Dark colour of A - band is due to :

Options

A. Overlapping of myosin filaments and actin filaments
B. Concentration of myosin filaments only
C. Concentration of actin filaments only
D. Concentration of troponin and tropomyosin

Answer

### Step-by-Step Solution:

1. **Understanding the Structure of Muscle Fibers**:
- Muscle fibers are composed of myofibrils, which contain repeating units called sarcomeres. Each sarcomere has distinct bands: the A band, I band, and H zone.

2. **Identifying the A Band**:
- The A band is the dark band in the sarcomere. It is primarily composed of thick myosin filaments and some overlapping thin actin filaments.

3. **Reason for the Dark Color of A Band**:
- The dark color of the A band is due to the overlapping of myosin and actin filaments. When these two types of filaments overlap, they create a denser area that appears darker under a microscope.

4. **Excluding Other Options**:
- Option B (concentration of myosin filament only) is incorrect because the A band is not solely made of myosin; it also includes overlapping actin.
- Option C (concentration of actin filament only) is incorrect because the A band is not formed by actin alone; it is lighter and corresponds to the I band.
- Option D (concentration of troponin and tropomyosin) is also incorrect as these proteins are regulatory and do not contribute to the dark appearance of the A band.

5. **Conclusion**:
- Therefore, the correct answer is that the dark color of the A band is due to the overlapping of myosin and actin filaments.

### Final Answer:
The dark color of the A band is due to the overlapping of myosin filament and actin filament.

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