What is the approximate thickness of the plasma membrane observed in thin section under an electron microscope?
A
75 Å (angstroms), formed of three trilamellar sheets.
B
5 nm, formed of a simple lipid layer.
C
20 nm, formed of four alternating layers.
D
100 nm, formed of a uniform glycoprotein matrix.
Explanation: explanation: The other values do not correspond to the actual thickness of the plasma membrane.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – structure
difficulty: easy
What is the lipid composition of the erythrocyte membrane?
A
55% phospholipids, 25% cholesterol, 20% glycolipids.
B
70% cholesterol, 20% phospholipids, 10% glycolipids.
C
50% glycolipids, 40% phospholipids, 10% cholesterol.
D
80% phospholipids, 10% cholesterol, 10% glycolipids.
Explanation: explanation: The proportion of membrane lipids is clearly defined in the cytology course.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – composition
difficulty: easy
Membrane cholesterol is present exclusively in:
A
Animal cell membranes.
B
Plant cell membranes only.
C
The membranes of Gram-positive bacteria.
D
All living eukaryotic and prokaryotic cells.
Explanation: explanation: Bacteria and plant cells use other sterols or have none at all.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – cholesterol
difficulty: easy
What is the effect of cholesterol on plasma membrane fluidity?
A
It increases fluidity by separating phospholipids.
B
It reinforces rigidity by immobilizing neighboring hydrocarbon chains.
C
It has no effect on membrane fluidity.
D
It increases fluidity only at low temperatures.
Explanation: explanation: The unsaturation of phospholipids increases fluidity; cholesterol has the opposite effect.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – fluidity
difficulty: medium
Glycophorin, an erythrocyte protein, belongs to which category of membrane proteins?
A
Peripheral extrinsic protein on the cytosolic side.
B
Monotopic transmembrane protein.
C
Polytopic transmembrane protein.
D
Protein anchored by a lipid via a GPI bond.
Explanation: explanation: Monotopic proteins cross the bilayer only once, polytopes cross it several times.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – proteins
difficulty: medium
The fluid mosaic model was proposed in 1972 by:
A
Davson and Danielli.
B
Singer and Nicholson.
C
Watson and Crick.
D
Robertson and Gorter.
Explanation: explanation: The fluid mosaic model remains the reference model of membrane structure.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – model
difficulty: medium
Which enzyme ensures the transverse movement (flip-flop) of phospholipids from one layer of the membrane to the other?
A
Membrane kinases.
B
Flippases.
C
Phospholipases A2.
D
TIM translocases.
Explanation: explanation: The spontaneous flip-flop is limited because the hydrophilic group must cross the internal hydrophobic zone; flippases catalyze this movement.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – flip-flop
difficulty: difficult
How many membranes does the mitochondria have?
A
A single outer membrane.
B
Two membranes: one external and one internal.
C
Three membranes separating four compartments.
D
A double fused membrane with no internal compartment.
Explanation: explanation: The internal membrane forms invaginations called cristae, the site of oxidative phosphorylation.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondrion – structure
difficulty: easy
Mitochondrial cristae are invaginations of:
A
The mitochondrial outer membrane.
B
The inner mitochondrial membrane in the matrix.
C
The nuclear envelope in the mitochondria.
D
The membrane of adjacent lysosomes.
Explanation: explanation: It is at the level of the cristae that the complexes of the respiratory chain and ATP synthase are found.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – cristae
difficulty: easy
What proportion of proteins does the inner mitochondrial membrane contain?
A
18% protein (like myelin).
B
52% protein.
C
80% protein.
D
40% protein.
Explanation: explanation: The internal membrane is involved in major biochemical reactions (respiratory chain, ATP), which justifies its richness in proteins.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – membrane composition
difficulty: medium
Human mitochondrial DNA codes for how many proteins?
A
37 total subunit proteins.
B
13 proteins, subunits of oxidative phosphorylation complexes.
C
22 ribosomal proteins.
D
200 matrix proteins.
Explanation: explanation: The 37 genes are divided into 13 proteins + 22 tRNAs + 2 rRNAs, or 37 in total.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – DNA
difficulty: medium
Which inner membrane mitochondrial enzyme is involved in the synthesis of steroid hormones?
A
ATP synthase.
B
Cytochrome P450.
C
Glucose-6-phosphatase.
D
Glycosyl transferase.
