thin and thick filaments

there are two types of filaments . One of them is thick (a band) and another is thin ( i band) filaments.

1. THICK FILAMENT (A band)
myosin: 6 polypeptides twisted to form fiber helix with globular end, which has ATPase activity & an affinity to bind to actin

2. THIN FILAMENT (I band)
G-actin: globular protein which polymerizes into polymeric fiber, each globular actin unit contains a myosin binding site

3. Tropomyosin: fiber-like protein which wraps helically around thin filament

4. Troponin: globular protein complex which binds Ca+2 & initiates contraction cycle

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