Spindle fibres are chemically formed of ...................

Explanation

**Step-by-Step Solution:**

1. **Understanding Spindle Fibers**: Spindle fibers are essential structures that play a crucial role during cell division, specifically in the process of mitosis.

2. **Composition of Spindle Fibers**: Spindle fibers are primarily composed of proteins. The main protein that makes up these fibers is called tubulin.

3. **Types of Tubulin**: There are two types of tubulin proteins that combine to form the spindle fibers: alpha-tubulin and beta-tubulin. These proteins polymerize to form a structure known as polymeric tubulin.

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