Nucleation (lag phase), elongation (growth phase) and steady state (equil. phase)

ATP-G-actin monomers (G for globular) form stable oligomers of actin (filament nuclei) slowly. Nuclei are rapidly elongated by addition of subunits to both ends. In the third phase the ends of F-actin filaments (F for filament) are in a steady state with monomeric G-actin. This is achieved at the critical concentration Cc. Below the Cc the filament will disassemble, above the Cc the filament will get longer. If preformed filaments are added there is no lag (nucleation) phase but the Cc remains constant.