protein targeting Flashcards
1
Q
Which protein makes up actin microfilaments?
A
G-actin (globular actin), which polymerises to F-actin.
2
Q
What is meant by actin filament polarity?
A
Filaments have a (+) end for fast growth and a (–) end for slower growth or shrinkage.
3
Q
Q: What is ‘treadmilling’ in actin filaments?
A
A: Continuous addition of actin at the (+) end and loss at the (–) end.
4
Q
Q: What is the role of Rho GTPases in actin dynamics?
A
A: They control when and where actin filaments assemble.
5
Q
Q: What does Thymosin β4 do?
A
A: Sequesters G-actin to prevent polymerisation into F-actin.
6
Q
Q: What does Villin do in actin structures?
A
cross-links actin into parallel bundles, particularly in the formation of microvilli on epithelial cells. These parallel bundles give structural stability and increase the surface area for absorption, especially in the small intestine.
7
Q
Q: How does Filamin help actin function?
A
A: Cross-links actin at angles to form a mesh; supports the cell cortex.
8
Q
Q: What is the function of Gelsolin?
A
A: Severs actin filaments and caps the (+) end to regulate stiffness.
9
Q
Q: What are lamellipodia?
A
A: Actin-based protrusions that help cells move across surfaces.
10
Q
Q: What is the basic unit of intermediate filaments?
A
A: A tetramer formed from two coiled-coil dimers.
11
Q
Q: Are intermediate filaments polar or non-polar?
A
A: Non-polar.
12
Q
Q: What is the role of keratin in epithelial cells?
A
A: Provides resistance to mechanical stress.
13
Q
Q: What disease is linked to defective keratin?
A
A: Epidermolysis bullosa simplex (EBS).
14
Q
Q: What do neurofilaments do in neurons?
A
A: Maintain axon shape and diameter.
15
Q
Q: Which intermediate filament proteins support the nuclear envelope?
A
A: Lamins A, B, and C.
16
Q
Q: What does plectin do?
A
A: Connects intermediate filaments to actin and microtubules.
17
Q
Q: What is ALS and how is it related to intermediate filaments?
A
A: A neurodegenerative disease linked to neurofilament abnormalities.
18
Q
Q: What are microtubules made from?
A
A: α- and β-tubulin dimers.
19
Q
Q: How many protofilaments make up a microtubule?
A
A: 13 protofilaments.
20
Q
Q: What energy molecule is needed for microtubule polymerisation?
A
GTP
21
Q
Q: What is the MTOC and what is its function?
A
microtubule organising centre, nucleates microtubule growth
22
Q
Q: What do centrosomes contain?
A
a pair of centrioles and nucleating material
23
Q
Q: Which motor protein moves towards the (+) end of microtubules?
A
Kinesin
24
Q
Q: Which motor protein moves towards the (–) end of microtubules?
A
Dynein
25
Q: What is the structure of motile cilia?
A: 9 microtubule doublets around 2 central microtubules.
26
Q: What causes situs inversus?
A: Defects in cilia-mediated signalling during embryonic development.
27
Q: What does colchicine do?
A: Binds tubulin to inhibit microtubule polymerisation.
28
Q: How does Taxol act as an anti-cancer drug?
A: It stabilises microtubules, preventing cell division.
29
❓Question 2:
What is the composition of actin microfilaments?
A. Alpha-helical crosslinked alpha tubulin
B. Covalently linked globular actin units
C. G-actin polymerised in a head-to-head polarity
D. Triple helical strands of polymers of actin
E. Two chains of polymerised actin subunits
✅ Correct answer: E. Two chains of polymerised actin subunits
🧠 Explanation:
Actin microfilaments are made of two long chains of F-actin (filamentous actin), which are themselves formed from polymerised G-actin (globular actin) monomers.
These two chains twist around each other to form a double-helical filament.
C is incorrect because polymerisation happens in a head-to-tail orientation, not head-to-head.
