The cytoskeleton is a network of protein filaments that extends throughout the cytoplasm. It provides structure and organization to the cell, determining shape and positioning organelles. The three main types of filaments are actin filaments, intermediate filaments, and microtubules. Actin filaments are the thinnest filaments and form structures like filopodia, lamellipodia, and stress fibers. Microtubules are hollow cylinders composed of tubulin dimers and originate from the centrosome. They are involved in processes like cell division, organelle transport, and motility. Cilia and flagella project from the cell surface and use microtubule motors for movement.

2. CYTOSKELETON

3. • The cytoskeleton,is the network of protein filaments
extending throughout the cytoplasm of all eukaryotic
cells
• The cytoskeleton provides a structural framework for
the cell, that determines cell shape,the positions of
organelles, and the general organization of the
cytoplasm
• The cytoskeleton is composed of three principal types
of protein filaments such as Actin filaments,
Intermediate filaments, and Microtubules

5. FUNCTIONS OF CYTOSKELETON
• Establishing cell shape
• Providing mechanical strength
• Locomotion
• Chromosome separation in mitosis and meiosis
• Intracellular transport of organelles

6. • The major cytoskeletal protein of most cells is Actin
• which polymerizes to form actin filaments- thin,
flexible fibers approximately 7 nm in diameter and
up to several micrometers in length
• Being the thinnest of the cytoskeletal filaments,This
are also called Microfilaments

7. (Actin cytoskeleton stained with phalloidin

8. • In skeletal muscle fibers they are called Thin
filaments
• Generate Cytoplasmic streaming in some cells
• Generate Locomotion in cells such as white blood
cells
• Interact with myosin ("thick") filaments in skeletal
muscle fibres to provide the force of muscular
contraction

9. ACTIN STRUCTURE

10. • Filopodia (also called Microspikes) are long, thin and
transient structures that extend out from the cell surface.
• Bundles of parallel actin filaments, with their plus ends
oriented toward the filopodial tip
• Microvilli are shorter and more numerous protrusions of
the cell surface found in some cells.
• Tightly bundled actin filaments within these structures
their plus ends oriented toward the tip
• Small cross-linking proteins such as fimbrin and
villin bind actin filaments together within microvilli
provide stiffness

12. Lamellipodia are thin but broad projections at the
edge of a mobile cell. Lamellipodia
are dynamic structures, constantly changing shape.

13. • Stress fibers form when a cell makes stable
connections to a substrate
• Bundles of actin filaments extend from the cell
surface through the cytosol. The actin filaments,
whose plus ends are oriented toward the cell
surface
• Myosin mediates sliding of anti-parallel actin
filaments during contraction of stress fibers
• -Actinin may cross-link actin filaments within
stress fibers

15. • Some cells have a cytoskeletal network just inside the
plasma membrane that includes actin along with various
other proteins such as spectrin
• This cytoskeleton has a role in maintaining cell shape.
An example is found in erythrocytes
• Spectrin is an actin-binding protein that forms an
elongated tetrameric complex having an actin-binding
domain at each end
• With short actin filaments, spectrin forms a cytoskeletal
network on the cytosolic surface of the plasma
membrane of erythrocytes and some other cells

16. Spectrin - actin-binding

17. ORGANISATION OF ACTIN FILAMENTS
• Individual actin filaments are assembled into two general types
of structures called Actin bundles and Actin networks,
which play different roles in the cell.
• The first type of bundle, containing closely spaced actin
filaments aligned in parallel supports projections of the plasma
membrane, such as microvilli
• In these bundles, all the filaments have the same polarity, with
their barbed ends adjacent to the plasma membrane
• An example of a bundling protein involved in the formation of
these structures is fimbrin
• which was first isolated from intestinal microvilli and later
found in surface projections of a wide variety of cell types

19. • The second type of actin bundle is composed of
filaments that are more widely spaced, allowing the
bundle to contract
• In contrast to fimbrin, actin binds to actin as a
dimer, each subunit of which is a 102 kd protein
containing a single actin-binding site

20. • The actin filaments in networks are held together by large actin-
binding proteins, such as filamin.
• Filamin binds actin as a dimer of two 280 kd subunits. The
actin-binding domains and dimerization domains are at opposite
ends of each subunit
• so the filamin dimer is a flexible V-shaped molecule with actin-
binding domains at the ends of each arm.
• As a result, filamin forms cross-links between orthogonal actin
filaments, creating a loose three-dimensional mesh .

