GRE Biochemistry, Cell and Molecular Biology Subject Test — Questions and Answers

Question 1: Which fat-soluble vitamin acts as the most important lipid-soluble antioxidant in membranes?

Vitamin A (retinol)
Vitamin E (alpha-tocopherol) (Correct answer)
Vitamin K (phylloquinone)
Vitamin D (calcitriol)

Correct answer: Vitamin E (alpha-tocopherol)

Vitamin E (alpha-tocopherol) is the primary chain-breaking antioxidant in lipid membranes, donating a hydrogen atom to lipid peroxyl radicals to terminate lipid peroxidation chain reactions.

Question 2: CRISPR-Cas9 gene editing creates targeted double-strand DNA breaks using which component to guide it to the correct genomic location?

A DNA-binding zinc finger protein
A TAL effector nuclease (TALEN)
A single guide RNA (sgRNA) complementary to the target sequence (Correct answer)
A restriction endonuclease with sequence specificity

Correct answer: A single guide RNA (sgRNA) complementary to the target sequence

The single guide RNA (sgRNA) base-pairs with the complementary target DNA sequence, directing the Cas9 nuclease to cut precisely at that location in the genome.

Question 3: Which monosaccharide is the primary fuel for the brain under normal conditions?

Galactose
Fructose
Glucose (Correct answer)
Ribose

Correct answer: Glucose

Glucose is the brain's primary energy substrate under normal fed conditions, as the blood-brain barrier limits the transport of most other fuels.

Question 4: The fluid mosaic model of the cell membrane was proposed by:

Lipmann and Krebs
Watson and Crick
Singer and Nicolson (Correct answer)
Mitchell and Moyle

Correct answer: Singer and Nicolson

Singer and Nicolson (1972) proposed the fluid mosaic model, describing the membrane as a dynamic lipid bilayer with embedded proteins that can move laterally.

Question 5: The enzyme lactase cleaves lactose into which two monosaccharides?

Galactose + fructose
Glucose + fructose
Glucose + galactose (Correct answer)
Glucose + glucose

Correct answer: Glucose + galactose

Lactase (beta-galactosidase) hydrolyzes the beta(1→4) glycosidic bond of lactose to release glucose and galactose, which are absorbed in the small intestine.

Question 6: Methotrexate inhibits nucleotide synthesis by targeting which enzyme?

PRPP synthetase
Ribonucleotide reductase
Thymidylate synthase
Dihydrofolate reductase (DHFR) (Correct answer)

Correct answer: Dihydrofolate reductase (DHFR)

Methotrexate competitively inhibits DHFR, preventing reduction of dihydrofolate to tetrahydrofolate; without THF cofactors, one-carbon transfer reactions required for both purine ring synthesis and dTMP formation are blocked.

Question 7: Eicosanoids (prostaglandins, thromboxanes, leukotrienes) are derived from which fatty acid precursor?

Palmitic acid (16:0)
Linoleic acid (18:2)
Stearic acid (18:0)
Arachidonic acid (20:4) (Correct answer)

Correct answer: Arachidonic acid (20:4)

Arachidonic acid (20:4, ω-6) is released from membrane phospholipids by phospholipase A2 and serves as the substrate for cyclooxygenases and lipoxygenases to produce eicosanoids.

Question 8: Which of the following nitrogenous bases is classified as a purine?

Adenine (Correct answer)
Cytosine
Thymine
Uracil

Correct answer: Adenine

Adenine and guanine are purines, characterized by a bicyclic structure of a pyrimidine ring fused to an imidazole ring, while cytosine, uracil, and thymine are monocyclic pyrimidines.

Question 9: The saponification number (saponification value) of a fat is a measure of:

The number of free hydroxyl groups
The degree of unsaturation
The amount of glycerol present
The average molecular weight of fatty acids (Correct answer)

Correct answer: The average molecular weight of fatty acids

The saponification value reflects the amount of alkali required to saponify a fat; higher values indicate shorter-chain fatty acids, since more moles (and thus more alkali) are needed per gram.

Question 10: Sphingolipids differ from glycerophospholipids in that their backbone is:

Serine
Sphingosine (Correct answer)
Cholesterol
Glycerol

Correct answer: Sphingosine

Sphingolipids are built on a sphingosine backbone (an amino alcohol) rather than glycerol, with a fatty acid attached via an amide bond to form ceramide.

Question 11: The cooperative binding of oxygen to hemoglobin is best described by which model?

Lock-and-key model
Induced fit model
Concerted (MWC) model (Correct answer)
Michaelis-Menten model

Correct answer: Concerted (MWC) model

The concerted (Monod-Wyman-Changeux) model proposes that all subunits switch simultaneously between T (tense, low affinity) and R (relaxed, high affinity) states.

Question 12: A signal peptide on a newly synthesized protein directs it to which cellular location for processing?

Cytosol
Nucleus
Rough endoplasmic reticulum (Correct answer)
Mitochondria

Correct answer: Rough endoplasmic reticulum

Signal peptides are N-terminal hydrophobic sequences recognized by signal recognition particle (SRP), which directs the ribosome to the rough ER for co-translational insertion.

Question 13: Which chromatographic technique separates molecules based on their specific binding interaction with an immobilized ligand?

Affinity chromatography (Correct answer)
Size-exclusion chromatography
Reverse-phase HPLC
Ion-exchange chromatography

Correct answer: Affinity chromatography

Affinity chromatography exploits specific, reversible binding between a target molecule and an immobilized ligand (e.g., His-tag/Ni-NTA, antibody/antigen) to achieve high-purity protein isolation.

Question 14: The Bohr effect refers to which property of hemoglobin?

Decreased O₂ affinity at lower pH and higher CO₂ (Correct answer)
Increased O₂ affinity when 2,3-BPG binds
Increased O₂ affinity at low temperature
Decreased O₂ affinity at high altitude

Correct answer: Decreased O₂ affinity at lower pH and higher CO₂

The Bohr effect describes hemoglobin's decreased oxygen affinity in acidic, high-CO₂ environments (active tissues), facilitating O₂ release where it is needed most.

Question 15: Scurvy results from Vitamin C deficiency primarily because without ascorbate:

The immune system cannot produce antibodies
Iron cannot be absorbed from the diet
Coagulation factors cannot be gamma-carboxylated
Prolyl and lysyl hydroxylases cannot function, impairing collagen synthesis (Correct answer)

Correct answer: Prolyl and lysyl hydroxylases cannot function, impairing collagen synthesis

Ascorbate is required to maintain the Fe²⁺ state of prolyl and lysyl hydroxylases; without it, hydroxylation of proline and lysine in collagen is deficient, preventing proper triple helix formation.

Question 16: Ribonucleotide reductase reduces ribonucleotides to deoxyribonucleotides using which catalytic mechanism?

Pyridoxal phosphate-mediated transamination
FAD-mediated hydride transfer
A stable tyrosyl radical stabilized by a dinuclear iron center (Correct answer)
NAD+-dependent oxidation

Correct answer: A stable tyrosyl radical stabilized by a dinuclear iron center

Ribonucleotide reductase class I uses a stable tyrosyl radical (generated by a diferric iron center in the R2 subunit) to initiate the radical-based mechanism that removes the 2'-OH of ribonucleotides.

Question 17: Collagen's unique triple-helix structure is stabilized primarily by which amino acid's repeated occurrence?

Glycine
Both B and C (Correct answer)
Proline and hydroxyproline
Alanine

Correct answer: Both B and C

Collagen's Gly-X-Y repeat sequence requires glycine (smallest residue) at every third position, and proline/hydroxyproline at X and Y positions to stabilize the helix with hydrogen bonds.

