Question

Study the following statements regarding contractile proteins of muscle. Identify the correct statements: I. Actin is a protein present in thin filament present in I-band of a sarcomere. II. Myosin forms the thick filament and contains ATPase activity in its tail region. III. Troponin binds calcium ions and helps in exposing the active sites on thin filament. IV. Tropomyosin actively hydrolyses ATP during muscle contraction.

• A. I, II and III
• B. I and IV
• C. I and III
• D. I, III and IV

Hint:

Actin = thin, Myosin = thick/ATPase-head.

Answer 1

The Correct Option is C

Step 1: The task.
We check four statements about muscle proteins and keep only the correct ones.

Step 2: Statement I, actin.
Actin makes up the thin filament, and the thin filament lies in the I-band of the sarcomere. This is true.

Step 3: Statement II, myosin.
Myosin does form the thick filament, but its ATPase (the part that splits ATP) is in the head, not the tail. The head also forms cross-bridges with actin. So this statement is wrong.

Step 4: Statement III, troponin.
Troponin binds calcium ions. When it does, it shifts tropomyosin away and uncovers the active sites on actin so the myosin heads can attach. This is true.

Step 5: Statement IV, tropomyosin.
Tropomyosin only blocks or unblocks the binding sites. It does not break down ATP. ATP is hydrolysed by myosin. So this statement is wrong.

Step 6: Conclusion.
Only I and III are correct, so the answer is "I and III". \[ \boxed{\text{I and III}} \]