Actin filaments consist of two strands of polymerized actin monomers, twisted into a helix with 13.5 molecules per turn. Consider how the shape of this molecule might relate to its ability to polymerize. Electron micrographs indicate the parallel double-helical form of these filaments. The structure is indicated below. Each sphere represents a globular actin monomer.
Actin filaments are the major component of thin filaments of muscle tissue.
Microtubules Microtubules are composed of dimers of the protein tubulin. The two subunits are homologous. In vivo, most microtubules contain thirteen protofilaments. A ring of nine doublet microtubules, with two singlet microtubules at the centre, forms the basis of cilia and flagella . In many cases, this nucleocapsid is enveloped by a lipid bilayer and other proteins.