Question 18: Pyruvate dehydrogenase complex (PDC) converts pyruvate to acetyl-CoA with release of:

CO₂ and NADH (Correct answer)
H₂O and NADPH
CO₂ and FADH₂
CO₂ and ATP

Correct answer: CO₂ and NADH

PDC catalyzes the oxidative decarboxylation of pyruvate: pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH, using five cofactors including thiamine pyrophosphate.

Question 19: The net ATP yield from complete oxidation of one molecule of glucose through glycolysis and oxidative phosphorylation is approximately:

2 ATP
8 ATP
36-38 ATP
30-32 ATP (Correct answer)

Correct answer: 30-32 ATP

Using current estimates of the P/O ratio, complete oxidation of glucose yields approximately 30–32 ATP: 2 from glycolysis, 2 from the TCA cycle substrate-level phosphorylation, and ~26–28 from oxidative phosphorylation.

Question 20: Vitamin C (ascorbic acid) acts as an antioxidant primarily by:

Inhibiting lipid peroxidation by binding membranes
Activating superoxide dismutase
Chelating iron to prevent Fenton reactions
Donating electrons to neutralize free radicals (Correct answer)

Correct answer: Donating electrons to neutralize free radicals

Ascorbic acid is a potent reducing agent that donates electrons to neutralize reactive oxygen species and can regenerate other antioxidants such as vitamin E.

Question 21: Protein denaturation by heat or chemicals disrupts all levels of structure EXCEPT which?

Tertiary
Quaternary
Primary (Correct answer)
Secondary

Correct answer: Primary

Denaturation disrupts non-covalent interactions and disulfide bonds but does not break the covalent peptide bonds that define primary structure.

Question 22: During the signal transduction process, what role does a second messenger, such as cAMP, play?

Degrades the signaling molecule
Directly activates the receptor on the cell surface
Amplifies and transmits the signal inside the cell (Correct answer)
Acts as the primary messenger outside the cell

Correct answer: Amplifies and transmits the signal inside the cell

In signal transduction, a second messenger like cAMP amplifies and transmits the signal received by a cell surface receptor to various target molecules inside the cell. It acts as an intracellular relay, propagating the initial signal throughout the cytoplasm and initiating a cascade of biochemical events. This amplification ensures a robust and coordinated cellular response to external stimuli.

Question 23: Carnitine plays an essential role in fatty acid oxidation by:

Activating fatty acids to acyl-CoA in the cytosol
Regenerating CoA from acetyl-CoA
Transporting long-chain fatty acyl groups across the inner mitochondrial membrane (Correct answer)
Serving as a coenzyme for beta-oxidation enzymes

Correct answer: Transporting long-chain fatty acyl groups across the inner mitochondrial membrane

Carnitine shuttles long-chain fatty acyl groups across the inner mitochondrial membrane as acylcarnitine, which is essential because the inner membrane is impermeable to acyl-CoA.

Question 24: In the integration of metabolism, which pathway is the primary source of ATP during intense physical activity?

Pentose phosphate pathway
Fatty acid oxidation
Glycolysis (Correct answer)
Oxidative phosphorylation

Correct answer: Glycolysis

During intense physical activity, oxygen supply to muscles can become limited, leading to anaerobic conditions. In this scenario, glycolysis becomes the primary source of ATP, rapidly producing a small amount of energy without requiring oxygen. While less efficient than oxidative phosphorylation, it provides the quick bursts of energy necessary for high-intensity exercise.

Question 25: Which type of fatty acid contains one double bond?

Trans fatty acid
Polyunsaturated fatty acid
Monounsaturated fatty acid (Correct answer)
Saturated fatty acid

Correct answer: Monounsaturated fatty acid

Monounsaturated fatty acids contain exactly one carbon-carbon double bond, such as oleic acid (18:1, Δ9), and are liquid at room temperature.

Question 26: 2,3-Bisphosphoglycerate (2,3-BPG) affects hemoglobin oxygen binding by:

Increasing oxygen affinity by stabilizing the R state
Blocking the heme iron from binding oxygen
Promoting dissociation of hemoglobin subunits
Decreasing oxygen affinity by stabilizing the T state (Correct answer)

Correct answer: Decreasing oxygen affinity by stabilizing the T state

2,3-BPG binds in the central cavity of deoxyhemoglobin and stabilizes the low-affinity T state, promoting oxygen release to tissues.

Question 27: Reverse transcriptase PCR (RT-PCR) begins with which step?

Conversion of mRNA to complementary DNA (cDNA) (Correct answer)
Denaturation of double-stranded DNA
Ligation of DNA fragments
Amplification of genomic DNA

Correct answer: Conversion of mRNA to complementary DNA (cDNA)

RT-PCR starts by using reverse transcriptase to synthesize a complementary DNA (cDNA) copy of the mRNA template, which can then be amplified by conventional PCR.

Question 28: An ELISA (enzyme-linked immunosorbent assay) detects a target molecule using:

Fluorescent dyes that stain all proteins nonspecifically
Enzyme-conjugated antibodies producing a colorimetric signal (Correct answer)
Electric current to detect protein migration
Radioactively labeled antibodies (RIA method)

Correct answer: Enzyme-conjugated antibodies producing a colorimetric signal

ELISA uses antibodies conjugated to enzymes (e.g., HRP or alkaline phosphatase) that convert a substrate into a colored product, allowing quantitative detection of antigens.

Question 29: Ubiquitin-mediated proteasomal degradation requires that ubiquitin chains be attached to which amino acid residue of the target protein?

Serine
Cysteine
Lysine (Correct answer)
Aspartate

Correct answer: Lysine

Ubiquitin is attached via an isopeptide bond to lysine residues on target proteins; a chain of at least four ubiquitin molecules targets the protein to the 26S proteasome.

Question 30: The Cori cycle involves the transfer of metabolites between muscle and liver. Muscle exports lactate; the liver converts it back to:

Glucose via gluconeogenesis, which is returned to muscle (Correct answer)
Glycogen stored permanently in the liver
Pyruvate only, which is excreted
Acetyl-CoA for ketogenesis

Correct answer: Glucose via gluconeogenesis, which is returned to muscle

In the Cori cycle, lactate from anaerobic muscle glycolysis travels to the liver, where gluconeogenesis converts it back to glucose, which is released into the blood for muscle use.

Question 31: Competitive inhibition of an enzyme:

Increases both Km and Vmax
Decreases both Km and Vmax
Decreases Vmax without changing Km
Increases apparent Km without changing Vmax (Correct answer)

Correct answer: Increases apparent Km without changing Vmax

A competitive inhibitor competes with substrate for the active site, increasing apparent Km (reduced affinity) but leaving Vmax unchanged because excess substrate can overcome inhibition.

Question 32: Which of the following statements about enzymes is TRUE?

All enzymes require cofactors to function.
Enzymes lower the activation energy of a reaction. (Correct answer)
Enzyme activity is independent of temperature and pH.
Enzymes are consumed during the reaction.

Correct answer: Enzymes lower the activation energy of a reaction.

Enzymes are biological catalysts that accelerate biochemical reactions by lowering their activation energy. They achieve this by binding to specific substrates at their active site, stabilizing the transition state, and facilitating product formation. Enzymes are not consumed during the reaction and can be reused multiple times.

Question 33: Mass spectrometry in proteomics identifies proteins by measuring:

Their UV absorbance spectra
Their fluorescence emission wavelength
The mass-to-charge ratio (m/z) of peptide fragments (Correct answer)
Their migration speed in an electric field without gel

Correct answer: The mass-to-charge ratio (m/z) of peptide fragments

Mass spectrometry measures the m/z ratio of ionized peptide fragments with high precision; the resulting mass spectra are matched against protein databases for identification.

