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MCAT Question A Day - 8/4/13 - Answer!

The proteins primarily responsible for preventing contraction in a resting muscle cell are

A.  myosin and troponin.
B.  myosin and tropomyosin.
C.  troponin and tropomyosin.
D.  myosin and actin.

The correct answer is (C). The troponin-tropomyosin complex binds to actin when a muscle cell is at rest so that the myosin heads (cross-bridges) cannot make contact with the active site along the actin. When calcium is released from the sarcoplasmic reticulum, the troponin-tropomyosin complex will shift to bind with calcium, thus allowing contact between the myosin heads and actin that is necessary for contraction to occur.