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Chapter
5 |
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The
Cytoskeleton
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| 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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