Practice important
Thin Myofilaments
MCQ questions and answers.
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MCQ Questions
1. Which structural feature of a thin myofilament most directly permits its interaction with myosin during skeletal muscle contraction?
A.
The arrangement of globular actin monomers into a helical filament
B.
The presence of myosin heads along the filament
C.
The storage of calcium ions inside tropomyosin
D.
The attachment of thick filaments to troponin
Correct Answer:
The arrangement of globular actin monomers into a helical filament
Explanation:
Thin myofilaments contain actin arranged as two intertwined chains of globular actin monomers. Each actin monomer has a myosin-binding site that participates in cross-bridge formation.
2. In a relaxed skeletal muscle fibre, what directly prevents myosin heads from binding to actin?
A.
Troponin covers the ATP-binding sites on myosin
B.
Tropomyosin lies across the myosin-binding sites on actin
C.
Actin dissociates into individual globular monomers
D.
Calcium binds to actin and blocks its active sites
Correct Answer:
Tropomyosin lies across the myosin-binding sites on actin
Explanation:
In the relaxed state, tropomyosin is positioned along actin so that it masks the myosin-binding sites on the actin filament, preventing cross-bridge formation.
3. Which sequence correctly describes the role of calcium ions in regulating thin myofilaments during skeletal muscle contraction?
A.
Calcium binds actin, causing tropomyosin to hydrolyse ATP
B.
Calcium binds troponin, producing a change that moves tropomyosin away from actin's binding sites
C.
Calcium binds tropomyosin, causing actin to detach from the thin filament
D.
Calcium binds myosin, exposing binding sites on tropomyosin
Correct Answer:
Calcium binds troponin, producing a change that moves tropomyosin away from actin's binding sites
Explanation:
Calcium ions bind to the troponin complex on the thin filament. This changes troponin's conformation and shifts tropomyosin, exposing the myosin-binding sites on actin.
4. Which combination correctly identifies the principal protein components of a skeletal muscle thin myofilament?
A.
Actin, tropomyosin, and troponin
B.
Myosin, titin, and nebulin
C.
Actin, myosin, and titin
D.
Tropomyosin, myosin, and titin
Correct Answer:
Actin, tropomyosin, and troponin
Explanation:
The principal components of a thin myofilament are actin, which provides binding sites; tropomyosin, which regulates their exposure; and troponin, which responds to calcium.
5. If calcium ions are absent from the cytoplasm of a skeletal muscle fibre, which condition of the thin myofilament is expected?
A.
Troponin remains calcium-bound and tropomyosin exposes actin sites
B.
Tropomyosin remains positioned over actin's myosin-binding sites
C.
Actin changes into myosin and forms thick filaments
D.
Troponin hydrolyses ATP to expose the actin sites
Correct Answer:
Tropomyosin remains positioned over actin's myosin-binding sites
Explanation:
Without calcium, troponin does not undergo the conformational change required to shift tropomyosin. Therefore, the myosin-binding sites on actin remain covered.
6. Which statement best distinguishes the regulatory roles of troponin and tropomyosin in a thin myofilament?
A.
Troponin binds calcium, whereas tropomyosin directly covers or exposes actin's myosin-binding sites
B.
Troponin forms the actin backbone, whereas tropomyosin supplies ATP
C.
Troponin attaches actin to the Z-line, whereas tropomyosin forms myosin heads
D.
Troponin shortens actin, whereas tropomyosin lengthens the sarcomere
Explanation:
Troponin acts as the calcium-sensitive regulatory complex, while tropomyosin physically changes its position on actin to regulate access to myosin-binding sites.
7. A mutation prevents calcium from binding to the calcium-sensitive component of a thin myofilament. Which immediate molecular event would be impaired?
A.
Formation of the actin double helix
B.
Movement of tropomyosin away from actin's myosin-binding sites
C.
Polymerisation of myosin into thin filaments
D.
Binding of ATP to the actin monomer
Correct Answer:
Movement of tropomyosin away from actin's myosin-binding sites
Explanation:
Calcium binding to troponin is required for the conformational change that shifts tropomyosin. If calcium cannot bind, the actin sites remain blocked.
8. Which arrangement of proteins in a thin myofilament is most accurate?
A.
Two actin strands are associated with tropomyosin and periodically positioned troponin complexes
B.
Two tropomyosin strands surround a central chain of myosin heads
C.
Troponin forms two helices, while actin binds calcium at regular intervals
D.
Myosin heads form the backbone, with actin and troponin attached externally
Correct Answer:
Two actin strands are associated with tropomyosin and periodically positioned troponin complexes
Explanation:
A thin myofilament consists mainly of two helical actin strands, with tropomyosin lying along them and troponin complexes attached at intervals.
9. Why is the position of tropomyosin on actin important for skeletal muscle contraction?
A.
It determines whether myosin can access the binding sites on actin
B.
It determines whether actin can synthesize ATP
C.
It determines whether calcium can be stored inside myosin
D.
It determines whether thin filaments can become thick filaments
Correct Answer:
It determines whether myosin can access the binding sites on actin
Explanation:
Tropomyosin acts as a movable regulatory strand. Its position either blocks or exposes the sites on actin that interact with myosin heads.
10. Which change in a thin myofilament is most directly associated with the transition from relaxation to contraction?
A.
Calcium binds troponin, causing tropomyosin to uncover the actin-binding sites
B.
Actin monomers separate, eliminating all myosin-binding sites
C.
Troponin detaches permanently from actin and is replaced by myosin
D.
Tropomyosin binds ATP and converts into a contractile filament
Correct Answer:
Calcium binds troponin, causing tropomyosin to uncover the actin-binding sites
Explanation:
The transition begins when calcium binds troponin. The resulting movement of tropomyosin exposes actin's myosin-binding sites, allowing interaction with myosin heads.
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