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 Sarcomere Structure**:
- The A band is a part of the sarcomere, which is the basic contractile unit of skeletal muscle. The sarcomere is made up of thick and thin filaments.

2. **Identifying Filaments**:
- The thick filaments are primarily composed of myosin, while the thin filaments are primarily composed of actin.

3. **Analyzing the A Band**:
- The A band appears dark under a microscope because it contains both thick (myosin) and thin (actin) filaments that overlap.

4. **Evaluating the Options**:
- **Option 1**: Overlapping of the Myosin Filament and the Actin Filament.
- This is correct because the overlap of these filaments contributes to the dark appearance of the A band.
- **Option 2**: Concentration of the Myosin Filaments only.
- This is incorrect; the concentration of myosin filaments alone is not responsible for the A band’s dark color.
- **Option 3**: Concentration of the Actin Filaments only.
- This is also incorrect; the concentration of actin filaments alone corresponds to the I band, not the A band.
- **Option 4**: Concentration of Troponin and Tropomyosin.
- This is incorrect as well; troponin and tropomyosin are regulatory proteins associated with actin but do not determine the color of the A band.

5. **Conclusion**:
- The dark color of the A band is due to the overlapping of myosin and actin filaments. Therefore, the correct answer is **Option 1**.