Practice important
Thick Myofilaments
MCQ questions and answers.
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MCQ Questions
1. Which structural arrangement of myosin molecules gives a skeletal-muscle thick filament its bipolar organization?
A.
Myosin heads point toward the center, while tails project toward both ends
B.
Myosin tails are packed centrally, while the globular heads project outward in opposite directions
C.
All myosin heads point toward one end, producing a unipolar filament
D.
Myosin heads and tails alternate regularly along the filament circumference
Correct Answer:
Myosin tails are packed centrally, while the globular heads project outward in opposite directions
Explanation:
In a thick filament, myosin tails form the central shaft and the globular heads project toward both ends. This opposite orientation produces a bipolar filament.
2. The central bare zone of a skeletal-muscle thick filament is characterized primarily by the absence of:
A.
Myosin tails
B.
M-line proteins
C.
Myosin globular heads
D.
Titin molecules
Correct Answer:
Myosin globular heads
Explanation:
The bare zone lies around the center of the thick filament and lacks projecting myosin heads, although myosin tails and associated structural proteins remain present.
3. Which part of a myosin molecule directly forms cross-bridges with actin during contraction?
A.
The coiled-coil tail
B.
The light meromyosin region
C.
The globular head
D.
The central bare-zone shaft
Correct Answer:
The globular head
Explanation:
The globular head of myosin contains the actin-binding site and ATPase activity, allowing it to form and cycle through cross-bridges.
4. Which pairing correctly relates a myosin structural region to its principal function in a thick filament?
A.
Heavy meromyosin tail — anchors the thick filament directly to the Z disc
B.
Globular myosin head — binds actin and hydrolyses ATP
C.
Light meromyosin head — catalyses ATP hydrolysis
D.
M-line segment — binds calcium ions to activate myosin
Correct Answer:
Globular myosin head — binds actin and hydrolyses ATP
Explanation:
The globular head, part of heavy meromyosin, contains both the actin-binding region and the ATPase site essential for cross-bridge cycling.
5. Why does the thick filament retain a central region without projecting myosin heads?
A.
The central region consists mainly of overlapping actin molecules
B.
Myosin heads are arranged only at the two polar regions, leaving a bare zone near the center
C.
ATP prevents myosin heads from forming in the central region
D.
Titin actively removes myosin heads from the center during contraction
Correct Answer:
Myosin heads are arranged only at the two polar regions, leaving a bare zone near the center
Explanation:
The myosin heads project from the thick-filament shaft in opposite directions on either side of the center. Their absence near the center creates the bare zone.
6. Which statement best distinguishes the two major portions of a myosin molecule in relation to thick-filament organization?
A.
The head is fibrous and forms the shaft, whereas the tail is globular and binds ATP
B.
The tail forms the central filament backbone, whereas the head projects outward and performs motor functions
C.
Both head and tail are globular and independently form separate thick filaments
D.
The tail binds actin directly, whereas the head connects adjacent thick filaments
Correct Answer:
The tail forms the central filament backbone, whereas the head projects outward and performs motor functions
Explanation:
Myosin tails associate to build the thick-filament backbone. The projecting globular heads interact with actin and use ATP to generate force.
7. A mutation prevents myosin tails from assembling into a stable central shaft but leaves their globular heads functional. What would be most directly disrupted?
A.
Formation of an organized thick filament
B.
ATP binding by individual myosin heads
C.
Actin's ability to form thin filaments
D.
Calcium storage inside the sarcoplasmic reticulum
Correct Answer:
Formation of an organized thick filament
Explanation:
Myosin-tail interactions are required to assemble individual myosin molecules into a stable thick filament. Functional heads alone cannot produce normal thick-filament architecture.
8. Which feature most directly explains why myosin heads on opposite halves of a thick filament pull actin toward the center during contraction?
A.
The heads are arranged with opposite polarity on the two halves
B.
The tails contain two independent calcium-binding sites
C.
The bare zone contains contractile actin heads
D.
The M line reverses the direction of ATP hydrolysis
Correct Answer:
The heads are arranged with opposite polarity on the two halves
Explanation:
Myosin heads have opposite orientations on the two halves of the bipolar thick filament. Their power strokes therefore draw actin filaments toward the central region.
9. Which component is most closely associated with maintaining the longitudinal alignment and elastic centering of a thick filament within the sarcomere?
A.
Titin
B.
Myosin ATPase site
C.
The actin-binding cleft of the myosin head
D.
The myosin neck alone
Correct Answer:
Titin
Explanation:
Titin is an elastic structural protein associated with thick filaments. It helps center and align them while contributing to passive elasticity.
10. If a thick filament were examined from one end toward its center, which sequence would be expected?
A.
Bare zone followed by myosin heads followed by myosin tails
B.
Projecting myosin heads followed by a head-free central bare zone
C.
Myosin tails followed by actin heads followed by the M line
D.
Titin heads followed by myosin heads followed by a Z disc
Correct Answer:
Projecting myosin heads followed by a head-free central bare zone
Explanation:
Each half of a bipolar thick filament contains projecting myosin heads that extend toward the center, but the central portion itself lacks heads and forms the bare zone.
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