Explanation: explanation: Cytochromes P450 are hydroxylases using O₂ and NADPH to synthesize hormonal precursors.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – steroid hormones
difficulty: difficult
What are the three types of fibers constituting the cytoskeleton of a eukaryotic cell?
A
Actin microfilaments, microtubules and intermediate filaments.
B
Myosin filaments, microfilaments and nuclear lamins.
C
Microtubules, ribosomes and keratin filaments only.
D
Microfilaments, collagen filaments and microtubules.
Explanation: explanation: Each of these three components has a distinct diameter and functions.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – components
difficulty: easy
What is the approximate diameter of actin microfilaments?
A
25 nanometers.
B
7 nanometers.
C
10 nanometers.
D
50 nanometers.
Explanation: explanation: Diameter is a diagnostic characteristic to distinguish the three components of the cytoskeleton.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – microfilaments
difficulty: easy
Microtubules are made up of how many protofilaments arranged in a ring?
A
9 protofilaments.
B
7 protofilaments.
C
13 protofilaments.
D
20 protofilaments.
Explanation: explanation: The number of 13 protofilaments is a constant structural characteristic of microtubules.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – microtubules
difficulty: medium
Which intermediate filament is characteristic of epithelial cells?
A
Vimentin.
B
Neurofilaments.
C
Keratin.
D
Desmin.
Explanation: explanation: Each cell type expresses specific intermediate filaments: vimentin in many cells, keratin in the epithelium, neurofilaments in neurons.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – intermediate filaments
difficulty: medium
How many microtubule triplets is each centriole made up of?
A
6 triplets.
B
7 triplets.
C
9 triplets.
D
11 triplets.
Explanation: explanation: The rule of 9 triplets (9+0) is specific to centrioles, unlike cilia (9+2 doublets).
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – centrioles
difficulty: medium
Which motor protein ensures the transport of vesicles from the endoplasmic reticulum to the cell periphery along microtubules?
A
Dynein.
B
Kinesin.
C
Myosin.
D
G-actin.
Explanation: explanation: Dynein ensures transport in the opposite direction (from the periphery to the center). Kinesin = centrifugal, dynein = centripetal.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – intracellular transport
difficulty: difficult
What is the distinctive structural feature of the rough endoplasmic reticulum (REG)?
A
Its cytosolic side is lined with ribosomes.
B
Its membrane is devoid of ribosomes and rich in lipid enzymes.
C
It forms smooth tubules without flattened cisternae.
D
It is only present in muscle cells.
Explanation: explanation: REL, on the other hand, does not carry ribosomes and is involved in the synthesis of lipids and steroid hormones.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Endomembrane system – endoplasmic reticulum
difficulty: easy
What is the main role of the smooth endoplasmic reticulum (SER)?
A
Synthesis of membrane and exported proteins.
B
Synthesis of lipids and steroid hormones.
C
Mitochondrial DNA replication.
D
Assembly of ribosomal subunits.
Explanation: explanation: The biosynthesis of membrane phospholipids is carried out essentially at the REL level.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Endomembrane system – REL
difficulty: medium
Protein N-glycosylation begins in which organelle?
A
The Golgi apparatus faces trans.
B
The rough endoplasmic reticulum (REG).
C
The cell nucleus.
D
Lysosomes.
Explanation: explanation: The N-glycosidic bond involves asparagine in an Asn-X-Ser/Thr consensus sequence.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Endomembrane system – glycosylation
difficulty: medium
The labeling of lysosomal hydrolases in the Golgi apparatus involves which specific marker?
A
Mannose-6-phosphate (Man-6P).
B
Glucose-6-phosphate.
C
Sialic acid.
D
N-acetylgalactosamine.
Explanation: explanation: This Man-6P system is essential for targeting enzymes to lysosomes.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Endomembrane system – Golgi
difficulty: difficult
What is the main role of nuclear pores?
A
Ensure nuclear DNA replication.
B
Ensure molecular exchanges between the cytoplasm and the nucleoplasm.
C
Synthesize ribosomal proteins.
D
Produce transfer RNAs exclusively.
Explanation: explanation: Pores allow selective bidirectional traffic between the nucleus and the cytoplasm.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Core – structure
difficulty: easy
What is the main function of the nucleolus?
A
Mitochondrial DNA replication.
B
Synthesis of ribosomal RNAs and assembly of ribosomal subunits.
C
Nuclear calcium storage.
D
Degradation of misfolded proteins.
