Question

In a resting muscle fibre, tropomyosin partially covers
A.Ca-binding sites on action
B.Ca-binding sites on troponin
C.Actin binding sites on myosin
D.Myosin binding sites on actin

Answer 1

Hint: Muscle contraction is initiated by a signal sent by the central nervous system which causes the release of calcium ions in the sarcoplasm.These calcium ions binds to the troponin subunit.

Complete step by step answer:
Tropomyosin forms a covering for myosin binding sites on actin molecules, in an attempt to prevent cross-bridge formation, which in turn prevents contraction during a muscle without nervous input. The protein complex troponin binds to tropomyosin, helping to position it on the actin molecule.
When the muscle is at rest, tropomyosin prevents the interactions between thin and thick filaments. Tropomyosin may be a double-helical protein that covers the active sites for myosin on the individual actin molecules, thus preventing the actin-myosin interaction necessary for the contraction of muscle. To initiate contraction, Ca++ ions are needed to move the regulatory proteins, tropomyosin and troponin, to show the myosin-binding site on an actin filament to permit cross-bridge formation between the actin and myosin microfilaments.

So, the right answer is option D. Myosin binding sites on Actin.

Note:
 It is important to note that for the thinner filaments to still slide past thick filaments during contraction, myosin heads must pull the action at the binding sites, detach, then attach to more binding sites, pull, detach. Each cycle requires energy by the expenditure of ATP.