30
❓Question 3:
The processive movement of kinesin is essential for which process?
A. Beating of cilia and flagella
B. Chemotaxis
C. Determination of axon diameter
D. Muscle contraction
E. Transport of cargo along axons
E. Transport of cargo along axons
Verdict: ✅ Correct!
🧠 Why this is correct:
Kinesin is a motor protein that "walks" along microtubules using ATP.
It moves toward the plus (+) end, which typically means away from the nucleus and towards the cell periphery — such as in long axons.
This is essential in neurons, where materials must be transported from the cell body all the way down the axon.
31
Which of the following best describes the structural organisation of intermediate filaments?
A. A single filament of coiled α-helix tubulin
B. Two actin filaments arranged in a helix
C. A tetramer of antiparallel dimers stacked end-to-end
D. A triple-stranded polymer of G-actin
E. 13 protofilaments surrounding a central lumen
✅ Correct Answer: C. A tetramer of antiparallel dimers stacked end-to-end
🧠 Explanation:
Intermediate filaments (IFs) are made from tetramers, which are formed by pairing two coiled-coil dimers in an antiparallel fashion. These tetramers then assemble end-to-end and side-by-side to form the rope-like structure of the filament.
32
Which of the following proteins links actin microfilaments into a flexible, mesh-like network beneath the plasma membrane?
A. Gelsolin
B. Villin
C. Filamin
D. Thymosin β4
E. Keratin
✅ Correct Answer: C. Filamin
🧠 Explanation:
Filamin links actin filaments together at angles, forming a mesh-like network just beneath the plasma membrane. This helps the cell maintain its shape and adapt to mechanical stress.
Gelsolin doesn’t link filaments — it severs them and caps the (+) end to stop further growth.
33
Which of the following best describes the role of dynein?
A. Severs actin filaments in response to calcium
B. Moves vesicles toward the cell periphery along actin
C. Binds intermediate filaments to the plasma membrane
D. Transports cargo along microtubules toward the nucleus
E. Pulls chromosomes apart during anaphase by binding kinetochore microtubules
✅ Correct Answer: D. Transports cargo along microtubules toward the nucleus
🧠 Explanation:
Dynein is a microtubule motor protein that moves toward the minus (–) end, which usually points toward the nucleus.
It carries organelles, vesicles, and even whole chromosomes in that direction.
B is incorrect because vesicle movement toward the cell periphery is typically done by kinesin (and along microtubules, not actin).
34
🧪 Question 4/30:
What is the role of the basal body in cilia?
A. It severs and reorganises microtubules to allow cilia to beat
B. It is a centriole-derived MTOC that anchors and initiates cilia formation
C. It attaches dynein arms to the ciliary membrane
D. It stores ATP for ciliary movement
E. It allows actin to assemble beneath the plasma membrane
✅ Correct!
Your Answer: B. It is a centriole-derived MTOC that anchors and initiates cilia formation
Verdict: ✅ Spot on!
🧠 Explanation:
The basal body is a type of microtubule-organising centre (MTOC) that sits at the base of each cilium.
It originates from a centriole and provides the foundation from which the cilium grows.
It anchors the cilium to the cell membrane and organises the “9+2” microtubule structure inside.
35
Question 5/30:
Which of the following is a key feature of intermediate filaments, distinguishing them from actin filaments and microtubules?
A. They polymerise in a head-to-tail fashion with ATP
B. They require GTP for polymerisation and depolymerisation
C. They are non-polar and do not exhibit dynamic instability
D. They are composed of tubulin heterodimers
E. They sever and cap filaments in response to calcium
C
Your Answer: C. They are non-polar and do not exhibit dynamic instability
Verdict: ✅ Spot on again!
🧠 Explanation:
Intermediate filaments (IFs) are unique in that they are non-polar — they don’t have distinct (+) or (–) ends like actin or microtubules.
They are also very stable and not dynamic, meaning they don’t grow and shrink rapidly.
Their main job is to provide durable structural support, not transport or motility.
