1|P a g e
USABO OPEN COMPREHENSIVE ACTUAL EXAM
PREP 2026 ALL QUESTIONS AND CORRECT
DETAILED ANSWERS WITH RATIONALES ALREADY
A GRADED WITH EXPERT FEEDBACK |NEW AND
REVISED
SECTION 1: CELL BIOLOGY & BIOCHEMISTRY
Question 1
Which of the following best describes a nucleosome?
A. A region in bacterial cells where DNA is located
B. DNA wrapped around a core of eight histone proteins
C. A structure found only in prokaryotic cells
D. A complex of RNA and proteins involved in splicing
Rationale: A nucleosome is the fundamental repeating unit of
chromatin, consisting of approximately 146 base pairs of DNA
wrapped around a core of eight histone proteins (an octamer). This
structure is found in eukaryotic cells and is essential for DNA
packaging. Bacterial DNA is not organized into nucleosomes.
Question 2
The endomembrane system includes all of the following EXCEPT:
A. Endoplasmic reticulum
B. Golgi apparatus
C. Lysosomes
D. Mitochondria
,2|P a g e
Rationale: The endomembrane system includes the nuclear envelope,
endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and the
plasma membrane. Mitochondria are not part of the endomembrane
system; they are double-membrane organelles with their own DNA
and are thought to have originated from endosymbiotic bacteria.
Question 3
Which organelle is responsible for the synthesis of lipids and
detoxification of drugs?
A. Rough endoplasmic reticulum
B. Smooth endoplasmic reticulum
C. Golgi apparatus
D. Peroxisomes
Rationale: The smooth endoplasmic reticulum (SER) is involved in
lipid synthesis, carbohydrate metabolism, and detoxification of drugs
and poisons. The rough endoplasmic reticulum (RER) is involved in
protein synthesis and has ribosomes attached. The Golgi apparatus
modifies and packages proteins, and peroxisomes are involved in fatty
acid oxidation and hydrogen peroxide metabolism.
Question 4
The fluid mosaic model describes the structure of which cellular
component?
A. The nuclear envelope
B. The plasma membrane
C. The cell wall
D. The cytoskeleton
Rationale: The fluid mosaic model describes the plasma membrane as
a lipid bilayer with proteins embedded within it. The membrane is fluid
because lipids and proteins can move laterally within the bilayer. This
model was proposed by Singer and Nicolson in 1972 and remains the
accepted model for membrane structure.
,3|P a g e
Question 5
Which of the following is a characteristic of active transport?
A. Movement of molecules down their concentration gradient
B. Movement of molecules against their concentration gradient with
energy expenditure
C. Movement of molecules through ion channels
D. Movement of water across a semipermeable membrane
Rationale: Active transport requires energy (typically ATP) to move
molecules against their concentration gradient. This is in contrast to
passive transport, which moves molecules down their concentration
gradient without energy expenditure. Ion channels facilitate passive
transport, and osmosis is the passive movement of water.
Question 6
The sodium-potassium pump (Na⁺/K⁺-ATPase) transports:
A. Sodium into the cell and potassium out of the cell
B. Sodium out of the cell and potassium into the cell
C. Sodium and potassium into the cell
D. Sodium and potassium out of the cell
Rationale: The sodium-potassium pump uses ATP to transport three
sodium ions out of the cell and two potassium ions into the cell. This
maintains the concentration gradients necessary for the resting
membrane potential and action potentials. The pump is essential for
nerve impulse transmission and other cellular functions.
Question 7
Which of the following is a difference between prokaryotic and
eukaryotic cells?
A. Prokaryotic cells have a nucleus; eukaryotic cells do not
B. Prokaryotic cells lack membrane-bound organelles; eukaryotic
cells have membrane-bound organelles
, 4|P a g e
C. Prokaryotic cells are larger than eukaryotic cells
D. Prokaryotic cells have mitochondria; eukaryotic cells do not
Rationale: Prokaryotic cells lack membrane-bound organelles,
including a nucleus, mitochondria, and endoplasmic reticulum. Their
DNA is located in a nucleoid region. Eukaryotic cells have a true
nucleus and membrane-bound organelles. Prokaryotic cells are
generally smaller than eukaryotic cells.
Question 8
What is the primary function of the Golgi apparatus?
A. Protein synthesis
B. Modification, sorting, and packaging of proteins
C. Cellular respiration
D. Lipid synthesis
Rationale: The Golgi apparatus modifies, sorts, and packages proteins
and lipids for secretion or delivery to other organelles. Proteins are
received from the rough endoplasmic reticulum, modified (e.g., by
glycosylation), and then packaged into vesicles for transport. Protein
synthesis occurs at ribosomes, cellular respiration in mitochondria,
and lipid synthesis in the smooth ER.
Question 9
Which of the following is an example of a post-translational
modification?
