Microtubules

      As the name implies, microtubules are hollow, cylindrical structures that occur in nearly every eukaryotic cell. Microtubules are components of a diverse array of structures besides the cytoskeleton, including the mitotic spindle of dividing cells and the core of cilia and flagella.
      They are essential in differents domains, specially for the mitosis.

Description :
      Microtubules have an outer diameter of 24 nm, a wall thickness of approximatly 5 nm. The wall is a polymer composed of globular subunits (13), each of them consist of a single molecule of the protein tubulin. The subunits are arranged in longitudinal rows, termed protofilaments, that are aligned parallel to the long axis of the tubule.
      The assembly of a microtubule occurs by the incorporation of dimeric building blocks. Each of these building blocks is an assembly unit that consists of two tubulin molecules, one a and one b (heterodimer). The entire protofilament has an asymmetric structure with an a-tubulin at one end and a b-tubulin at the other end. That’s why, the polymer is a polarised structure with the one end distinguishable from the other end (the plus end and the minus end).


      The formation of microtubules from ab-tubulin dimers occurs in two distinct phases : a slower phase of nucleation in which a small portion of microtubule is initially formed, and a more rapid phase of elongation. The nucleation of microtubules in vivo occurs in association with a variety of specialised structures that, because of their role in microtubule formation, are called microtubule organising centers (or MTOCs). The centrosomes are MTOCs. A centrosome is a complex structure that countains two barrel-shaped centrioles. It is typically situated near the center of the cell, just outside of the nucleus. It is a site of convergence of large numbers of microtubules. The polarity of these microtubules is always the same; the minus end is associated with the centrosome and the plus end is situated at the opposite end of the polymer. This behaviour of microtubules, of switching back and forth between growing and shortening phases, is described as dynamic instability. Dynamic instability leads to the rapid exchange between tubulin subunits and polymers. Subunits become incorporated into a microtubule by addition to the ends of the polymer, but assembly and disassembly both occur more rapidly at the plus end -the end located opposite the centrosome). The proprieties of the extremities in growth are different from the extremities in shrinkage. Every dimer of tubulin has indeed two molecules of GTP (Trisphosphate Guanine) linked ; one of them is hydrolysed in GDP shortly after the dimer is incorporated into a microtubule. When almost all the molecules of tubuline that constitute one of the extremity contain GDP, the microtubule is instable and depolymerised. But, if other tubulins-GTP are added to the extremity before the hydrolyse of the GTP linked (or before the dissociation of the tubulin linked to the GDP), this one becomes stable and grows. Thus, an high concentration of tubulin favours the growth and a weak concentration leads to a “cap” of GDP, which induces depolymerisation. Moreover, an hydrolyse slowed of the GTP in GDP favours the growth of the microtubule, whereas an high hydrolyse favours its shrinkage.
      Thus, dynamic instability is something essential during the mitosis (cell division).