36
🧪 Question 6/30:
Which cytoskeletal component is directly involved in forming the contractile ring during cytokinesis?
A. Microtubules
B. Intermediate filaments
C. Keratin
D. Actin filaments
E. Cilia
Correct Answer: D. Actin filaments
🧠 Explanation:
The contractile ring that pinches a cell in two during cytokinesis is made of actin filaments and myosin.
These filaments form a ring beneath the plasma membrane at the centre of the dividing cell, and their contraction causes the cell to split.
Microtubules (A) are involved in spindle formation, not the contractile ring.
37
🧪 Question 7/30:
Which cytoskeletal structure is composed of 13 protofilaments arranged in a hollow tube?
A. Actin microfilament
B. Stereocilium
C. Intermediate filament
D. Axoneme
E. Microtubule
Correct Answer: E. Microtubule
🧠 Explanation:
A microtubule is built from 13 protofilaments arranged side-by-side to form a hollow cylindrical tube.
This structure is key for their rigidity and ability to serve as tracks for transport and scaffolds for structures like cilia.
Intermediate filaments (C) do not have a defined number of protofilaments and are rope-like, not tubular.
38
🧪 Question 8/30:
What is the role of gelsolin in actin dynamics?
A. Nucleates actin filament formation
B. Cross-links actin into bundles
C. Binds and severs actin filaments in response to calcium
D. Anchors actin to the nuclear membrane
E. Caps actin at the minus end to promote elongation
Correct Answer: C. Binds and severs actin filaments in response to calcium
🧠 Explanation:
Gelsolin is an actin-severing protein that is activated by calcium.
It cuts actin filaments and then caps the new (+) end, preventing further polymerisation.
This helps convert the actin cytoskeleton from a stiff, gel-like state to a more fluid, sol-like one — useful for cell movement or shape change.
39
🧪 Question 9/30:
Which of the following best describes the function of kinesin?
A. Transports vesicles toward the nucleus along actin
B. Carries cargo toward the plus end of microtubules
C. Cross-links intermediate filaments to the ECM
D. Nucleates microtubule growth from the centrosome
E. Causes contraction of the contractile ring
✅ Correct!
Your Answer: B. Carries cargo toward the plus end of microtubules
Verdict: ✅ Nailed it!
🧠 Explanation:
Kinesin is a motor protein that uses ATP to walk toward the (+) end of microtubules.
This usually means moving cargo away from the nucleus, toward the cell periphery.
It’s essential for things like axonal transport in neurons.
40
🧪 Question 10/30:
Which of the following cytoskeletal components is the most stable and least dynamic?
A. Actin microfilaments
B. Microtubules
C. Intermediate filaments
D. Mitotic spindles
E. Primary cilia
C
✅ Correct!
Your Answer: C. Intermediate filaments
Verdict: ✅ Spot on!
🧠 Explanation:
Intermediate filaments are designed for long-term structural support.
They are non-polar and do not undergo rapid assembly/disassembly, unlike actin and microtubules.
This makes them the most stable part of the cytoskeleton — perfect for reinforcing cell shape and resisting stress.
41
🧪 Question 11/30:
Which of the following correctly describes the structure of motile cilia?
A. A solid bundle of actin filaments in a 9+0 pattern
B. 13 microtubules arranged in a hollow tube
C. 9 microtubule doublets surrounding 2 central microtubules
D. 9 singlet microtubules arranged in a ring
E. 9+2 arrangement of actin–myosin complexes
C
✅ Correct!
Your Answer: C. 9 microtubule doublets surrounding 2 central microtubules
Verdict: ✅ Perfect!
🧠 Explanation:
This classic "9+2" arrangement is found in motile cilia and flagella.
The structure allows coordinated bending powered by dynein arms — essential for moving mucus in airways or sperm propulsion.
Primary cilia by contrast have a 9+0 pattern (no central pair) and are usually non-motile.