Question 34: In enzyme kinetics, the Michaelis constant (Km) is defined as:

The rate constant for product release
The equilibrium constant for enzyme-substrate binding
The maximum reaction velocity
The substrate concentration at half-maximal velocity (Correct answer)

Correct answer: The substrate concentration at half-maximal velocity

Km is the substrate concentration at which reaction velocity equals Vmax/2; it is a measure of substrate affinity (lower Km = higher affinity) but is not a true equilibrium dissociation constant.

Question 35: Fatty acid synthesis occurs in which cellular compartment?

Smooth ER
Mitochondrial matrix
Cytosol (Correct answer)
Peroxisome

Correct answer: Cytosol

Fatty acid synthesis by fatty acid synthase (FAS) occurs in the cytosol, using NADPH as the reducing agent and malonyl-CoA as the two-carbon donor.

Question 36: Which technique separates proteins based on their isoelectric points?

Isoelectric focusing (Correct answer)
SDS-PAGE
Size-exclusion chromatography
Affinity chromatography

Correct answer: Isoelectric focusing

Isoelectric focusing separates proteins in a pH gradient; each protein migrates until it reaches the pH equal to its pI, where it has no net charge and stops.

Question 37: What is the final end product of purine catabolism in humans?

Allantoin
Uric acid (Correct answer)
Xanthine
Hypoxanthine

Correct answer: Uric acid

Humans lack uricase and therefore cannot convert uric acid to allantoin as most other mammals do; uric acid is thus the terminal product of purine breakdown, and its accumulation causes gout.

Question 38: Which of the following best describes passive transport?

Movement of substances against their concentration gradient using energy
Endocytosis of large molecules into the cell
Use of ATP to pump ions across the membrane
Diffusion of molecules down their concentration gradient without energy input (Correct answer)

Correct answer: Diffusion of molecules down their concentration gradient without energy input

Passive transport describes the movement of substances across a cell membrane down their concentration gradient, meaning from an area of higher concentration to an area of lower concentration. This process does not require the cell to expend metabolic energy (ATP) because it is driven by the natural tendency of molecules to spread out. Examples include simple diffusion and facilitated diffusion.

Question 39: Which prosthetic group is found in myoglobin and hemoglobin, enabling oxygen binding?

Pyridoxal phosphate (PLP)
Flavin adenine dinucleotide (FAD)
Heme group (Correct answer)
Biotin

Correct answer: Heme group

The heme group contains an iron atom (Fe²⁺) coordinated within a porphyrin ring, which reversibly binds molecular oxygen.

Question 40: Gel filtration (size-exclusion) chromatography separates proteins such that:

Proteins elute in order of hydrophobicity
Proteins elute in order of charge
Larger proteins elute first (Correct answer)
Smaller proteins elute first

Correct answer: Larger proteins elute first

Larger proteins cannot enter the pores of the gel matrix and pass through the column faster (elute first), while smaller proteins enter the pores and take longer to elute.

Question 41: Which level of protein structure refers to the sequence of amino acids in a polypeptide chain?

Tertiary structure
Secondary structure
Quaternary structure
Primary structure (Correct answer)

Correct answer: Primary structure

Primary structure is the linear sequence of amino acids linked by peptide bonds, which ultimately determines all higher levels of protein structure.

Question 42: Two-dimensional gel electrophoresis (2D-PAGE) separates proteins first by isoelectric point, then by:

Charge using ion-exchange
Net charge at pH 7
Molecular weight using SDS-PAGE (Correct answer)
Hydrophobicity using reverse-phase electrophoresis

Correct answer: Molecular weight using SDS-PAGE

2D-PAGE uses isoelectric focusing (IEF) in the first dimension to separate proteins by pI, then SDS-PAGE in the second dimension to separate by molecular weight, giving a two-dimensional protein map.

Question 43: In a non-competitive inhibition mechanism, the inhibitor:

Binds only to the enzyme-substrate complex
Competes with substrate for the active site
Binds an allosteric site on either free enzyme or ES complex, reducing Vmax without changing Km (Correct answer)
Covalently modifies the active site irreversibly

Correct answer: Binds an allosteric site on either free enzyme or ES complex, reducing Vmax without changing Km

A non-competitive inhibitor binds an allosteric site independent of substrate binding, reducing Vmax without affecting Km because substrate binding and inhibitor binding are independent events.

Question 44: The anomeric carbon of glucose is carbon number:

C-3
C-1 (Correct answer)
C-6
C-2

Correct answer: C-1

C-1 of glucose is the anomeric carbon, the former carbonyl carbon that becomes the new chiral center when the open-chain form cyclizes to form alpha or beta glucopyranose.

Question 45: Which enzyme is responsible for unwinding the DNA double helix replication?

Helicase (Correct answer)
Ligase
DNA polymerase
Topoisomerase

Correct answer: Helicase

Helicase is the enzyme responsible for unwinding the DNA double helix during replication. It breaks the hydrogen bonds between complementary base pairs, separating the two strands to create a replication fork. This unwinding is essential to expose the single-stranded DNA templates, allowing DNA polymerase to synthesize new DNA strands.

Question 46: During oxidative phosphorylation, the final electron acceptor in the electron transport chain is:

FAD
NAD⁺
Oxygen (O₂) (Correct answer)
Water (H₂O)

Correct answer: Oxygen (O₂)

During oxidative phosphorylation, the electron transport chain passes electrons through a series of protein complexes, ultimately delivering them to oxygen. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. This crucial step drives the entire electron transport process and enables ATP synthesis.

Question 47: Vitamin C (ascorbate) is required for collagen synthesis because it is needed for:

Hydroxylation of proline and lysine residues (Correct answer)
Secretion of procollagen from fibroblasts
Glycosylation of collagen fibers
Cross-linking of collagen fibrils

Correct answer: Hydroxylation of proline and lysine residues

Ascorbate is a cofactor for prolyl and lysyl hydroxylases, which add hydroxyl groups to proline and lysine residues essential for helix stability and cross-linking.

Question 48: Galactosemia results from accumulation of galactose-1-phosphate due to deficiency of which enzyme?

Phosphoglucomutase
Galactokinase
UDP-galactose 4-epimerase
Galactose-1-phosphate uridylyltransferase (GALT) (Correct answer)

Correct answer: Galactose-1-phosphate uridylyltransferase (GALT)

Classic galactosemia is caused by GALT deficiency, which prevents conversion of galactose-1-phosphate to UDP-galactose, leading to toxic accumulation of galactose-1-phosphate in the liver, brain, and kidneys.

Question 49: Which of the following best describes the function of tRNA in translation?

Catalyzes peptide bond formation between amino acids.
Transports mRNA from the nucleus to the cytoplasm.
Carries amino acids to the ribosome and matches them to the mRNA codon. (Correct answer)
Acts as the template for protein synthesis.

Correct answer: Carries amino acids to the ribosome and matches them to the mRNA codon.

Transfer RNA (tRNA) plays a crucial role in translation by acting as an adaptor molecule. Each tRNA carries a specific amino acid and has an anticodon that base-pairs with a complementary codon on the mRNA molecule. This ensures that the correct amino acid is delivered to the ribosome and incorporated into the growing polypeptide chain according to the mRNA sequence.

Question 50: Iron absorption from the diet is enhanced by Vitamin C because ascorbate:

Reduces Fe³⁺ to Fe²⁺, which is better absorbed by DMT1 (Correct answer)
Binds transferrin to increase iron transport
Forms a chelate with iron that prevents oxidation
Increases hepcidin levels to regulate iron stores

Correct answer: Reduces Fe³⁺ to Fe²⁺, which is better absorbed by DMT1

Ascorbate reduces dietary non-heme iron from the insoluble Fe³⁺ (ferric) form to the soluble Fe²⁺ (ferrous) form, which is the substrate for the divalent metal transporter DMT1 in intestinal cells.

