The thick filament of a myofibril is part of the muscle fibre. Select the incorrect statement regarding it.

Options

A. Each thick filament is a polymerized protein of monomeric meromyosins.
B. Each meromyosin has two important parts, a globular head with a short arm and a tail.
C. The heavy meromyosin components from the cross arms of thick filaments.
D. Short arms of the light meromyosin has an active ATPase enzyme activity.

Answer

To solve the question regarding the incorrect statement about the thick filament of a myofibril, we will analyze each provided option step by step.

### Step-by-Step Solution:

1. **Understanding Muscle Structure**:
- Skeletal muscles consist of myofibrils, which are made up of thick and thin filaments.
- The thick filaments are primarily composed of the protein myosin, while the thin filaments are composed of the protein actin.

2. **Identifying Thick Filaments**:
- The thick filament is formed by polymerized units of myosin, specifically monomeric units known as meromyosin.
- Myosin can be divided into two parts: heavy meromyosin and light meromyosin.

3. **Analyzing the Options**:
- **Option A**: "Each thick filament is a polymerized form of the monomeric meromyosins."
- This statement is true because thick filaments are indeed made up of polymerized myosin units (meromyosins).

- **Option B**: "Each meromyosin has two important parts: a globular head with a short arm and a tail."
- This statement is also true. The structure of meromyosin includes a globular head (which has ATPase activity) and a tail.

- **Option C**: "The heavy meromyosin components form the cross arm of thick filaments."
- This statement is true as well. Heavy meromyosin contributes to the structure of the thick filament, including the cross-bridges that interact with actin.

- **Option D**: "The short arms of the light meromyosin have an active ATPase enzyme activity."
- This statement is incorrect. The ATPase activity is associated with the heavy meromyosin, not the light meromyosin. The light meromyosin does not have ATPase activity.

4. **Conclusion**:
- The incorrect statement regarding the thick filament of a myofibril is **Option D**: "The short arms of the light meromyosin have an active ATPase enzyme activity."