42
🧪 Question 12/30:
What is the effect of colchicine on microtubules?
A. Inhibits tubulin synthesis
B. Binds and stabilises microtubules to prevent disassembly
C. Promotes spindle formation during mitosis
D. Binds tubulin and prevents microtubule polymerisation
E. Cross-links microtubules into bundles
Correct Answer: D. Binds tubulin and prevents microtubule polymerisation
🧠 Explanation:
Colchicine is an anti-mitotic drug that binds to free tubulin dimers, preventing them from polymerising into microtubules.
This disrupts the mitotic spindle and blocks cell division — it’s often used to treat conditions like gout.
The drug that stabilises microtubules and prevents disassembly is Taxol (Paclitaxel), not colchicine.
💊 Clinical Use of Colchicine
Colchicine is primarily used for conditions involving inflammation due to microtubule-dependent immune cell activity.
yes
43
🧪 Question 13/30:
Which cytoskeletal element is most directly responsible for the formation of lamellipodia?
A. Intermediate filaments
B. Microtubules
C. Actin filaments
D. Tubulin dimers
E. Lamin proteins
C
Your Answer: C. Actin filaments
Verdict: ✅ Well done!
🧠 Explanation:
Lamellipodia are thin, sheet-like projections at the leading edge of migrating cells.
They are formed by rapid polymerisation of actin filaments, creating a mesh that pushes the plasma membrane forward.
This structure helps cells crawl — crucial in wound healing, immune responses, and development.
44
🧪 Question 15/30:
Which of the following best describes the function of lamins?
A. Anchor actin filaments in lamellipodia
B. Stabilise the spindle during mitosis
C. Support the nuclear envelope from within
D. Link intermediate filaments to the plasma membrane
E. Form the core of primary cilia
C
Correct Answer: C. Support the nuclear envelope from within
🧠 Explanation:
Lamins are a type of intermediate filament protein found inside the nucleus.
They form a dense meshwork called the nuclear lamina, which supports the inner surface of the nuclear envelope.
They also play roles in DNA organisation and nuclear reassembly after mitosis.
A refers to actin-associated proteins, not lamins.
45
🧪 Question 16/30:
Which of the following is a structural feature of stereocilia in the inner ear?
A. 9+2 arrangement of microtubules
B. Central pair of singlet microtubules
C. Bundles of actin filaments
D. Arrangement of intermediate filaments
E. Core of dynein-linked tubulin dimers
C
Correct Answer: C. Bundles of actin filaments
🧠 Explanation:
Despite the name, stereocilia are not true cilia — they don’t contain microtubules.
Instead, they’re supported by actin filaments, arranged in stiff bundles to keep them rigid.
Found in the inner ear, they detect sound vibrations by bending — this bending opens ion channels, leading to nerve signals.
46
🧪 Question 17/30:
What is the most accurate description of Taxol’s mechanism of action?
A. Promotes actin polymerisation in microvilli
B. Cross-links keratin filaments into stable sheets
C. Binds and stabilises microtubules, preventing disassembly
D. Blocks kinesin from binding to cargo
E. Disrupts G-actin polymerisation into F-actin
C
Correct Answer: C. Binds and stabilises microtubules, preventing disassembly
🧠 Explanation:
Taxol (paclitaxel) is a chemotherapy drug that binds to microtubules and stabilises them.
This prevents the microtubules from breaking down, which disrupts mitosis and blocks cell division — especially in rapidly dividing cancer cells.
It doesn’t interact with keratin (which is an intermediate filament), so B is unrelated.
47
🧪 Question 18/30:
What is the primary structural component of microvilli?
A. Microtubules
B. Intermediate filaments
C. Actin filaments
D. Tubulin singlets
E. Stereocilia
C
Your Answer: C. Actin filaments
Verdict: ✅ Great job!
🧠 Explanation:
Microvilli are small, finger-like projections on the surface of epithelial cells (especially in the intestine).
They are supported internally by parallel bundles of actin filaments, which are cross-linked by proteins like villin and fimbrin.