Question 51: SDS-PAGE separates proteins primarily based on:

Molecular weight (size) (Correct answer)
Isoelectric point
Net charge at physiological pH
Hydrophobicity

Correct answer: Molecular weight (size)

SDS denatures proteins and coats them with uniform negative charge proportional to mass, so migration in the polyacrylamide gel reflects molecular weight only.

Question 52: In glycolysis, which enzyme catalyzes the conversion of glucose to glucose-6-phosphate?

Aldolase
Hexokinase (Correct answer)
Pyruvate kinase
Phosphofructokinase

Correct answer: Hexokinase

Hexokinase is the enzyme that catalyzes the first committed step of glycolysis, converting glucose into glucose-6-phosphate. This phosphorylation traps glucose inside the cell, preventing its diffusion out and initiating its breakdown for energy. This reaction is crucial for cellular glucose metabolism.

Question 53: Which technique measures real-time amplification of DNA during PCR by detecting fluorescent signal proportional to DNA amount?

Quantitative PCR (qPCR) (Correct answer)
Southern blot
DNA microarray
Gel electrophoresis

Correct answer: Quantitative PCR (qPCR)

Quantitative PCR (qPCR) monitors fluorescent signal (from SYBR Green or TaqMan probes) after each amplification cycle, allowing real-time quantification of starting template DNA or cDNA.

Question 54: Thiamine (Vitamin B1) deficiency causes which neurological and cardiovascular condition?

Scurvy
Rickets
Pellagra
Beriberi (Correct answer)

Correct answer: Beriberi

Thiamine deficiency causes beriberi, characterized by peripheral neuropathy (dry beriberi) or cardiovascular dysfunction (wet beriberi), due to impaired pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase.

Question 55: Hemoglobin is an example of a protein with which level of structural organization?

Tertiary
Primary
Quaternary (Correct answer)
Secondary

Correct answer: Quaternary

Quaternary structure describes the assembly of two or more polypeptide subunits, as seen in hemoglobin's four globin chains.

Question 56: What is the isoelectric point (pI) of a protein?

The pH at which the protein carries no net charge (Correct answer)
The pH at which the protein has maximum solubility
The pH at which the protein has maximum enzymatic activity
The pH at which the protein denatures

Correct answer: The pH at which the protein carries no net charge

The isoelectric point is the pH at which the net charge of the protein is zero, meaning positive and negative charges are balanced.

Question 57: Adenosine deaminase (ADA) deficiency causes which clinical condition, and why?

Severe combined immunodeficiency (SCID), due to toxic dATP accumulation in lymphocytes (Correct answer)
Hemolytic anemia, due to erythrocyte purine depletion
Orotic aciduria, due to blocked pyrimidine conversion
Gout, due to uric acid overproduction from adenosine catabolism

Correct answer: Severe combined immunodeficiency (SCID), due to toxic dATP accumulation in lymphocytes

ADA deficiency allows deoxyadenosine to accumulate and be phosphorylated to dATP, which is selectively toxic to lymphocytes (inhibits ribonucleotide reductase and induces apoptosis), resulting in SCID.

Question 58: Which of the following processes requires energy input in the form of ATP?

Facilitated diffusion
Osmosis
Active transport (Correct answer)
Simple diffusion

Correct answer: Active transport

Active transport is the process that requires energy input in the form of ATP to move molecules across a cell membrane. Unlike passive transport, active transport moves substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. This energy-dependent process is crucial for maintaining specific ion concentrations and nutrient uptake.

Question 59: Tay-Sachs disease results from accumulation of which lipid in neurons due to a missing enzyme?

Glucocerebroside
Ceramide
Sphingomyelin
GM2 ganglioside (Correct answer)

Correct answer: GM2 ganglioside

Tay-Sachs is caused by deficiency of hexosaminidase A, leading to accumulation of GM2 ganglioside in neuronal lysosomes, causing progressive neurodegeneration.

Question 60: The polymerase chain reaction (PCR) requires which enzyme?

RNA polymerase
Thermostable DNA polymerase (Taq polymerase) (Correct answer)
DNA ligase
Reverse transcriptase

Correct answer: Thermostable DNA polymerase (Taq polymerase)

PCR uses a thermostable DNA polymerase (typically Taq from Thermus aquaticus) that retains activity through the high-temperature denaturation cycles required to separate DNA strands.

Question 61: PRPP (5-phosphoribosyl-1-pyrophosphate) is best described as which of the following?

A substrate exclusive to pyrimidine de novo synthesis
The final catabolic product of both purines and pyrimidines
An allosteric inhibitor of purine salvage enzymes
An activated ribose donor used in both de novo synthesis and salvage pathways (Correct answer)

Correct answer: An activated ribose donor used in both de novo synthesis and salvage pathways

PRPP donates the activated ribose-5-phosphate moiety in both de novo nucleotide synthesis and salvage pathways for purines (HGPRT, APRT) and pyrimidines (OPRT).

Question 62: PRPP amidotransferase, which catalyzes the first committed step of de novo purine synthesis, is allosterically inhibited by which molecules?

IMP and XMP only
AMP, ADP, ATP, GMP, GDP, and GTP (Correct answer)
Orotate and UMP
Carbamoyl phosphate and aspartate

Correct answer: AMP, ADP, ATP, GMP, GDP, and GTP

PRPP amidotransferase is subject to cumulative feedback inhibition by the end-product purine nucleotides (AMP/ADP/ATP and GMP/GDP/GTP), ensuring synthesis shuts down when purine pools are replete.

Question 63: Which hormone primarily regulates the metabolic processes in the fed state?

Glucagon
Epinephrine
Cortisol
Insulin (Correct answer)

Correct answer: Insulin

Insulin is the primary hormone that regulates metabolic processes in the fed state, when blood glucose levels are elevated after a meal. It promotes the uptake of glucose by cells, stimulates the synthesis of glycogen in the liver and muscles, and encourages fat storage. Insulin effectively lowers blood glucose and directs nutrients towards storage.

Question 64: Familial hypercholesterolemia is caused by a defect in:

HMG-CoA reductase activity
Cholesterol ester transfer protein (CETP)
LDL receptor function (Correct answer)
Bile acid synthesis

Correct answer: LDL receptor function

Familial hypercholesterolemia is typically caused by loss-of-function mutations in the LDL receptor gene, impairing LDL uptake and causing elevated plasma LDL cholesterol.

Question 65: Insulin stimulates glucose uptake in muscle and adipose tissue primarily by:

Promoting translocation of GLUT4 transporters to the plasma membrane (Correct answer)
Activating GLUT2 transporters already in the membrane
Increasing hexokinase gene transcription directly
Inhibiting glucagon receptor signaling

Correct answer: Promoting translocation of GLUT4 transporters to the plasma membrane

Insulin signaling via PI3K/Akt promotes the translocation of GLUT4-containing vesicles from intracellular stores to the plasma membrane, rapidly increasing glucose uptake.

Question 66: In the DNA double helix, adenine forms hydrogen bonds with which complementary base?

Guanine
Uracil
Thymine (Correct answer)
Cytosine

Correct answer: Thymine

Adenine pairs with thymine in DNA via two hydrogen bonds (A=T), following Chargaff's rules, while guanine pairs with cytosine via three hydrogen bonds.

Question 67: Folate (Vitamin B9) deficiency causes megaloblastic anemia primarily because folate is required for synthesis of:

Thymidylate (dTMP) for DNA synthesis (Correct answer)
Globin protein chains
Heme precursors
Iron-binding proteins

Correct answer: Thymidylate (dTMP) for DNA synthesis

Folate as 5,10-methylene-THF is required for thymidylate synthase to convert dUMP to dTMP; without sufficient folate, DNA synthesis is impaired, causing large, immature red blood cells.

