In a resting muscle fibre, tropomyosin partially covers

Options

A. Ca-binding sites on actin
B. Ca-binding sites on troponin
C. Actin binding sites on myosin
D. Myosin binding sites on actin.

Answer

To solve the question regarding what tropomyosin partially covers in a resting muscle fiber, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Muscle Fiber Structure**:
- Muscle fibers contain two main types of filaments: actin (thin filaments) and myosin (thick filaments).
- Actin filaments have specific binding sites for myosin to facilitate muscle contraction.

2. **Role of Tropomyosin**:
- Tropomyosin is a regulatory protein that winds around the actin filaments.
- In a resting muscle fiber, tropomyosin covers specific binding sites on the actin filament.

3. **Binding Sites**:
- The binding sites that tropomyosin covers are crucial for muscle contraction. These are the sites where myosin heads attach to actin during the contraction process.

4. **Interaction with Troponin**:
- Tropomyosin works in conjunction with another protein called troponin. When calcium ions bind to troponin, it causes a conformational change that moves tropomyosin away from the binding sites on actin.

5. **Conclusion**:
- Therefore, in a resting muscle fiber, tropomyosin partially covers the myosin binding sites on actin. This prevents myosin from binding to actin, thus inhibiting muscle contraction until the appropriate signal (calcium ions) is received.

### Final Answer:
In a resting muscle fiber, tropomyosin partially covers the **myosin binding sites on actin**.

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