Their function is to increase surface area for absorption — not to move, like cilia.
48
🧪 Question 19/30:
Which actin-binding protein sequesters G-actin, preventing it from polymerising into F-actin?
A. Profilin
B. Villin
C. Thymosin β4
D. Filamin
E. Gelsolin
C
✅ Correct!
Your Answer: C. Thymosin β4
Verdict: ✅ Nailed it!
🧠 Explanation:
Thymosin β4 binds to G-actin (the monomer form of actin) and prevents it from polymerising into F-actin.
It keeps a reserve pool of actin monomers available for rapid filament assembly when needed.
It’s like keeping bricks on standby rather than building the wall right away!`
49
🧪 Question 20/30:
Which of the following correctly matches the motor protein with the filament system it travels along?
A. Dynein – Actin
B. Myosin – Microtubules
C. Kinesin – Microtubules
D. Kinesin – Intermediate filaments
E. Myosin – Intermediate filaments
C✅ Correct!
Your Answer: C. Kinesin – Microtubules
Verdict: ✅ Excellent!
🧠 Explanation:
Kinesin is a motor protein that "walks" along microtubules, specifically toward the plus (+) end — usually heading toward the cell periphery.
It plays a major role in transporting organelles, vesicles, and proteins across the cell.
Myosin walks along actin filaments, not microtubules.
50
🧪 Question 21/30:
Which of the following conditions is linked to mutations in keratin intermediate filaments?
A. Amyotrophic lateral sclerosis (ALS)
B. Epidermolysis bullosa simplex (EBS)
C. Situs inversus
D. ADPKD
E. Hereditary spherocytosis
B
✅ Correct!
Your Answer: B. Epidermolysis bullosa simplex (EBS)
Verdict: ✅ Spot on!
🧠 Explanation:
EBS is a skin-blistering condition caused by mutations in keratin genes, which are a type of intermediate filament found in epithelial cells.
These mutations make the skin more fragile and prone to tearing with even minor trauma.
ALS (A) is linked to neurofilament abnormalities, another type of intermediate filament.
51
🧪 Question 22/30:
Which type of microtubule attaches to the centromere of chromosomes during mitosis?
A. Polar microtubules
B. Aster microtubules
C. Lamellipodial microtubules
D. Kinetochore microtubules
E. Basal microtubules
D
✅ Correct!
Your Answer: D. Kinetochore microtubules
Verdict: ✅ Perfect!
🧠 Explanation:
Kinetochore microtubules attach to a specific structure on the chromosome called the kinetochore, which is found at the centromere.
These microtubules pull the sister chromatids apart during anaphase of mitosis.
Aster microtubules help anchor the spindle poles to the cell cortex.
52
🧪 Question 23/30:
What cytoskeletal structure is disrupted by defects in dynein arms, leading to immotile cilia and conditions like situs inversus?
A. Axoneme
B. Lamellipodium
C. Contractile ring
D. Keratin meshwork
E. Mitotic spindle
Correct Answer: A. Axoneme
🧠 Explanation:
The axoneme is the core structural component of cilia and flagella.
It has the 9+2 microtubule arrangement and includes dynein arms, which allow for the bending motion needed for ciliary beating.
Defects in dynein arms impair ciliary motion, causing disorders like Primary Ciliary Dyskinesia — which can result in situs inversus due to disrupted embryonic signalling.
53
🧪 Question 24/30:
Which of the following proteins helps cross-link actin filaments into tight, parallel bundles in structures like microvilli?
A. Filamin
B. Thymosin β4
C. Gelsolin
D. Villin
E. Dynein
D
✅ Correct!
Your Answer: D. Villin
Verdict: ✅ Excellent!
🧠 Explanation:
Villin is an actin-binding protein that organises actin filaments into tight, parallel bundles, especially in microvilli.
These bundles provide rigid structural support and increase surface area for absorption, particularly in the small intestine.