Explanation: explanation: The genes encoding rRNAs (distributed on chromosomes 13, 14, 15, 21, 22) are grouped in the nucleolar organizers.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Nucleus – nucleolus
difficulty: easy
Euchromatin corresponds to:
A
The condensed form of chromatin during interphase, transcriptionally inactive.
B
The decondensed form of chromatin during interphase, the active zone of transcription.
C
The DNA constituting centromeres, never transcribed.
D
Sex chromatin (Barr corpuscle).
Explanation: explanation: Heterochromatin is the condensed form corresponding to inactive areas.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Nucleus – chromatin
difficulty: medium
During which phase of the cell cycle does DNA replication take place?
A
Phase G1.
B
Phase S.
C
Phase G2.
D
Phase M (mitosis).
Explanation: explanation: The phase order is G1 → S → G2 → M, with replication exclusively in S phase.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Cell cycle – phases
difficulty: medium
During which phase of mitosis are the chromosomes aligned on the equator of the cell?
A
Prophase.
B
Metaphase.
C
Anaphase.
D
Telophase.
Explanation: explanation: During anaphase, the chromatids separate and migrate towards the poles; during telophase, cytodieresis takes place.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Cell cycle – mitosis
difficulty: medium
The G0 phase of the cell cycle is defined as:
A
A rapid growth phase preceding the S phase.
B
A phase of mitotic quiescence where the cell stops its multiplication.
C
The cell division phase itself.
D
A subphase of mitotic metaphase.
Explanation: explanation: Many differentiated neurons or cardiomyocytes are in phase G0, explaining their low regeneration capacity.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Cell cycle – G0
difficulty: difficult
Which biologist discovered the Golgi apparatus (Nobel Prize 1906)?
A
Rudolf Virchow.
B
Camillo Golgi.
C
Robert Brown.
D
Thomas Hunt Morgan.
Explanation: explanation: The Golgi apparatus is made up of a set of parallel flattened cisternae or saccules associated with secretory vesicles.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Golgi apparatus – structure
difficulty: easy
Which side of the Golgi apparatus is in the vicinity of the endoplasmic reticulum and receives the transitional vesicles?
A
The trans (internal) side.
B
The cis (external) face.
C
The middle face.
D
The trans-Golgi network (TGN).
Explanation: explanation: Vesicular trafficking follows the cis → medial → trans direction in the Golgi apparatus.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Golgi apparatus – polarity
difficulty: medium
Which cells in the body lack a nucleus?
A
Hepatocytes, osteoclasts and osteocytes.
B
Red blood cells, superficial keratinocytes and platelets.
C
Neurons, lymphocytes and monocytes.
D
Smooth muscle cells only.
Explanation: explanation: Red blood cells lose their nucleus during terminal erythropoiesis to maximize hemoglobin transport.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Nucleus – cells without nucleus
difficulty: easy
What is the shape of the nucleus in polynuclear neutrophils?
A
Spherical.
B
Ovoid.
C
Polylobed.
D
In single band.
Explanation: explanation: The polylobed shape of the polynuclear nucleus allows them to pass through small capillaries during diapedesis.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Core – form
difficulty: easy
With what other organelle do mitochondria share the role of main reservoir of intracellular calcium?
A
The Golgi apparatus.
B
The smooth endoplasmic reticulum.
C
Lysosomes.
D
Peroxisomes.
Explanation: explanation: Mitochondrial calcium plays a role in apoptosis and regulation of oxidative phosphorylation.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – calcium
difficulty: medium
What role does the signal peptide play during cotranslational protein synthesis in the REG?
A
It directs the protein to the mitochondria.
B
It constitutes a transfer initiation signal, allowing translocation of the protein across the REG bilayer.
C
It glycosylates the protein in the cytosol.
D
It recruits ribosomes to the plasma membrane.
Explanation: explanation: After translocation, the signal peptide is cut by peptidases and degraded by ER proteases.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Endoplasmic reticulum – signal peptide
difficulty: difficult
What is the proportion of proteins in the erythrocyte plasma membrane?
A
18% (like myelin).
B
52 %.
C
76% (like the inner mitochondrial membrane).
D
30 %.
Explanation: explanation: The quantity of membrane proteins is inversely proportional to the structural role of the membrane: myelin (structural role) = 18%, the mitochondrial inner membrane (intense enzymatic activity) = 76%.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Cell – erythrocyte membrane composition
difficulty: easy
The genes encoding ribosomal RNAs (rRNAs) are distributed over how many pairs of chromosomes in the human diploid genome?