Question 68: Which pyrimidine base is present in RNA but absent from DNA?

Cytosine
Uracil (Correct answer)
Hypoxanthine
Thymine

Correct answer: Uracil

Uracil replaces thymine in RNA; thymine contains a methyl group at C-5 that uracil lacks, and uracil pairs with adenine in RNA just as thymine does in DNA.

Question 69: Beta-oxidation of fatty acids occurs primarily in which cellular compartment?

Peroxisome only
Mitochondrial matrix (Correct answer)
Cytosol
Smooth ER

Correct answer: Mitochondrial matrix

Beta-oxidation of most fatty acids occurs in the mitochondrial matrix, where fatty acyl-CoA undergoes sequential rounds of oxidation to produce acetyl-CoA, NADH, and FADH₂.

Question 70: Flow cytometry can be used to:

Measure enzyme kinetics in real time
Separate proteins by molecular weight
Count and sort cells based on fluorescent markers (Correct answer)
Determine protein tertiary structure

Correct answer: Count and sort cells based on fluorescent markers

Flow cytometry passes cells single-file through a laser beam and detects fluorescent signals from labeled antibodies or dyes, allowing rapid analysis and sorting of cell populations.

Question 71: Which reaction does hexokinase catalyze in the first step of glycolysis?

Glucose → glucose-6-phosphate (using ATP) (Correct answer)
Glucose-6-phosphate → fructose-6-phosphate
Glucose → glucose-1-phosphate
Fructose-6-phosphate → fructose-1,6-bisphosphate

Correct answer: Glucose → glucose-6-phosphate (using ATP)

Hexokinase catalyzes the phosphorylation of glucose to glucose-6-phosphate using ATP, trapping glucose in the cell and committing it to metabolism.

Question 72: Alpha-helices and beta-pleated sheets are examples of which level of protein structure?

Quaternary
Secondary (Correct answer)
Primary
Tertiary

Correct answer: Secondary

Secondary structure describes local folding patterns stabilized by hydrogen bonds between backbone amide and carbonyl groups.

Question 73: Lesch-Nyhan syndrome results from a deficiency of which enzyme in purine metabolism?

Purine nucleoside phosphorylase
Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (Correct answer)
Adenosine deaminase
Xanthine oxidase

Correct answer: Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

HGPRT deficiency impairs the purine salvage pathway, leading to massive uric acid overproduction, causing gout, intellectual disability, and self-mutilating behavior characteristic of Lesch-Nyhan syndrome.

Question 74: Heme biosynthesis requires which vitamin as a cofactor for the enzyme delta-aminolevulinate synthase?

Pyridoxal phosphate (B6) (Correct answer)
Folate (B9)
Vitamin B12 (cobalamin)
Biotin

Correct answer: Pyridoxal phosphate (B6)

Delta-aminolevulinate synthase, which catalyzes the first committed step of heme synthesis, requires PLP (pyridoxal phosphate) as a cofactor for the condensation of glycine and succinyl-CoA.

Question 75: Which metabolic pathway is activated during prolonged starvation to supply energy to the brain?

Glycolysis
Citric acid cycle
Beta-oxidation
Ketogenesis (Correct answer)

Correct answer: Ketogenesis

During prolonged starvation, the body activates ketogenesis to provide an alternative energy source for the brain when glucose reserves are severely depleted. The liver produces ketone bodies from fatty acids, which can then be utilized by the brain for energy. This metabolic adaptation helps conserve muscle protein by reducing the need for gluconeogenesis from amino acids.

Question 76: S-adenosylmethionine (SAM) is the principal methyl donor in biochemical methylation reactions. How is SAM synthesized?

From GTP and methionine by methionine adenosyltransferase
From AMP and homocysteine by methyltransferase
From PRPP and methionine by a transferase
From ATP and methionine by methionine adenosyltransferase (Correct answer)

Correct answer: From ATP and methionine by methionine adenosyltransferase

Methionine adenosyltransferase transfers the adenosyl group of ATP to methionine, forming SAM and releasing inorganic pyrophosphate and phosphate; SAM then donates its methyl group in reactions from DNA methylation to creatine synthesis.

Question 77: During fasting, which process provides glucose for the body when glycogen stores are depleted?

Lipogenesis
Gluconeogenesis (Correct answer)
Ketogenesis
Glycogenesis

Correct answer: Gluconeogenesis

During fasting, when the body's glycogen stores are depleted, gluconeogenesis becomes the primary process to synthesize new glucose. This pathway, mainly in the liver, produces glucose from non-carbohydrate precursors like amino acids, lactate, and glycerol. Gluconeogenesis is vital for maintaining blood glucose levels to supply energy to glucose-dependent tissues, such as the brain.

Question 78: What distinguishes a nucleotide from a nucleoside?

Nucleotides contain one or more phosphate groups; nucleosides do not (Correct answer)
Nucleosides contain a phosphate group; nucleotides do not
Nucleosides contain deoxyribose; nucleotides contain ribose
Nucleotides are found only in DNA; nucleosides are found only in RNA

Correct answer: Nucleotides contain one or more phosphate groups; nucleosides do not

A nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups, whereas a nucleoside contains only the base and the sugar without any phosphate.

Question 79: Chaperone proteins assist in which process?

Protein degradation
Post-translational glycosylation
Correct protein folding (Correct answer)
mRNA translation initiation

Correct answer: Correct protein folding

Molecular chaperones bind to unfolded or misfolded proteins and provide a protected environment that facilitates correct folding, preventing aggregation.

Question 80: Which technique is used to determine the three-dimensional structure of proteins at atomic resolution using X-rays?

Cryo-electron microscopy (cryo-EM)
Nuclear magnetic resonance (NMR) spectroscopy
X-ray crystallography (Correct answer)
Circular dichroism (CD) spectroscopy

Correct answer: X-ray crystallography

X-ray crystallography diffracts X-rays through protein crystals and uses the resulting diffraction pattern to calculate electron density maps, from which atomic coordinates are determined.

Question 81: Which disaccharide is composed of glucose and fructose joined by an alpha(1→2) glycosidic bond?

Maltose
Lactose
Sucrose (Correct answer)
Cellobiose

Correct answer: Sucrose

Sucrose (table sugar) consists of glucose and fructose linked by an unusual alpha(1→2) glycosidic bond involving both anomeric carbons, making it a non-reducing sugar.

Question 82: Which nucleotide intermediate serves as the branch point from which both AMP and GMP are synthesized in the purine de novo pathway?

XMP
Adenylosuccinate
PRPP
IMP (inosine monophosphate) (Correct answer)

Correct answer: IMP (inosine monophosphate)

IMP is the first complete purine nucleotide formed in de novo synthesis and serves as the branch-point intermediate from which AMP and GMP diverge via separate enzymatic routes.

Question 83: In transcription, which of the following statements is correct?

RNA polymerase synthesizes RNA in the 5' to 3' direction. (Correct answer)
Both DNA strands are transcribed into RNA.
The newly synthesized RNA remains permanently bound to the DNA template.
Transcription occurs in the cytoplasm of eukaryotic cells.

Correct answer: RNA polymerase synthesizes RNA in the 5' to 3' direction.

During transcription, RNA polymerase synthesizes RNA in the 5' to 3' direction, reading the DNA template strand in the 3' to 5' direction. This directional synthesis is a fundamental characteristic of nucleic acid polymerization. Only one DNA strand serves as the template for RNA synthesis, and in eukaryotes, this process occurs in the nucleus.

Question 84: Which enzyme catalyzes the committed (first unique) step of pyrimidine biosynthesis?