Filamin, by contrast, forms a looser, angled meshwork, not bundles.
54
🧪 Question 25/30:
Which of the following best describes the structural core of motile cilia?
A. A ring of 9 actin filaments
B. A 13-protofilament microtubule bundle
C. 9 microtubule singlets in a circular arrangement
D. A 9+2 arrangement of microtubule doublets
E. 9+0 arrangement of actin cross-links
✅ Correct!
Your Answer: D. A 9+2 arrangement of microtubule doublets
Verdict: ✅ Nailed it!
🧠 Explanation:
Motile cilia have a 9+2 arrangement: 9 doublets of microtubules arranged in a ring around 2 central singlets.
This structure is essential for generating the bending motion needed for moving mucus, fluid, or sperm.
The 9+0 arrangement is found in primary (non-motile) cilia.
55
🧪 Question 26/30:
Which protein links intermediate filaments to actin and microtubules, helping integrate the entire cytoskeleton?
A. Dynein
B. Plectin
C. Profilin
D. Lamin A
E. Vinculin
✅ Correct!
Your Answer: B. Plectin
Verdict: ✅ Brilliant!
🧠 Explanation:
Plectin is a large cross-linking protein that connects intermediate filaments to actin filaments, microtubules, and even to membrane junctions.
It acts like a “cytoskeletal glue,” ensuring mechanical integrity and coordination across different filament systems.
56
🧪 Question 27/30:
What is the function of the centrosome during mitosis?
A. Contracts the cleavage furrow during cytokinesis
B. Stabilises the nuclear lamina
C. Acts as the basal body for stereocilia
D. Serves as the microtubule-organising centre for spindle formation
E. Forms the core of gap junctions
Correct Answer: D. Serves as the microtubule-organising centre for spindle formation
🧠 Explanation:
The centrosome is a microtubule-organising centre (MTOC) that duplicates and moves to opposite poles of the cell during mitosis.
It organises the mitotic spindle, which is essential for pulling chromatids apart.
A refers to the role of the contractile ring, which is made of actin and myosin, not the centrosome.
57
🧪 Question 28/30:
Which of the following is not a typical function of microtubules?
A. Intracellular transport of organelles
B. Formation of the mitotic spindle
C. Structural core of cilia
D. Anchoring of intermediate filaments to desmosomes
E. Arrangement of organelles within the cell
D
✅ Correct!
Your Answer: D. Anchoring of intermediate filaments to desmosomes
Verdict: ✅ Great job!
🧠 Explanation:
Desmosomes anchor intermediate filaments, not microtubules.
All other listed roles (A, B, C, E) are key functions of microtubules, including intracellular transport, mitotic spindle formation, and organelle positioning.
58
🧪 Question 29/30:
What cytoskeletal structure is disrupted in Amyotrophic Lateral Sclerosis (ALS)?
A. Actin filaments in the contractile ring
B. Microtubules in the mitotic spindle
C. Neurofilaments in axons
D. Lamins in the nuclear envelope
E. Keratin filaments in epithelial cells
✅ Correct!
Your Answer: C. Neurofilaments in axons
Verdict: ✅ Spot on!
🧠 Explanation:
ALS is associated with abnormal expression or accumulation of neurofilaments, which are a type of intermediate filament found in neurons.
These disruptions impair axonal transport and structure, contributing to motor neuron degeneration and progressive muscle weakness.
59
🧪 Question 30/30 (Final One!):
What happens to primary cilia during cell division?
A. They contract to pull chromosomes apart
B. They form the cleavage furrow
C. They are converted into stereocilia
D. They are disassembled during mitosis
E. They initiate the mitotic spindle
Correct Answer: D. They are disassembled during mitosis
🧠 Explanation:
Primary cilia are non-motile sensory structures present on most cells during interphase.
Before mitosis begins, they are disassembled so that the centrosome (which acts as their basal body) can be used to organise the mitotic spindle.
They are reassembled after cell division is complete.
