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The eukaryotic cells possess a skeletal system
called cytoskeleton that has got analogous
The cytoskeleton is composed of 3 well defined
filamentous structurs – microtubules,
microfilaments and intermediate filaments with
distinct functions .
Each filaments are made of protein subunits
held together by weak non covalent bonds.
This type of construction allows rapid assembly
and disasembly contolled by cell regulation
They are components of a diverse array of substances including the mitotic
spindles of dividing cells and core of flagella and cilia
STRUCTURE AND COMPOSITION
Have an outer diameter of 25nm and a wall thickness of 4nm and may extend
across the length and breadth of the cell
The wall of microtubule is composed of globular proteins arranged in
longitudinal rows called protofilaments that are allinged parallel to the long axis
of the tubule .
When veiwed in cross section they are seen
to have 13 protofilaments allinged side by
side in a circular pattern
Each protofilament is assembled from
dimeric building blocks consisting of one
alpha and one beta subunits
The protofilament is asymmetric with alpha
subunit on one end and beta on the other
One end of the protofilament is known as
the plus end is terminated by a row of beta
tubulin units and the mnus end is
terminated by the alpha tubulin units
1. ACT AS STRUCTURAL SUPPORT AND
They are stiff enough to resist the forces that can
bend or compress the fibre
The distribution of microtubules through the
cytoplasm of a cell determines the shape of a cell
eg: in coloumnar epithelial cells the microtubules
are alligned along the axis of the cell
Maintain a key role in the internal organization of
2.ACT AS AGENTS OF
Involved in the movement of
the cytoplasm throught the cell
Eg: AXONAL TRANSPORT: proteins
such as neurotransmittors are
secreated and packed in membranous
vesicles by golgi body and
endoplasmic reticulum of the cell
body are transported through the
axon which consists of a number of of
microtubules and motor proteins
which takes it down the axon
MOTOR PROTEINS :
they convert chemical energy into mechanical energy that
is used to generate force or move the Types of cargo include
vesicles,chromosomes , mitochondria, proteins etc
They can be classified into 3 types mainly : kinesins and
dyneins that move along the microtubules and myosin that
move along microfilaments
The binding of ATP and its hydrolysis provides energy to
them to travel
cargo attached to the motor
KINESINS: is a tetramer constructed by 2 identical
heavy chains and 2 identical light chains, has a
globular head that binds ATP ,a neck a stalk and a fan
shaped tail that binds to the cargo to be transported
DYENEINS: it is a huge protein composed of two identical
heavy chains and a variety of intermediate and light
chains. They are responsible for the movement of cilia and
flagella .move towards the minus end of the microtubule.
They act as:
As a force-generating agent in positioning the spindle and
moving chromosomes during mitosis
As a minus end–directed microtubular motor with a role
in positioning the centrosome and Golgi complex and
moving organelles,vesicles,and particles through the
1. CENTROSOME: In animal cells, the
centrosome has a pair of centrioles,
each with nine triplets of microtubules
arranged in a ring.
• the centroles are surrounded by an
electron rich pericentriolar matrix
Microtubules are major
components of spindle
fibre used to pull apart
chromosomes during cell
2. BASAL BODIES
the outer microtubules in a cillia or flagella arise from
basal bodies attached to the base of cillia or flagella
Microtubules are the central structural supports in
cilia and flagella.
Both can move unicellular and small multicellular
organisms by propelling water past the organism.
If these structures are anchored in a large structure,
they move fluid over a surface.
For example, cilia sweep mucus carrying trapped debris from
A flagellum has an undulatory movement.
Force is generated parallel to the flagellum’s axis.
Cilia move more like oars with alternating power and
They generate force perpendicular to the cilia’s axis.
In spite of their differences, both cilia and flagella have
the same ultrastructure.
Both have a core of microtubules sheathed by the plasma
Nine doublets of microtubules arranged around a pair at
the center, the “9 + 2” pattern.
Flexible “wheels” of proteins connect outer doublets to
each other and to the core.
The outer doublets are also connected by motor
The cilium or flagellum is anchored in the cell by a basal
body, whose structure is identical to a centriole.
The bending of cilia and flagella is driven by the arms of
a motor protein, dynein.
Addition to dynein of a phosphate group from ATP and
its removal causes conformation changes in the protein.
Dynein arms alternately
grab, move, and release
the outer microtubules.
Protein cross-links limit
sliding and the force is
expressed as bending.
the thinnest class of the cytoskeletal fibers, are
solid rods of the globular protein actin.
An actin microfilament consists of a twisted double
chain of actin subunits.
Microfilaments are designed to resist tension.
With other proteins, they form a three-
dimensional network just inside the plasma
In muscle cells, thousands of actin filaments are
arranged parallel to one another.
Thicker filaments, composed of a motor protein,
myosin, interdigitate with the thinner actin fibers.
Myosin molecules walk along the actin filament, pulling
stacks of actin fibers together and shortening
In other cells, these actin-myosin aggregates are less
organized but still cause localized contraction.
A contracting belt of microfilaments divides the
cytoplasm of animals cells during cell division.
Localized contraction also drives amoeboid movement.
Pseudopodia, cellular extensions, extend and contract
through the reversible assembly and contraction of actin
subunits into microfilaments.
In plant cells (and others), actin-myosin interactions
and sol-gel transformations drive cytoplasmic
This creates a circular flow of cytoplasm in the cell.
This speeds the distribution of materials within the cell.