In a myofibril, the active sites are present on

Options

A. Actin filaments
B. Myosin filaments
C. Troponin
D. Trypomuosin

Answer

To answer the question "In a myofibril, the active sites are present on," we need to follow these steps:

### Step-by-Step Solution:

1. **Understanding Myofibrils**:
- Myofibrils are the basic rod-like units of a muscle cell (muscle fiber) and are composed of long, parallel filaments. These filaments are primarily made up of two types of proteins: actin and myosin.

2. **Identifying the Filaments**:
- The myofibrils consist of light bands and dark bands. The light bands are primarily composed of actin filaments, while the dark bands consist of myosin filaments.

3. **Structure of Actin Filaments**:
- Actin filaments, also known as thin filaments, are made up of two strands of F-actin that are twisted around each other.

4. **Role of Tropomyosin and Troponin**:
- Tropomyosin is a protein that wraps around the actin filaments and serves to block the active sites on actin. Troponin is another protein that is attached to tropomyosin and helps regulate the interaction between actin and myosin.

5. **Active Sites on Actin**:
- The active sites for myosin binding are located on the actin filaments. However, in a resting state, these active sites are masked by tropomyosin, which is held in place by troponin.

6. **Conclusion**:
- Therefore, the active sites that myosin binds to are present on the actin filaments, specifically masked by the tropomyosin in the resting state.

### Final Answer:
In a myofibril, the active sites are present on **actin filaments**.

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