Images removed due to copyright considerations See Chapter 2, pp 70-72 in [Alberts-ECB Alberts, B, et al. Essential Cell Biology. New York: Garland Publishing, 1988Images removed due to copyright considerations. See Chapter 2, pp. 70-72 in [Alberts-ECB]. Alberts, B., et al. Essential Cell Biology. New York: Garland Publishing, 1988