They are found in the cytoplasmic
matrix of all eukaryotic cell, organelles like cilia, flagella etc but
absent in prokaryotic cell (except Anabaena), Amoeba and Slime moulds etc. The
microtubules are hollow unbranched cylindrical, 25 nm wide and 0.2 -25um long.
They may mainly occur singly or in bundles and radiate from the centrioles to
the periphery of the cell.
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| Microtubules |
The wall of a microtubule is composed
of 13 parallel protofilaments that enclose a central lumen about 150 A0 wide.
Each protofilaments is made up of a row of globular subunits formed mainly of
protein tubulin, each contains one alpha-tubulin molecule and one beta-tubulin
molecule. Microtubules are absent in prokaryotes.
What are microtubule associated
proteins(MAPs)?
MAPs are proteins on microtubule
surface. Two major classes of MAPs are
1)HMW proteins high molecular weight
proteins MW of 200,000 -300,000 D
2)tau proteins MW 40,000-60000 D
Both the classes of proteins have two
domains one binds to microtubules and the other binds to other cell components.
What are microtubule organising centres
(MTOCs)?
In animal cells,
centrosomes are also called MTOCs. It is the site where microtubules are
produced. Centrosome has a pair of small organelles called centrioles, each
made up of a ring of nine groups of microtubules. During cell division, centrosome
divides and centrioles replicates resulting in two centrosomes with its own
centrioles. Centrsome moves opposite to the nucleus. From each centrosome,
microtubules grow into a spindle which separates replicated chromosomes into 2
daughter cells.
Functions:
- Microtubules provide support to the cell and
its processes. Microtubules are basic structures of spindle apparatus,
centrioles ,basal bodies, cilia and flagella .They are also present
in other cellular structures like sensory hair, nerve processes,
sperm tail etc
- They form the motile elements of cilia and
flagella, form spindle in cell division and movement of chromosomes during
anaphase and pigment granules in the chromatophores.
- They control orientation of the cellulose
microfibrils in the plant cell wall.
- They serve as tracks for the oriented support
of macromolecules and organelles and play a role in cell differentiation
and in the determination of polarity of the cell.
Microtubule is an important target for
anti cancer drugs, why?
The drug Colchicine inhibits
the polymerisation of microtubules, thus blocking the cell processes such as
cell division that depend on functioning microtubules. Colchicine is used for
inducing polyploidy in cultured cells.
Taxol, stabilizes tubulin
in microtubules and promotes polymerisation.
Taxol is used as anticancer drug since
it blocks the proliferation of rapidly dividing cells by interfering with the
mitotic spindle.
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Tags:
anti cancer drugs
Colchicine
MAPs
Microtubules Function
Microtubules Structure
MTOCs
Taxol
Tubulin