Table of Contents
Microtubules are a part of a cell’s cytoskeleton. They are hollow tubes made of the proteins alpha and beta tubulins. Microtubulin form a network of protein filaments extended throughout the cell that gives the cell shape.
Microtubules are thicket among all the structures in the cytoskeleton, their thickness is about 24 nanometers. The microtubules helps in cell division cell movement and in transportation of materials within cells.
The structure of microtubule is made of dimers of alpha and beta tubulin. They form hollow cylinders of repeating protein structures. The proteins α-tubulin and β-tubulin bind to each other and form complexes of dimers.
They always bind in alternating manner and form a chain known as protofilament. A microtubule is formed by thirteen protofilaments that arrange into a cylindrical pattern. The dimers constantly add and remove and thus microtubules constantly assemble and disassemble.
Their state is maintained even though the individual molecules themselves are constantly changing thus they are said to be in a state of dynamic equilibrium.
Microtubules have positive and negative charges ends the positive charged end grows relatively fast than the negatively charged end. The arrangement of protofilaments is parallel to each other, so the beta subunits are always exposed in the positive end of the microtubule, while the alpha subunits are always exposed in the negative end.
The microtubules assembles in a specific way and do correct function due to its polarity. Microtubules radiate outwards from an organelle called centrosome in animal cells. Centrosome is located in the center of the cell and called microtubule organizing center (MTOC). Centrosomes are not found in plants and fungi and MTOC surrounds the cell nucleus instead of nuclear membrane.
i. Cell Movement
The structure like cilia and flagella are formed by microtubules. A small protuberances of a cell is called cilia. In humans, the linings of trachea consist cilia that prevent materials like mucus and dirt from entering the lungs.
The fallopian tubes of females also have cilia that helps in the movement of egg released from the ovary to the uterus. Another tail like appendages are called flagella that also helps in movement.
Several bacteria and human sperms contain flagella which allows them to move. With the help of microtubules, whole cell can crawl or migrate from one place to another by contracting at one end and expanding at another.
ii. Cell Division
The other important function of microtubule is in the formation of mitotic spindle, which is also known as spindle apparatus. During mitosis, the structure is formed in eukaryotic cells which helps in separation of chromosomes equally during cell division.
It organizes the chromosomes so that they can be partitioned into separate daughter cells. Microtubules MTOCs, and microtubule-associated proteins are the components of spindle apparatus.
Microtubules are categorized in three subgroups- astral, polar, and kinetochore microtubules that aid in process of mitosis. Astral microtubules radiates from the MTOCs of the cell and it keeps the mitotic spindle in place.
The chromosomes are separated by the polar microtubules that also interwine between two MTOCs. The chromosomes are pulled apart towards opposite poles by the kinetochore microtubules. The kinetochore is a complex of proteins attached to the microtubule.
iii. Cell Transport
Microtubule is a part of cytoskeleton, thus it also play role in movement of organelles inside a cell’s cytoplasm. It include all of the cell’s contents except for its nucleus. Various areas of cell can communicate with each other with the help of microtubule and it also provide shape and structure to the cell.
Other Cytoskeletal Components
The eukaryotic cytoskeleton is made of microtubule along with microfilaments and intermediate filaments. The diameter of microfilaments is about 7nm, which is smaller than microtubules.
They also have various vital functions in the cell such as cytokinesis, role in cytoplasmic streaming (flow of cytoplasm) throughout the cell. Intermediate filaments provide the shape and structural support to the cell and they are larger than microfilaments but smaller than microtubules.
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