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Chapter 5

The Cytoskeleton

 
The cytoskeleton, a fundamental component of all eukaryotic cells, comprises filamentous protein polymers of three types: intermediate filaments, actin filaments, and micro-tubules. Intermediate filaments are the most “skeletal” in nature, giving the cytoplasm mechanical strength. Actin filaments and microtubules not only are skeletal but also “muscular”; they are responsible for the movements of cytoplasm and organelles. Actin filaments and microtubules are dynamic, turning over many times during their functional lifetimes. These polymers also are polar, in the sense of having different biochemical properties at each end. Their dynamic behavior and polarity are enhanced by free energy from the hydrolysis of nucleoside triphosphates. The biochemical properties of cytoskeletal polymers themselves appear to be widely conserved among eukaryotes. The different functions of the cytoskeleton are mediated by various accessory proteins, which assemble, disassemble, bundle, sever, cap, cross-link, or move the polymers. Actin filaments play starring roles in organelle movement. Microtubules are important for cell polarity and for controlling the direction of cell expansion. The cytoskeleton provides the machinery that separates the replicated chromosomes at mitosis and that partitions the daughter cells at cytokinesis. Mitosis depends on the mitotic spindle, a structure built largely from microtubules. Spindle microtubules interact with chromosomes at the kinetochore, a specialized protein-rich region that forms at the centromere. The dynamic properties of the microtubules, the architecture of the spindle, and the force-generating proteins at the kinetochore participate jointly in chromosome movement. The importance of each may vary at different phases of mitosis and in different cell types. Cytokinesis depends on cytoskeletal structures to ensure the appropriate orientation of the cell’s division and to divide the cell. During interphase, the phragmosome, a raft of cytoskeletal-rich cytoplasmic strands, often forms in the plane in which the cell division will take place. In prophase, the preprophase band, a ring of microtubules and actin filaments, usually marks the location where the new cell plate will later fuse with the parental cell wall. During cytokinesis itself, the phragmoplast, a structure built from microtubules and actin filaments, orchestrates the intense secretory activity needed to build the forming cell plate and guides the growing cell plate to the parent cell wall.

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