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Actin is one of the proteins composing the cytoskeleton. Actin filaments are polymers made up of globular actin monomers called G-actin. When these polymerise, using ATP, they form actin filaments, referred to as F-actin; filamentous actin. Under the right conditions, G-actin will self-assemble and form F-actin. Two actin filaments then form a double helix.&nbsp; 

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The function of actin filaments is to provide the cell with strength, shape and movement. Actin underlies the plasma membrane, and can form many cell surface projections such as lamellopodia, filopodia (for movement), stereocilia (eg response to sound in the inner ear) and microvilli (to increase the surface area to make absorption more efficient). Actin filaments are also responsible for the division of cells after nuclear division has occured. This happens by actin forming a contractile ring, separating the two cells. 

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Actin filaments are also extremely important in muscle contraction, as this is where myosin binds to induce contraction. 

<br> '''References''' Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., Walter, P., (2008) Molecular biology of the cell, 5th edition. New York: Garland Science 

Berg, J.M., Tymoczko J.L., Stryer, L., (2012), Biochemistry, 7th edition, Houndsmills: W.H. Freeman and Company