Dihydroorotase
PRPP synthetase
Carbamoyl phosphate synthetase II (CPS II)
Aspartate transcarbamoylase (ATCase) (Correct answer)

Correct answer: Aspartate transcarbamoylase (ATCase)

ATCase (aspartate transcarbamoylase) catalyzes the condensation of carbamoyl phosphate with aspartate, forming carbamoyl aspartate, which is the committed step unique to pyrimidine biosynthesis.

Question 85: Which class of proteins contains immunoglobulin-fold domains and is central to adaptive immunity?

Fibrous proteins
Nuclear receptors
G-protein coupled receptors
Antibodies (immunoglobulins) (Correct answer)

Correct answer: Antibodies (immunoglobulins)

Antibodies are glycoproteins with characteristic immunoglobulin folds consisting of beta-sandwich domains, forming variable regions that bind specific antigens.

Question 86: Which of the following is a key regulatory enzyme in the citric acid cycle?

Citrate synthase
Malate dehydrogenase
Isocitrate dehydrogenase (Correct answer)
Succinate dehydrogenase

Correct answer: Isocitrate dehydrogenase

Isocitrate dehydrogenase is a key regulatory enzyme in the citric acid cycle, controlling its overall rate based on the cell's energy demands. Its activity is allosterically inhibited by high levels of ATP and NADH, signaling ample energy, and activated by ADP, indicating a need for more energy. This regulation ensures efficient energy production.

Question 87: Which of the following statements about the Cori cycle is TRUE?

It occurs between the liver and brain.
Lactate produced in the muscles is converted back to glucose in the liver. (Correct answer)
It involves the direct synthesis of ATP in muscle cells.
It only operates in the presence of oxygen.

Correct answer: Lactate produced in the muscles is converted back to glucose in the liver.

The Cori cycle describes the metabolic pathway where lactate produced during anaerobic glycolysis in active muscles is transported to the liver. In the liver, this lactate is converted back into glucose through gluconeogenesis. This newly synthesized glucose can then be released into the bloodstream to be used by muscles or other tissues, effectively recycling the lactate.

Question 88: What is the primary function of the phospholipid bilayer in the cell membrane?

Production of ATP
Formation of a selective barrier for molecules (Correct answer)
Transport of proteins into the nucleus
Structural support for the cytoskeleton

Correct answer: Formation of a selective barrier for molecules

The primary function of the phospholipid bilayer in the cell membrane is to form a selective barrier that controls the movement of molecules into and out of the cell. Its hydrophobic interior prevents the free passage of most polar molecules, while its overall structure maintains cellular integrity and homeostasis. This selectivity is vital for cell function.

Question 89: In the pyrimidine salvage pathway, uridine is converted directly to UMP by which enzyme?

Uridine phosphorylase
Uridine kinase (Correct answer)
CTP synthetase
UMP synthase

Correct answer: Uridine kinase

Uridine kinase phosphorylates uridine (using ATP) to produce UMP in the salvage pathway, allowing cells to recycle preformed pyrimidine nucleosides without de novo synthesis.

Question 90: Which of the following is true about the DNA replication?

It is semiconservative, where each daughter DNA molecule has one parent and one new strand. (Correct answer)
It does not require a primer to start synthesis.
It is conservative, where both parent strands remain together.
It occurs only during mitosis.

Correct answer: It is semiconservative, where each daughter DNA molecule has one parent and one new strand.

DNA replication is a semiconservative process, meaning that each new double helix consists of one original (parental) strand and one newly synthesized strand. This mechanism ensures that genetic information is accurately copied and passed on to daughter cells. The parental strand serves as a template for the synthesis of the new complementary strand.

Question 91: Von Gierke disease (Type I glycogen storage disease) is caused by deficiency of:

Glycogen phosphorylase
Glycogen synthase
Debranching enzyme
Glucose-6-phosphatase (Correct answer)

Correct answer: Glucose-6-phosphatase

Glucose-6-phosphatase deficiency in Von Gierke disease prevents the liver from releasing free glucose from glycogen (or gluconeogenesis), causing severe fasting hypoglycemia.

Question 92: NAD+ is synthesized in cells using which vitamin as the key precursor of the nicotinamide ring?

Pyridoxine (B6)
Thiamine (B1)
Riboflavin (B2)
Niacin (B3) (Correct answer)

Correct answer: Niacin (B3)

Niacin (vitamin B3), supplied as nicotinic acid or nicotinamide, provides the nicotinamide moiety of NAD+ and NADP+; deficiency causes pellagra and impairs all NAD+-dependent redox reactions.

Question 93: Cholesterol in animal cell membranes functions to:

Increase membrane permeability to ions
Modulate membrane fluidity (Correct answer)
Provide receptor sites for hormones only
Serve as the primary energy storage lipid

Correct answer: Modulate membrane fluidity

Cholesterol inserts between phospholipid tails, reducing fluidity at high temperatures and preventing crystallization at low temperatures, thereby buffering membrane fluidity.

Question 94: Which enzyme is the rate-limiting step in cholesterol biosynthesis?

HMG-CoA synthase
Lanosterol synthase
Squalene synthase
HMG-CoA reductase (Correct answer)

Correct answer: HMG-CoA reductase

HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate and is the primary regulated, rate-limiting step in cholesterol synthesis; statins inhibit this enzyme.

Question 95: Glycogen synthase is activated when it is in which phosphorylation state?

Phosphorylated (inactive) and glycogen-bound
Phosphorylated (active)
Unrelated to phosphorylation
Dephosphorylated (active) (Correct answer)

Correct answer: Dephosphorylated (active)

Glycogen synthase is active in its dephosphorylated form; phosphorylation by kinases such as GSK-3 inhibits the enzyme, reducing glycogen synthesis.

Question 96: Which vitamin deficiency is associated with Wernicke-Korsakoff syndrome, commonly seen in chronic alcoholics?

Niacin (B3)
Folate (B9)
Vitamin B12
Thiamine (B1) (Correct answer)

Correct answer: Thiamine (B1)

Thiamine deficiency due to poor diet and impaired absorption in alcoholics causes Wernicke-Korsakoff syndrome, characterized by ophthalmoplegia, ataxia, and confabulation from neuronal damage.

Question 97: Which sugar transporter (GLUT) is expressed in pancreatic beta-cells and serves as a glucose sensor?

GLUT4
GLUT1
GLUT5
GLUT2 (Correct answer)

Correct answer: GLUT2

GLUT2 in pancreatic beta-cells has a high Km for glucose, allowing glucose uptake proportional to blood glucose concentration, enabling beta-cells to sense and respond to glucose levels.

Question 98: The Bradford assay measures protein concentration using which dye?

BCA (bicinchoninic acid)
Lowry reagent
Coomassie Brilliant Blue G-250 (Correct answer)
Biuret reagent (copper sulfate)

Correct answer: Coomassie Brilliant Blue G-250

The Bradford assay uses Coomassie Brilliant Blue G-250, which shifts its absorbance maximum from 465 nm to 595 nm upon binding to proteins; the absorbance at 595 nm is proportional to protein concentration.

Question 99: Riboflavin (Vitamin B2) is the precursor for which two coenzymes?

NAD⁺ and NADP⁺
Biotin and lipoic acid
CoA and acyl-CoA
FAD and FMN (Correct answer)

Correct answer: FAD and FMN

Riboflavin is the precursor for flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which serve as prosthetic groups for flavoenzymes in oxidative metabolism.

Question 100: Which amino acid can form disulfide bonds that stabilize protein structure?

Cysteine (Correct answer)
Serine
Threonine
Methionine

Correct answer: Cysteine

Cysteine residues contain thiol (-SH) groups that can be oxidized to form covalent disulfide (-S-S-) bonds between chains or within a single chain.

