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( Treadmilling)
Treadmilling is a phenomenon observed in many cellular cytoskeletal filaments, especially in actin filaments and microtubules. It occurs when one end of a filament grows in length while the other end shrinks resulting in a section of filament seemingly "moving" across a stratum or the cytosol. This is due to the constant removal of the protein subunits from these filaments at one end of the filament while protein subunits are constantly added at the other end[1]. The cytoskeleton is a highly dynamic part of a cell. Cell movement (i.e. a macrophage moving to engulf a foreign object) is often mediated by the synthesis of filaments in one direction of the cell with the front denoted as the leading edge. Filaments are able to move through the synthesis of subunits. Although both sides of a filament are able to grow in length through this subunit addition, one end of the filament (denoted as the positive end) will always be more dynamic and be able to grow faster than the other end (a.k.a. the negative end)[2] This difference is resultant from the fact that the negative end requires that the subunit undergo a conformational change to attach onto the end and consequently, filamental subunits are directionalized (a.k.a. polarized[3]) and must be added onto the filament in the correct orientation for synthesis to occur. In the very same way, subunit removal (shrinkage) is mediated by the same process, with the positive end being able shrink faster than the negative end. What determines whether the end grows or shrinks is entirely dependent on the concentration of available subunit monomers in the surrounding area[4]. Hence there is concentration of the subunit called the critical concentration (CC) at which the growth rate is balanced by the shrinkage rate, and where
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