Which two proteins form the contractile filaments in skeletal muscle?

• A. Myosin and Actin
• B. Collagen and Elastin
• C. Tubulin and Dynein
• D. Keratin and Myosin

Answer: (A)

In skeletal muscle, movement comes from the interaction of thick and thin filaments inside the sarcomere. The thick filaments are mainly made of myosin, while the thin filaments are primarily actin. When contraction is triggered, calcium ions expose binding sites on actin, allowing myosin heads to attach, pull the actin filaments toward the center of the sarcomere (the power stroke), and use ATP to detach and re-cock for another cycle. This sliding filament mechanism shortens the sarcomere and generates force. Therefore, actin and myosin form the contractile filaments.

Other protein pairs listed don’t form the muscle’s contractile filaments: collagen and elastin are structural proteins in connective tissue; tubulin and dynein are components of microtubules involved in intracellular transport; keratin is an epithelial structural protein, and while myosin is present in muscle, it doesn’t pair with keratin to form the contractile filaments.