In a contracted skeletal muscle fiber,

Options

A. M line disappears
B. H-zone elongates
C. I-band remains constant
D. A - band disappears

Answer

To solve the question about the contracted skeletal muscle fiber and the statements regarding the sarcomere, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding the Sarcomere Structure**:
- The sarcomere is the basic contractile unit of skeletal muscle fibers. It is composed of thick and thin filaments, primarily myosin (thick) and actin (thin).

2. **Analyzing Statement 1**: "A band is present in the middle of sarcomere."
- The A band is indeed located in the middle of the sarcomere. It contains the entire length of the thick filaments (myosin) and overlaps with thin filaments (actin) at the edges.
- **Conclusion**: This statement is **true**.

3. **Analyzing Statement 2**: "H zone is present in the middle of A band."
- The H zone is a region within the A band where there is no overlap between thick and thin filaments; it is the central part of the A band.
- **Conclusion**: This statement is **true**.

4. **Analyzing Statement 3**: "M line is present in middle of H zone."
- The M line is a structural protein that holds the thick filaments together and is located in the center of the H zone.
- **Conclusion**: This statement is **true**.

5. **Final Conclusion**: Since all three statements are true, the correct answer is "all of these."

### Final Answer:
The correct answer is option 4: **All of these**.

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