Mitotic spindle is mainly composed of __ proteins.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Mitotic Spindle**: The mitotic spindle is a structure that is essential for cell division, specifically during the processes of mitosis and meiosis. It helps in the separation of chromosomes.

2. **Formation of the Mitotic Spindle**: The mitotic spindle is formed during the prophase stage of cell division. During this phase, certain structures called centrioles move to opposite ends of the cell.

3. **Role of Centrioles**: Centrioles are wheel-shaped protein structures that help in organizing the microtubules that make up the spindle apparatus. They establish the poles of the cell.

4. **Microtubules**: From the centrioles, protein fibers known as microtubules extend in various directions. These microtubules are crucial for forming the mitotic spindle.

5. **Composition of Microtubules**: Microtubules are primarily made up of proteins called tubulin. Tubulin is a type of protein that consists of small units, and it plays a significant role in the structure and function of the mitotic spindle.

6. **Final Answer**: Therefore, the mitotic spindle is mainly composed of **tubulin proteins**.

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