A
2 pairs (chromosomes 1 and 2).
B
5 pairs (chromosomes 13, 14, 15, 21, 22).
C
10 pairs of autosomal chromosomes.
D
On chromosome 17 alone.
Explanation: explanation: These regions form the nucleolar organizers (NOR) which come together to form the nucleolus.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Nucleus – chromosomes and nucleolus
difficulty: medium
The glycocalyx (cell-coat) is located:
A
On the inner (cytosolic) side of the plasma membrane.
B
On the outer dense layer of the plasma membrane, formed of perpendicular carbohydrate fibrils.
C
In the mitochondrial matrix.
D
On the surface of the inner nuclear membrane.
Explanation: explanation: The glycocalyx plays a role in cell recognition, adhesion and imparts a negative charge to the cell surface via sialic acid.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – glycocalyx
difficulty: easy
The GPI (glycosyl-phosphatidylinositol) anchoring of an integrated protein is done on which amino acid?
A
Glycine in the N-terminal position.
B
Amino acid in the C-terminal position.
C
Internal cysteine via a thioester bond.
D
Serine in central position.
Explanation: explanation: Distinct from myristoylation (N-terminal glycine), palmitylation (cysteine via thioester) and isoprenylation (C-terminal cysteine via thioester).
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – lipid anchors
difficulty: difficult
Which component important for phosphorylation is found in the mitochondrial intermembrane space?
A
Mitochondrial ribosomes (mitoribosomes).
B
Circular mitochondrial DNA.
C
H+ protons and cytochrome C molecules.
D
ATP synthases.
Explanation: explanation: The proton gradient between the inter-membrane space and the matrix is the motor of ATP synthase (Mitchell chemiosmosis).
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – inter-membrane space
difficulty: medium
The intermediate filaments of nerve cells are called:
A
Vimentine.
B
Neurofilaments.
C
Keratin.
D
Desmine.
Explanation: explanation: The designation of intermediate filaments is cell type specific, which is exploited in pathological anatomy to identify tumors.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – intermediate filaments
difficulty: easy
Which of the following statements about the chemical composition of the plasma membrane are correct?
A
Phospholipids have a hydrophilic head and a hydrophobic tail.
B
Cholesterol is present in the membranes of bacteria and animal cells.
C
Membrane carbohydrates participate in the negative charge via sialic acid.
D
Membrane proteins can be receptors, transporters or enzymes.
Explanation: explanation: Statement B is incorrect because cholesterol is only present in animal cell membranes.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – chemical composition
difficulty: medium
Which of the following physiological roles are performed by the mitochondria?
A
Synthesis of ATP by oxidative phosphorylation.
B
Synthesis of steroid hormones in cooperation with the REL.
C
Storage of intracellular calcium.
D
Synthesis of immunoglobulins.
Explanation: explanation: The mitochondrion is a central organelle with energetic, biosynthetic and calcium signaling functions.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – physiological roles
difficulty: medium
What functions are carried out by the cytoskeleton in the eukaryotic cell?
A
Maintenance of cell shape and anchoring of organelles.
B
Intracellular transport of vesicles along microtubules.
C
Migration of chromosomes during cell division.
D
Synthesis of ribosomal proteins.
Explanation: explanation: The microfilaments ensure contraction; microtubules transport and division; the intermediate filaments stability.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – functions
difficulty: easy
Which of the following functions are attributed to the Golgi apparatus?
A
Continuation of protein N-glycosylation started in the REG.
B
O-glycosylation of proteins (binding on serine or threonine).
C
Mitochondrial DNA replication.
D
Labeling of lysosomal hydrolases by mannose-6-phosphate.
Explanation: explanation: The Golgi apparatus is the center for sorting and post-translational modification of secreted or lysosomal proteins.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Endomembrane system – functions of the Golgi
difficulty: medium
Regarding heterochromatin, which statements are correct?
A
Constitutive heterochromatin constitutes centromeres and telomeres, and is never transcribed.
B
Facultative heterochromatin contains repressed genes that can be expressed in other cell types.
C
Heterochromatin is a decondensed form of chromatin corresponding to active areas of transcription.
D
Heterochromatin is the condensed form of chromatin during interphase.
Explanation: explanation: An example of facultative heterochromatin is the Barr corpuscle (inactivated X chromosome in female cells).
