A network of filamentous protein structures present in the protoplasm is collectively referred to as the cytoskeleton.
It provides shape and rigidity to the cell.
It is responsible for holding organelles in their proper position and the formation of cilia, flagella, and microvilli.
It involved in process of vascular transport and rigidity and flexibility of plasma membrane.
It is composed of three basic protein filaments –
(A)Microtubules: Microtubules occur in many several specialized cellular structures like cilia, flagella, centrioles, astral rays, spindle apparatus, chromosomes fibers, basal bodies, sperm tail, sensory hair, nerve processor. the diameter of microtubule is about 25nm, with a hollow core of about 15nm, surrounded by a wall of 5 nm two components of tubulin protein are α-tubulin and β-tubulin.
Tubulin is a dimer consisting of two closely related 55 kD polypeptide.
A small amount of non-tubulin protein also occurs in the microtubule. These are called microtubular associated protein (MAP).
The third type of tubulin γ – tubulin is present in centrosome where it initiates microtubule assembly.
Molecular motor remains associated with microtubules are two types –
Kinesin generally moves towards plus end of a microtubule away from centrosome and the dyneins move towards minus end near centrosome.
Drug Effect: –
The drug colchicine, vincristine, and vinblastine inhibit polymerization of tubulin at its positive end and taxol drug inhibit depolymerization of β-tubulin.
- Determine the shape of axon and dendrites of nerve cells.
- Cell polarity is established by the orientation of microtubules.
- The cell plate is formed in the region determined by a band of microtubules.
It is also called actin filaments because it is the polymer of protein actin which are components of muscle myofibrils.
Action protein in microfilament is G-Action or globular action. G-action have 43 kD molecular weight made of 375 amino acids.
They are present just below plasma membrane. They also play role in cyclosis
Formation of microfilaments occurs in three steps
- Nucleation – growth of actin filaments occurs by aggregation of action is called nucleation.
Phalloidin stabilizes action filament hence it is used in the study of the actin filament.
Cytochalasins bind to the plus end of actin filament and inhibit its polymerization and Phalloidin binds to negative end to prevent depolymerization.
- Cytokinesis occurs due to polymerization and depolymerization of actin filaments.
- RBC maintains its biconcave shape by microfilament.
- WBC move in the endothelial cell due to microfilament.
- The movement of microvilli is assisted by microfilament.
- They provide force for internalization of vesicles formed by the cell membranes during endocytosis
- Cytoskeleton forms network part of the cytoskeleton by getting cross-linked with the help of special protein eg. Filamin
- They form a sheet below plasma membrane to provide support to it.
- .Formation and retraction of pseudopodia are brought about by microfilaments.
(C) Intermediate filaments – it is non-living, non-contractile hollow filaments of 10 nm thickness. Made up of 50 different type of protein e.g. keratin, synemin, desmin, and vimentin. These protein are classified into six classes
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Intermediate filaments are divided into four classes –
a. keratin fibers – present in skin cells, helps in attachment of desmosome.
b. heterogeneous fibers – present in epidermal cells (Synonym fibers, desmifibres, and vimentin fibers.)
c. glial fibers – Present in astrocytes.
d. neuroglial fibers – present in the neuron.
Function:-Make a basket like structure around nucleus.
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References and source:-
Molecular Biology of the Cell-Alberts
Cell and Molecular Biology Concepts and Experiments.Geraldkarp
Cell Biology, Genetics, Molecular Biology, Evolution and Ecology by Verma, Agarwal