Final answer:
Actin filaments, essential parts of the cell's cytoskeleton, have a helical structure formed by polymerized actin subunits in a twisted double helix.
Step-by-step explanation:
The correct answer to the structure of an actin filament is a helical structure.
Actin filaments, also known as microfilaments, are integral components of the cell's cytoskeleton. These filaments have a diameter of approximately 7 nm and are composed of polymerized globular actin (G-actin) subunits. The linear polymers of G-actin, known as F-actin, combine to form a twisted double helix.
This helical structure is vital for the actin filament's function in muscle contraction and cell motility. When paired with motor proteins like myosin, actin filaments undergo ATP-driven interactions that result in the contraction of muscle fibers or changes in cell shape. The actin filament's helical structure is also fundamental for providing rigidity and maintaining the structural integrity of cells.