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Nucleus – heterochromatin
difficulty: difficult
When importing cytosolic proteins into the mitochondria, what steps are involved?
A
A cytosolic Hsp70 protein causes the protein to be imported to unwind.
B
The addressing sequence is recognized by an outer membrane receptor.
C
The protein crosses the TOM and TIM complexes of the outer and inner membranes.
D
The imported protein is synthesized directly in the mitochondrial matrix.
Explanation: explanation: Most mitochondrial proteins are encoded by the nuclear genome and imported from the cytosol.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – protein import
difficulty: medium
What phases constitute interphase of the cell cycle?
A
G1 phase (growth and protein synthesis).
B
S phase (DNA replication).
C
Phase G2 (second growth phase).
D
Phase M (mitosis itself).
Explanation: explanation: Interphase represents the largest part of the cell cycle, during which the cell prepares for division.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Cell cycle – interphase
difficulty: easy
Which of the following lipid modifications allowing the anchoring of proteins integrated into the membrane are cited in the course?
A
Myristoylation (myristic acid on N-terminal glycine).
B
Palmitylation (palmitic acid via a thioester bond on cysteine).
C
Glypiation (GPI anchor on C-terminal amino acid).
D
Ubiquitination on internal lysine.
Explanation: explanation: These four types of lipid anchor define the class of integrated lipid-anchored proteins.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – anchored proteins
difficulty: medium
During co-translational protein synthesis in the REG, which steps are correct?
A
The signal recognition particle (SRP) recognizes the signal peptide and temporarily blocks synthesis.
B
The PRS-signal peptide complex binds to the anchor protein (PRS receptor) on the ER membrane.
C
A membrane translocation pore opens to allow passage of the protein.
D
The signal peptide is retained in the mature secreted protein.
Explanation: explanation: This mechanism explains how proteins intended to be secreted or membrane bound are addressed to the ER.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: REG – cotranslational protein synthesis
difficulty: difficult
Which of the following characteristics corresponds to the telophase of mitosis?
A
Appearance of a constriction at the equator of the cell (cytodieresis).
B
Chromatin decondensation.
C
Reformation of the nuclear envelope.
D
Alignment of chromosomes on the equator (metaphase plate).
Explanation: explanation: Telophase is the last phase of mitosis, during which the two daughter cells physically separate.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Mitosis – stages
difficulty: medium
Describe the ultrastructural structure of the plasma membrane and list its main chemical constituents with their proportions.
Explanation: explanation: The plasma membrane is a trilamellar structure of approximately 75 Å, formed of two dark layers separated by a light layer. Its chemical constituents in the erythrocyte: 40% lipids, 52% proteins, 8% carbohydrates. Membrane lipids include 55% phospholipids, 25% cholesterol and 20% glycolipids. Proteins are intrinsic (transmembrane or lipid-anchored) or extrinsic. Carbohydrates form the glycocalyx on the outer side.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – structure and composition
difficulty: medium
Describe the structural organization of the mitochondria, specifying the compartments and their characteristics.
Explanation: explanation: The mitochondrion has two membranes delimiting three compartments: the outer membrane (lipid bilayer, rich in porins, permeable), the inter-membrane space (containing H+, cytochrome C), and the inner membrane (80% proteins, 20% lipids, forming ridges in the matrix). The matrix contains circular mitochondrial DNA, mitoribosomes, mRNAs, tRNAs and numerous enzyme systems.
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondrion – structure
difficulty: easy
Compare the three components of the cytoskeleton in terms of diameter, constituent proteins, and major functions.
Explanation: explanation: Microfilaments (actin, 7 nm) ensure muscle contraction, cell shape and cytodieresis. Microtubules (α and β tubulin, 25 nm, 13 protofilaments) ensure vesicular transport (kinesin/dynein), chromosome migration and shape maintenance. Intermediate filaments (8-10 nm, keratin, vimentin, neurofilaments depending on cell type) provide structural stability and resist stretching.
source_reference: Dr. Aouati, Cytoskeleton, Faculty of Medicine of Constantine 2022.
theme: Cytoskeleton – components
difficulty: medium
Describe the journey of a secreted protein from its synthesis to its extracellular secretion.
