $\frac{dN}{dt} = 0$, where $N$ represents population
size.
• (C) Mutations occur at five times the normal rate to balance the loss of genes by natural
selection.
• (D) The frequencies of the alleles in the population change over time.
• (E) The frequencies of genotypes in the population are stable over time.
Answer: E
,Expert Rationale: Hardy-Weinberg equilibrium describes a hypothetical, non-evolving
population where both allele frequencies ($p, q$) and genotype frequencies ($p^2, 2pq,
q^2$) remain constant and stable from generation to generation (Option E). Option A is
incorrect because recessive lethal alleles are masked in heterozygotes ($2pq$) and are
protected from negative natural selection, meaning they are not wiped out. Option B
describes zero population growth (ecological carrying capacity), which is independent of
genetic equilibrium. Option C is incorrect because Hardy-Weinberg assumes zero mutation
and zero natural selection. Option D directly contradicts the definition of genetic
equilibrium.
Question 3
Which of the following is NOT an accurate description of protein synthesis and folding?
• (A) All protein synthesis is initiated in the cytoplasm.
• (B) Many copies of a protein can be synthesized simultaneously from the same mRNA.
• (C) The 3D shape of a protein is determined largely by its amino acid sequence.
• (D) The entire chain of amino acids exits the ribosome before it begins folding into its 3D
shape.
• (E) The first encoded amino acid in every protein is methionine.
Answer: D
Expert Rationale: Protein folding is co-translational, meaning that the nascent polypeptide
chain begins to adopt secondary and tertiary structures dynamically as it exits the
ribosome tunnel, well before translation is complete. Therefore, waiting for the entire chain
to exit before folding starts is biologically inaccurate (Option D). Option A is correct; all
translation begins on free ribosomes in the cytosol before some are targeted to the rough
endoplasmic reticulum via a signal recognition particle (SRP). Option B is correct; multiple
ribosomes bind a single mRNA simultaneously to form a polyribosome (polysome). Option
C represents Anfinsen's dogma, where primary sequence dictates tertiary structure.
Option E is correct; the start codon (AUG) universally codes for methionine (or formyl-
methionine in prokaryotes).
, Question 4
Neurotransmitter released at a chemical synapse binds to classes of receptors. Select the correct
functional pairing.
• (A) Ionotropic receptors form an ion pore and open within milliseconds in response to a
ligand.
• (B) Metabotropic receptors form an ion pore that is gated by a secondary messenger
cAMP cascade.
• (C) Receptor tyrosine kinases act as ion channels that alter neuron voltage within
milliseconds.
• (D) Ionotropic receptors alter neuron function solely by changing gene transcription.
• (E) Steroid hormone receptors alter the membrane potential within milliseconds of
binding at the synaptic cleft.
Answer: A
Expert Rationale: Ionotropic receptors are ligand-gated ion channels. When a
neurotransmitter binds to them, they undergo an immediate conformational change to
open an intrinsic pore, allowing specific ions to flow down their electrochemical gradients
within milliseconds (Option A). Option B is incorrect because metabotropic receptors are
G-protein coupled receptors (GPCRs) that do not contain an intrinsic ion pore; instead,
they trigger downstream intracellular cascades. Option C is incorrect because receptor
tyrosine kinases drive enzyme-linked intracellular pathways, not rapid voltage changes.
Option D is incorrect because ionotropic receptors primarily drive rapid postsynaptic
potentials ($EPSPs$ or $IPSPs$). Option E is incorrect because steroid receptors are
primarily intracellular transcription factors operating on slower timescales (hours to days).
Question 5
Many proteins are transported around cells in vesicles. Which of the following molecules
dictates the specific matching and fusion of a transport vesicle with its intended target
membrane?