Question 101: Vitamin D3 (cholecalciferol) is hydroxylated first in the liver to 25-hydroxyvitamin D, then in the kidney to its active form:

1,24-dihydroxyvitamin D
7-dehydrocholesterol
24,25-dihydroxyvitamin D
1,25-dihydroxyvitamin D (calcitriol) (Correct answer)

Correct answer: 1,25-dihydroxyvitamin D (calcitriol)

1,25-dihydroxyvitamin D₃ (calcitriol) is the biologically active hormone that binds to nuclear vitamin D receptors to increase intestinal calcium absorption and regulate bone mineralization.

Question 102: Orotic aciduria type I is caused by a deficiency of which bifunctional enzyme?

Dihydroorotate dehydrogenase
UMP synthase (UMPS) (Correct answer)
CPS II
ATCase

Correct answer: UMP synthase (UMPS)

UMP synthase possesses both orotate phosphoribosyltransferase and OMP decarboxylase activities; its deficiency blocks the last two steps of pyrimidine synthesis, causing orotic acid accumulation and megaloblastic anemia.

Question 103: Which bond is primarily responsible for maintaining the tertiary structure of a protein in an aqueous environment?

Peptide bonds
Covalent disulfide bonds only
Hydrophobic interactions (Correct answer)
Ionic bonds only

Correct answer: Hydrophobic interactions

Hydrophobic interactions drive the burial of nonpolar side chains in the protein core and are the dominant force in tertiary structure stabilization.

Question 104: De novo purine nucleotide synthesis begins with which metabolite as the core scaffold?

Hypoxanthine
Ribose-5-phosphate (Correct answer)
Inosine monophosphate (IMP)
Xanthine

Correct answer: Ribose-5-phosphate

De novo purine synthesis starts with ribose-5-phosphate (from the pentose phosphate pathway), which is converted to PRPP (5-phosphoribosyl-1-pyrophosphate), the activated ribose donor for ring assembly.

Question 105: Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. Which of the following is NOT a gluconeogenic precursor?

Acetyl-CoA (Correct answer)
Oxaloacetate
Lactate
Glycerol

Correct answer: Acetyl-CoA

Acetyl-CoA cannot be converted to net glucose in mammals because the two carbons entering the TCA cycle as acetyl-CoA are lost as CO₂ before the cycle can regenerate oxaloacetate.

Question 106: Niacin (Vitamin B3) is a precursor for which essential electron carrier coenzymes?

Pyridoxal phosphate and pyridoxamine
FAD and FMN
NAD⁺ and NADP⁺ (Correct answer)
Coenzyme A and acetyl-CoA

Correct answer: NAD⁺ and NADP⁺

Niacin (nicotinic acid/nicotinamide) is the precursor for NAD⁺ and NADP⁺, which are crucial electron carriers in oxidative metabolism and anabolic reactions respectively.

Question 107: What is the term for the movement of a phospholipid from one leaflet of the bilayer to the other?

Vesicle budding
Lateral diffusion
Rotational diffusion
Flip-flop (transverse diffusion) (Correct answer)

Correct answer: Flip-flop (transverse diffusion)

Flip-flop (transverse diffusion) is the movement of a lipid across the bilayer, which is thermodynamically unfavorable and requires flippase enzymes to occur rapidly.

Question 108: Vitamin K is required for the gamma-carboxylation of which proteins critical for blood clotting?

Clotting factors II, VII, IX, and X (Correct answer)
Fibrinogen and thrombin
Von Willebrand factor
Platelet glycoproteins

Correct answer: Clotting factors II, VII, IX, and X

Vitamin K-dependent gamma-glutamyl carboxylase modifies prothrombin (factor II) and factors VII, IX, and X, adding extra carboxyl groups that allow Ca²⁺ binding essential for clot formation.

Question 109: Allopurinol treats hyperuricemia and gout by inhibiting which enzyme?

HGPRT
Xanthine oxidase (Correct answer)
Adenosine deaminase
Purine nucleoside phosphorylase

Correct answer: Xanthine oxidase

Allopurinol (and its oxidized form oxipurinol) inhibits xanthine oxidase, the enzyme that converts hypoxanthine to xanthine and xanthine to uric acid, thereby lowering serum urate levels.

Question 110: Which of the following statements about competitive inhibition is TRUE?

Competitive inhibitors reduce the Vmax of an enzyme.
Competitive inhibitors bind to the active site of the enzyme. (Correct answer)
Competitive inhibition cannot be overcome by increasing substrate concentration.
Competitive inhibitors form covalent bonds with the enzyme.

Correct answer: Competitive inhibitors bind to the active site of the enzyme.

Competitive inhibitors function by binding directly to the active site of an enzyme, competing with the natural substrate for access. Because they occupy the active site, they prevent the substrate from binding and undergoing catalysis. This type of inhibition can often be overcome by increasing the substrate concentration.

Question 111: Southern blotting detects:

Specific lipid molecules
Specific RNA sequences
Specific DNA sequences (Correct answer)
Specific protein antigens

Correct answer: Specific DNA sequences

Southern blotting (named after Edward Southern) involves electrophoresis of restriction-digested DNA, transfer to a membrane, and hybridization with a labeled DNA probe to detect specific sequences.

Question 112: Phosphofructokinase-1 (PFK-1) is the key regulatory enzyme of glycolysis. Which molecule allosterically activates it?

Citrate
AMP (Correct answer)
Acetyl-CoA
ATP

Correct answer: AMP

AMP (and ADP) allosterically activates PFK-1, signaling low energy status in the cell and stimulating glycolysis to increase ATP production.

Question 113: 5-Fluorouracil (5-FU) exerts its anticancer activity primarily by inhibiting which enzyme?

Thymidylate synthase (Correct answer)
Dihydrofolate reductase
CTP synthetase
Ribonucleotide reductase

Correct answer: Thymidylate synthase

5-FU is converted to 5-FdUMP, which forms a covalent ternary complex with thymidylate synthase and N5,N10-methyleneTHF, irreversibly blocking dTMP synthesis and depleting the thymidine pool for DNA replication.

Question 114: Which phospholipase cleaves the fatty acid at the sn-2 position of glycerophospholipids to release arachidonic acid?

Phospholipase D
Phospholipase C
Phospholipase A1
Phospholipase A2 (Correct answer)

Correct answer: Phospholipase A2

Phospholipase A2 cleaves the ester bond at the sn-2 position, releasing arachidonic acid (or other fatty acids) and a lysophospholipid.

Question 115: Vitamin B12 (cobalamin) is required for which two reactions in humans?

Methionine synthesis and methylmalonyl-CoA mutase reaction (Correct answer)
Fatty acid synthesis and glycolysis
Heme synthesis and bilirubin metabolism
DNA replication and RNA transcription

Correct answer: Methionine synthesis and methylmalonyl-CoA mutase reaction

Cobalamin is required for methionine synthase (converting homocysteine to methionine) and methylmalonyl-CoA mutase (converting methylmalonyl-CoA to succinyl-CoA) in the metabolism of odd-chain fatty acids and some amino acids.

Question 116: Which lipid class forms the structural backbone of the cell membrane bilayer?

Triglycerides
Sphingomyelin
Cholesterol
Phospholipids (Correct answer)

Correct answer: Phospholipids

Phospholipids are amphipathic molecules with a hydrophilic phosphate head group and hydrophobic fatty acid tails that spontaneously form bilayers in aqueous environments.

Question 117: The pentose phosphate pathway (PPP) primarily generates which two products needed for biosynthesis?

Acetyl-CoA and oxaloacetate
Pyruvate and lactate
ATP and NADH
NADPH and ribose-5-phosphate (Correct answer)

Correct answer: NADPH and ribose-5-phosphate

The oxidative PPP produces NADPH for reductive biosynthesis and antioxidant defense, and ribose-5-phosphate for nucleotide synthesis.

Question 118: During fatty acid synthesis, malonyl-CoA is formed from acetyl-CoA by which enzyme?