22. They have a diameter of about 25 nm
They are variable in length but can grow 1000 times as long as
they are wide
They are built by the assembly of dimers of alpha
tubulin and beta tubulin
They are straight, hollow cylinders whose wall is made up of a
ring of 13 "protofilaments”
They are found in both animal and plant cells.
In plant cells, microtubules are created at many sites scattered
through the cell.
In animal cells, the microtubules originate at the centrosome.

24. • The attached end is called the minus end; the other
end is the plus end
• grow at the plus end by the polymerization of tubulin
dimers (powered by the hydrolysis of GTP)
 Microtubules participate in a wide variety of cell
activities
• Most involve motion. The motion is provided by
protein "motors" that use the energy of ATP to move
along micotubule

26. Microtubule motors
• There are two major groups of microtubule motors:
• kinesins most of these move toward the plus end of the
microtubules
• dyneins which move toward the minus end

28. Some examples:
• The migration of chromosomes in mitosis and
meiosis takes place on microtubules that make up the
spindle fibres
• Both Kinesins and Dyneins are used as motors.
• The rapid transport of organelles, like vesicles and
mitochondria, along the axons of neurons takes
place along microtubules
• The motors used are kinesins

29. Vincristine, a drug found in the Madagascar
periwinkle (a wildflower), binds to tubulin dimers
preventing the assembly of microtubules. This halts
cells in metaphase of mitosis.
Taxol, a drug found in the bark of the Pacific yew,
prevents depolymerization of the microtubules of the
spindle fiber. This, in turn, stops chromosome
movement, and thus prevents the completion of
mitosis.
• Because the hallmark of cancer cells is uncontrolled
mitosis,both vincristine and Taxol are used as
anticancer drugs .

31. Madagascar periwinkle
Vincristine
Taxol
Pacific yew

32. STRUCTURE OF MICROTUBULE
• The building blocks of microtubules are tubulin dimers
consisting of two closely related 55 kd polypeptides: α-
tubulin and β-tubulin
• Like actin, both α and β tubulin are encoded by small
families of related genes
• In addition, a third type of tubulin (y-tubulin) is concentrated
in the centrosome where it plays a critical role in initiating
microtubule assembly
• Tubulin dimers can depolymerize as well as polymerize, and
micro-tubules can undergo rapid cycles of assembly and
disassembly.

34. ASSEMBLY OF MICROTUBULES
• In animal cells, most microtubules extend outward from the
centrosome
• It was first discovered by Theoder Boveri in 1888
• During mitosis Microtubules similarly extend outward from
duplicated centrosomes to form mititic spindle
• which is responsible for the separation and distribution of
chromosomes to daughter cells

35. • Microtubules grow by the addition of tubulin to
their plus ends
• which extend outward from the centrosome
toward the cell periphery, so the role of the
centrosome is to initiate microtubule growth.
• The centrosomes of most animal cells contain a
pair of centrioles, oriented perpendicular to each
other
• The centrioles are cylindrical structures based on
nine triplets of microtubules, similar to the basal
bodies of cilia and flagella.

36. Cilia and Flagella

37. • Cilia and flagella are microtubule-based projections
of the plasma membrane that are responsible for
movement of a variety of eukaryotic cells
• Many bacteria also have flagella, but these
prokaryotic flagella are quite different from those of
eukaryotes
• Bacterial flagella are protein filaments projecting
from the cell surface

39. • Eukaryotic cilia and flagella are very similar
structures, each with a diameter of approximately
0.25 pm
• The fundamental structure of both cilia and
flagella is the Axoneme,which is composed of
microtubules and their associated proteins
• The microtubules are arranged in a characteristic
"9 + 2" pattern in which a central pair of
microtubules is surrounded by nine outer micro-
tubule doublets