Explanation: explanation: 1) Synthesis on free ribosome → recognition of the signal peptide by PRS → docking to REG. 2) Co-translational translocation into the lumen of the REG → cleavage of the signal peptide → initial N-glycosylation. 3) Vesicular transport to the Golgi apparatus (cis side). 4) Post-translational modifications in the Golgi (complete N-glycosylation, O-glycosylation). 5) Packaging into secretory vesicles at the trans-Golgi network (TGN). 6) Fusion of vesicles with the plasma membrane and exocytosis.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Endomembrane system – fate of proteins
difficulty: medium
Describe the four phases of mitosis and specify the chromosomal events characteristic of each phase.
Explanation: explanation: Prophase: fragmentation of the nuclear envelope, condensation of chromatin into thick filaments, establishment of the division spindle. Metaphase: maximum condensation of chromosomes, alignment on the equatorial plate (metaphase plate), ideal stage for karyotype. Anaphase: separation of the two sister chromatids of each chromosome, migration towards opposite poles by shortening of the microtubules. Telophase: equatorial constriction (cytodieresis), decondensation of chromatin, reformation of the nuclear envelope, formation of two daughter cells.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Cell cycle – regulation
difficulty: difficult
Explain the role of the mitochondrial intermembrane space and cristae in ATP production.
Explanation: explanation: The ridges are the invaginations of the inner membrane into the matrix. They increase the membrane surface available for the complexes of the respiratory chain. These complexes pump H+ protons from the matrix to the inter-membrane space, creating an electrochemical gradient of protons (proton-motive force). ATP synthase uses this gradient to synthesize ATP from ADP + Pi, during the return of protons into the matrix (chemiosmosis).
source_reference: Dr. Aouati, Mitochondria, Faculty of Medicine of Constantine 2022.
theme: Mitochondria – oxidative phosphorylation
difficulty: medium
Distinguish euchromatin from heterochromatin by specifying their condensation state and their transcriptional activity.
Explanation: explanation: Euchromatin is the decondensed form of chromatin during interphase (weakly colored), corresponding to active DNA areas with high gene transcription. Heterochromatin is the condensed (strongly colored) form, corresponding to inactive areas. It is subdivided into constitutive heterochromatin (repetitive DNA that is never transcribed: centromeres, telomeres) and facultative (repressed genes that can be active in other cell types).
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Nucleus – nucleolus and chromatin
difficulty: easy
Explain Singer and Nicholson's (1972) fluid mosaic model and its implications for membrane lipid movements.
Explanation: explanation: The fluid mosaic model (Singer and Nicholson, 1972) describes the membrane as a fluid lipid bilayer in which proteins float, forming an asymmetric mosaic. Phospholipids can diffuse laterally in the same layer very easily (crossing of a bacteria in 1-2 seconds). The transverse movement (flip-flop) is very limited because the hydrophilic group must cross the hydrophobic zone; it requires flippases. Membrane proteins also diffuse laterally but more slowly.
source_reference: Dr. Aouati, Plasma membrane, Faculty of Medicine of Constantine 2022.
theme: Plasma membrane – fluid mosaic
difficulty: difficult
Describe the mechanism for targeting acid hydrolases to lysosomes via the Golgi apparatus.
Explanation: explanation: The labeling of lysosomal hydrolases begins in the cis saccules of the Golgi by the phosphorylation of mannose (addition of a phosphate on the 6th carbon: mannose-6-phosphate, Man-6P). Enzymes carrying Man-6P are recognized by specific receptors at the trans-Golgi network (TGN). Clathrin vesicles bud from the TGN, lose their clathrin coat, and then fuse with the endolysosome. At acidic pH, the enzyme dissociates from its receptor, Man-6P is removed by phosphatases (enzyme maturity), and the receptors are recycled to the Golgi.
source_reference: Dr. Aouati, Endomembrane system, Faculty of Medicine of Constantine 2022.
theme: Golgi apparatus – lysosomal labeling
difficulty: medium
Describe the interphase of the cell cycle, specifying the characteristics of each subphase.
Explanation: explanation: Interphase brings together G1, S and G2. G1 phase: the longest and the most variable depending on the cell type; phase of growth and synthesis of proteins necessary for the S phase. S phase: short phase of DNA replication, the quantity of DNA is doubled at the end. Phase G2: second phase of growth, the cell contains double the usual DNA and prepares for mitosis by synthesizing the necessary constituents.
source_reference: Dr. Aouati, Nucleus and cell cycle, Faculty of Medicine of Constantine 2020.
theme: Cell cycle – phases and durations
difficulty: easy