__________ and ______are contractile proteins of muscles.

Answer

**Step-by-Step Solution:**

1. **Understanding Muscle Composition**: Muscles are made up of muscle fibers, which contain smaller units called myofibrils. Each myofibril consists of different types of proteins.

2. **Types of Proteins in Myofibrils**: The proteins in myofibrils can be categorized into three main types: contractile proteins, regulatory proteins, and structural proteins.

3. **Identifying Contractile Proteins**: The question specifically asks for contractile proteins. The two primary contractile proteins found in muscles are **actin** and **myosin**.

4. **Function of Actin and Myosin**: Actin is known as the thin filament, while myosin is referred to as the thick filament. These proteins interact to facilitate muscle contraction.

5. **Structure of Actin**: Actin filaments are composed of two helical strands of filamentous actin (F-actin), which are made up of multiple units of globular actin (G-actin).

6. **Regulatory Proteins**: In addition to actin, regulatory proteins such as tropomyosin and troponin are present at regular intervals along the actin filament. These proteins play a crucial role in muscle contraction by covering the binding sites for myosin on actin during muscle relaxation.

7. **Structure of Myosin**: Myosin is composed of multiple units of myosin molecules, which have a globular head (heavy meromyosin) and a tail (light meromyosin). The heads of myosin project outwards and are involved in forming cross-bridges with actin during contraction.

8. **Final Answer**: Therefore, the two contractile proteins of muscles are **actin** and **myosin**.

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