Microtubule

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Microtubules are a component of the cells cytoskeleton, which is responsible for vital cell functions, such as cell structure, shape, and transport.

Structure of Microtubules

Microtubules are hollow cylinder-shaped polymers that are made up of globular protein subunits called tubulin. Each tubulin subunit, which are also known as heterodimers, consist of one alpha-tubulin and one beta tubulin[1]. Alpha-tubulin always attaches to beta-tubulin, with beta-tubulin always attaching to alpha-tubulin. Microtubules have both a positive end, where beta-tubulin is exposed and a negative end, where alpha tubulin is exposed[2]. This means that microtubules are polar structures because of the positive and negative ends.

Microtubules are 25 nm in length and 14 nm in width, making them the largest cytoskeletal filaments found in eukaryotic cells[3].

Function of Microtubules

Microtubules have different functions. They are most commonly found as cilia and flagella . Cilia, which are short, hair-like structures, are found in places such as the trachea and fallopian tube. The cilia are attached to the basal body and stick outwards of the cell surface, where they beat back and forth. This assists the removal of mucus in the trachea, and allows for an egg cell to move through the fallopian tube[4]. In flagella, microtubules form the long tail, which is necessary for sperm cells to propel themselves towards egg cells for fertilisation.

Both cilia and flagella have the 9+2 arrangement , which is observed in cross sections when the axoneme is cut. The 9+2 arrangement consists of a central singlet microtubule inside the cilia/flagella, and 9 surrounding doublet microtubules[5]. Dyneins attached to the inside and outside of the 9 doublet microtubules enable microtubules of the 9+2 arrangement to bend, which is why cilia can beat back and forth and why flagella, accompanied with ATP, can propel the sperm cell. Without the 9+2 arrangement, microtubules can only slide vertically on each other and cannot bend like cilia and flagella[6].

References

  1. Cooper GM. The Cell: A Molecular Approach. 2nd Ed. London: ASM Press. 2000.
  2. http://study.com/academy/lesson/microtubules-definition-functions-structure.html
  3. Cooper GM. The Cell: A Molecular Approach. 2nd Ed. London: ASM Press. 2000.
  4. Nigg EA. Centrioles, Centrosomes, and Cilia in Health and Disease. 2009. [cited 20 November 2016]; Available from: http://www.sciencedirect.com/science/article/pii/S0092867409013622
  5. Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. Molecular Cell Biology. 4th Ed. New York: W H Freeman and Company. 2000.
  6. Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J. Molecular Cell Biology. 4th Ed. New York: W H Freeman and Company. 2000.


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