A. Transcription
B. Translation
C. Phosphorylation
D. Replication
Rationale: Post-translational modifications are chemical
modifications that occur to proteins after translation. Examples
include phosphorylation, glycosylation, ubiquitination, and
USABO OPEN COMPREHENSIVE ACTUAL EXAM
PREP 2026 ALL QUESTIONS AND CORRECT
DETAILED ANSWERS WITH RATIONALES ALREADY
A GRADED WITH EXPERT FEEDBACK |NEW AND
REVISED
SECTION 1: CELL BIOLOGY & BIOCHEMISTRY
Question 1
Which of the following best describes a nucleosome?
A. A region in bacterial cells where DNA is located
B. DNA wrapped around a core of eight histone proteins
C. A structure found only in prokaryotic cells
D. A complex of RNA and proteins involved in splicing
Rationale: A nucleosome is the fundamental repeating unit of
chromatin, consisting of approximately 146 base pairs of DNA
wrapped around a core of eight histone proteins (an octamer). This
structure is found in eukaryotic cells and is essential for DNA
packaging. Bacterial DNA is not organized into nucleosomes.
Question 2
The endomembrane system includes all of the following EXCEPT:
A. Endoplasmic reticulum
B. Golgi apparatus
C. Lysosomes
D. Mitochondria
,2|P a g e
Rationale: The endomembrane system includes the nuclear envelope,
endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and the
plasma membrane. Mitochondria are not part of the endomembrane
system; they are double-membrane organelles with their own DNA
and are thought to have originated from endosymbiotic bacteria.
Question 3
Which organelle is responsible for the synthesis of lipids and
detoxification of drugs?
A. Rough endoplasmic reticulum
B. Smooth endoplasmic reticulum
C. Golgi apparatus
D. Peroxisomes
Rationale: The smooth endoplasmic reticulum (SER) is involved in
lipid synthesis, carbohydrate metabolism, and detoxification of drugs
and poisons. The rough endoplasmic reticulum (RER) is involved in
protein synthesis and has ribosomes attached. The Golgi apparatus
modifies and packages proteins, and peroxisomes are involved in fatty
acid oxidation and hydrogen peroxide metabolism.
Question 4
The fluid mosaic model describes the structure of which cellular
component?
A. The nuclear envelope
B. The plasma membrane
C. The cell wall
D. The cytoskeleton
Rationale: The fluid mosaic model describes the plasma membrane as
a lipid bilayer with proteins embedded within it. The membrane is fluid
because lipids and proteins can move laterally within the bilayer. This
model was proposed by Singer and Nicolson in 1972 and remains the
accepted model for membrane structure.
,3|P a g e
Question 5
Which of the following is a characteristic of active transport?
A. Movement of molecules down their concentration gradient
B. Movement of molecules against their concentration gradient with
energy expenditure
C. Movement of molecules through ion channels
D. Movement of water across a semipermeable membrane
Rationale: Active transport requires energy (typically ATP) to move
molecules against their concentration gradient. This is in contrast to
passive transport, which moves molecules down their concentration
gradient without energy expenditure. Ion channels facilitate passive
transport, and osmosis is the passive movement of water.
Question 6
The sodium-potassium pump (Na⁺/K⁺-ATPase) transports:
A. Sodium into the cell and potassium out of the cell
B. Sodium out of the cell and potassium into the cell
C. Sodium and potassium into the cell
D. Sodium and potassium out of the cell
Rationale: The sodium-potassium pump uses ATP to transport three
sodium ions out of the cell and two potassium ions into the cell. This
maintains the concentration gradients necessary for the resting
membrane potential and action potentials. The pump is essential for
nerve impulse transmission and other cellular functions.
Question 7
Which of the following is a difference between prokaryotic and
eukaryotic cells?
A. Prokaryotic cells have a nucleus; eukaryotic cells do not
B. Prokaryotic cells lack membrane-bound organelles; eukaryotic
cells have membrane-bound organelles
, 4|P a g e
C. Prokaryotic cells are larger than eukaryotic cells
D. Prokaryotic cells have mitochondria; eukaryotic cells do not
Rationale: Prokaryotic cells lack membrane-bound organelles,
including a nucleus, mitochondria, and endoplasmic reticulum. Their
DNA is located in a nucleoid region. Eukaryotic cells have a true
nucleus and membrane-bound organelles. Prokaryotic cells are
generally smaller than eukaryotic cells.
Question 8
What is the primary function of the Golgi apparatus?
A. Protein synthesis
B. Modification, sorting, and packaging of proteins
C. Cellular respiration
D. Lipid synthesis
Rationale: The Golgi apparatus modifies, sorts, and packages proteins
and lipids for secretion or delivery to other organelles. Proteins are
received from the rough endoplasmic reticulum, modified (e.g., by
glycosylation), and then packaged into vesicles for transport. Protein
synthesis occurs at ribosomes, cellular respiration in mitochondria,
and lipid synthesis in the smooth ER.
Question 9
Which of the following is an example of a post-translational
modification?
A. Transcription
B. Translation
C. Phosphorylation
D. Replication
Rationale: Post-translational modifications are chemical
modifications that occur to proteins after translation. Examples
include phosphorylation, glycosylation, ubiquitination, and
