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Figure 1.18 Organization of actin, microtubules, and intermediate filaments within a cell
(from http://www.sparknotes.com/biology/cellstructure/intracellularcomponents/section1.rhtml ).
The Structure of the Cytoskeleton
The cell-consuming cytoskeleton is a dynamic network of three types of protein fila-
ments, microtubules, actin filaments (microfilaments), and intermediate filaments
( Figure 1.18 ). Cytoskeleton occupies the whole volume of the cell. In many eukary-
otic unicellulars, it determines the formation of pseudopodia, which are locomotor
appendages. The term cytoskeleton was used first in 1903 by the Russian biologist
Nikolai K. Koltsov (1872-1940).
The cytoskeleton determines a cell's shape and changes of that shape. It is respon-
sible for cell motility. In eukaryotic unicellulars, it often forms locomotor append-
ages (pseudopods) by extending and contracting actin microfilaments, which make
the movement of the cell possible. For locomotion, other unicellulars use cilia and
flagella, which are also part of the cytoskeleton.
Microtubules
These are the longest cytoskeleton filaments, with an average diameter of 25 nm. These
self-assembling hollow tubules are formed by 13 protofilaments of heterodimers of
tubulin (α- and β-tubulin), globular protein molecules ( Figure 1.19 ). Microtubules are
also part of the structure of cilia and flagella. They are dynamic molecules that elon-
gate or shorten, according to the needs of molecule transportation, organelle locali-
zation, etc. In animal cells, the minus end of the microtubule is enucleated to the
centrioles of the centrosome (see later on their structure and function see Chapter 3,
section “Epigenetic modes of cell differentiation”), while the plus end is free in cyto-
plasm. The polymerization/depolymerization of microtubules is faster at the plus end.
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