Citrate lyase
Fatty acid synthase (FAS)
HMG-CoA reductase
Acetyl-CoA carboxylase (ACC) (Correct answer)

Correct answer: Acetyl-CoA carboxylase (ACC)

Acetyl-CoA carboxylase (ACC) catalyzes the ATP-dependent carboxylation of acetyl-CoA to malonyl-CoA, the committed step in fatty acid synthesis.

Question 119: Biotin serves as a cofactor for carboxylase enzymes by acting as a carrier of:

CO₂ (carboxyl groups) (Correct answer)
Methyl groups
Amino groups
Acyl groups

Correct answer: CO₂ (carboxyl groups)

Biotin is covalently attached to carboxylase enzymes and serves as a mobile carboxyl group carrier, transferring CO₂ in ATP-dependent carboxylation reactions such as those catalyzed by pyruvate carboxylase and ACC.

Question 120: Pellagra, characterized by the 4 D's (Dermatitis, Diarrhea, Dementia, Death), is caused by deficiency of:

Niacin (B3) (Correct answer)
Riboflavin (B2)
Pyridoxine (B6)
Thiamine (B1)

Correct answer: Niacin (B3)

Pellagra is caused by niacin (vitamin B3) deficiency, which impairs NAD⁺/NADP⁺-dependent redox reactions throughout the body, affecting skin, gut, and nervous system.

Question 121: Pyridoxal phosphate (PLP), the active form of Vitamin B6, is a cofactor for which class of enzymes?

Carboxylases
Kinases
Aminotransferases (transaminases) and other amino acid-metabolizing enzymes (Correct answer)
Oxidoreductases only

Correct answer: Aminotransferases (transaminases) and other amino acid-metabolizing enzymes

PLP forms a Schiff base with amino acid substrates and is essential for aminotransferases, amino acid decarboxylases, and many other enzymes involved in amino acid metabolism.

Question 122: In a Western blot, after SDS-PAGE and transfer to membrane, what is the purpose of the primary antibody?

To generate a detectable signal directly
To transfer protein from gel to membrane
To specifically recognize the target protein (Correct answer)
To block non-specific binding sites

Correct answer: To specifically recognize the target protein

The primary antibody binds specifically to the target protein (antigen) on the membrane, providing specificity; a secondary antibody conjugated to an enzyme or fluorophore then generates the detectable signal.

Question 123: Coenzyme A (CoA) is derived from which vitamin?

Thiamine (B1)
Riboflavin (B2)
Pantothenic acid (B5) (Correct answer)
Niacin (B3)

Correct answer: Pantothenic acid (B5)

Coenzyme A contains pantothenic acid (vitamin B5) as a core component, along with adenosine 3',5'-bisphosphate and a phosphopantetheine moiety that provides the reactive thiol group.

Question 124: Glycogen is the storage polysaccharide in animals. Its branching involves which type of linkage?

Alpha(1→4) in chains with alpha(1→6) at branch points (Correct answer)
Beta(1→4) linkages only
Beta(1→3) linkages at branch points
Alpha(1→2) linkages throughout

Correct answer: Alpha(1→4) in chains with alpha(1→6) at branch points

Glycogen has linear alpha(1→4)-linked glucose chains with alpha(1→6) branch points approximately every 8–12 residues, allowing rapid glucose release from multiple ends simultaneously.

Question 125: Ketone bodies are produced in the liver primarily from excess:

Glucose
Amino acids
Acetyl-CoA (Correct answer)
Glycerol

Correct answer: Acetyl-CoA

When acetyl-CoA from fatty acid oxidation exceeds the capacity of the TCA cycle (e.g., during fasting), the liver converts excess acetyl-CoA into ketone bodies (acetoacetate, β-hydroxybutyrate, acetone).

Question 126: In oxidative phosphorylation, ATP synthesis is powered by:

A proton gradient across the mitochondrial inner membrane (Correct answer)
The release of carbon dioxide from the citric acid cycle
Substrate-level phosphorylation
The transfer of electrons directly to ATP synthase

Correct answer: A proton gradient across the mitochondrial inner membrane

During oxidative phosphorylation, ATP synthesis is powered by a proton gradient established across the mitochondrial inner membrane. The electron transport chain pumps protons into the intermembrane space, creating an electrochemical gradient. Protons then flow back into the matrix through ATP synthase, and the energy released drives the phosphorylation of ADP to ATP.

Question 127: Lipid rafts in cell membranes are enriched in which components?

Cholesterol and sphingolipids (Correct answer)
Cardiolipin and phosphatidylserine
Lysophospholipids and free fatty acids
Phosphatidylethanolamine and unsaturated fatty acids

Correct answer: Cholesterol and sphingolipids

Lipid rafts are dynamic microdomains enriched in cholesterol and sphingolipids that form a more ordered (liquid-ordered) phase and serve as platforms for signaling proteins.

Question 128: Which of the following is NOT involved in post-transcriptional modification of eukaryotic mRNA?

5' capping
Addition of a primer (Correct answer)
Splicing out introns
Polyadenylation at the 3' end

Correct answer: Addition of a primer

Post-transcriptional modification of eukaryotic mRNA involves several crucial steps, including the addition of a 5' cap, splicing out introns, and polyadenylation at the 3' end. These modifications protect the mRNA, facilitate its export from the nucleus, and enhance its translation efficiency. The addition of a primer is a step in DNA replication, not mRNA processing.

Question 129: Under anaerobic conditions in muscle, pyruvate is converted to lactate to regenerate:

NAD⁺ (Correct answer)
Acetyl-CoA
FADH₂
ATP

Correct answer: NAD⁺

Lactate dehydrogenase reduces pyruvate to lactate, oxidizing NADH back to NAD⁺, which is required for glycolysis to continue producing ATP in the absence of oxygen.

Question 130: Lipoic acid (lipoate) functions as a cofactor in alpha-keto acid dehydrogenase complexes by:

Carrying amino groups between reactions
Donating CO₂ for carboxylation reactions
Acting as a transient carrier of acyl groups and electrons via its dithiol/disulfide couple (Correct answer)
Serving as an electron acceptor in the electron transport chain

Correct answer: Acting as a transient carrier of acyl groups and electrons via its dithiol/disulfide couple

Lipoic acid is covalently bound to a lysine residue and cycles between its oxidized (disulfide) and reduced (dithiol) forms to transfer acyl groups and electrons during oxidative decarboxylation.

Question 131: Cyclic AMP (cAMP) functions as a second messenger primarily by activating which intracellular target?

Protein kinase A (PKA) (Correct answer)
Phospholipase C
Protein kinase C (PKC)
Calmodulin-dependent kinase

Correct answer: Protein kinase A (PKA)

cAMP binds to the regulatory subunits of protein kinase A, causing their dissociation and releasing active catalytic subunits that phosphorylate serine/threonine residues on downstream target proteins.

GRE Biochemistry, Cell and Molecular Biology Subject Test

The GRE Biochemistry, Cell and Molecular Biology Subject Test assesses graduate-level knowledge across biochemistry, cell biology, and molecular biology and genetics for students applying to life sciences graduate programs. The exam consists of 180 multiple-choice questions drawn proportionally from three core content areas.

Questions

35 min

Time Limit

70%

To Pass

Checking for saved progress...

Exam Rules

You can skip questions and return to them later
Flag questions for review before submitting
No feedback shown until you submit the entire exam
Unanswered questions count as wrong — answer everything
10 pretest questions are mixed in and don't affect your score
Timer auto-submits when time runs out
Your progress is auto-saved every 30 seconds

These practice questions were written with AI tools. They are not real exam questions. We have not yet matched this exam's facts to an official source, so confirm fees, dates and rules with the exam body. Not reviewed by a subject-matter expert. Editorial policy