Which filament forms the thick filaments in a sarcomere?

• A. Actin
• B. Titin
• C. Nebulin
• D. Myosin

Answer: (D)

Thick filaments are formed by myosin. In a sarcomere, myosin molecules assemble into long, rod-like thick filaments with globular heads that protrude outward. These heads bind to the actin molecules of the thin filaments and, using energy from ATP hydrolysis, pull the actin filaments toward the center of the sarcomere to generate contraction. The other proteins play different roles: actin makes up the thin filament; nebulin helps set and stabilize thin filament length; and titin is a huge elastic protein that spans from the Z-disc to the M-line, supporting sarcomere structure and aligning thick filaments. Because the question asks which filament forms the thick filaments, myosin is the correct choice.