Microtubules are one of the components of the cytoskeleton. cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a Microtubules serve as structural components within cells and are involved in many cellular processes including mitosis, cytokinesis, and vesicular transport. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Cytokinesis is the process whereby the Cytoplasm of a single Eukaryotic cell is divided to form two daughter cells A vesicle is a small bubble of liquid within a cell A more formal definition in Cell biology, would be that a vesicle is a relatively small intracellular membrane-enclosed ^[1]

Contents 1 Structure 2 Organization within cells 3 Nucleation and growth 4 Dynamic instability 4.1 Chemical effects on microtubule dynamics 5 Motor proteins 6 Additional images 7 References

Structure

Microtubules are polymers of α- and β-tubulin dimers. A Tubulin is one of several members of a small family of globular Proteins The most common members of the tubulin family are α-tubulin and β-tubulin the proteins that A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces The tubulin dimers polymerize end to end in protofilaments. The protofilaments then bundle in hollow cylindrical filaments. Typically, the protofilaments arrange themselves in an imperfect helix with one turn of the helix containing 13 tubulin dimers each from a different protofilament. The image above illustrates a small section of microtubule, a few αβ dimers in length.

Another important feature of microtubule structure is polarity. "Polar molecule" and "Non-polar" redirect here Tubulin polymerizes end to end with the α subunit of one tubulin dimer contacting the β subunit of the next. A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds Therefore, in a protofilament, one end will have the α subunit exposed while the other end will have the β subunit exposed. These ends are designated (−) and (+) respectively. The protofilaments bundle parallel to one another, so in a microtubule, there is one end, the (+) end, with only β subunits exposed while the other end, the (−) end, only has α subunits exposed.

Organization within cells

Microtubules are nucleated and organized by the microtubule organizing centers (MTOCs), such as centrosomes and basal bodies. The microtubule-organizing center ( MTOC) is a structure found in eukaryotic cells from which Microtubules emerge In Cell biology, the centrosome is an Organelle that serves as the main Microtubule organizing center (MTOC of the animal cell as well A basal body (sometimes basal granule or kinetosome) is an Organelle formed from a Centriole, a short Cylindrical array They are part of a structural network (the cytoskeleton) within the cell's cytoplasm, but, in addition to structural support, microtubules take part in many other processes, as well. cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. They are capable of growing and shrinking in order to generate force, and there are also motor proteins that allow organelles to move along the microtubule. A notable structure involving microtubules is the mitotic spindle used by eukaryotic cells to segregate their chromosomes correctly during cell division. In Cell biology, the spindle apparatus (also called spindle fibers) is the structure that separate the Chromosomes into the daughter cells during Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Microtubules are also part of the cilia and flagella of eukaryotic cells (prokaryote flagella are entirely different). A cilium (plural cilia) is an Organelle found in eukaryotic cells Cilia are tail-like projections extending approximately A flagellum ( plural flagella) is a tail-like structure that projects from the Cell body of certain Prokaryotic and Eukaryotic cells and it Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other

Nucleation and growth

Polymerization of microtubules is nucleated in a microtubule organizing center. Contained within the MTOC is another type of tubulin, γ-tubulin, which is distinct from the α and β subunits which compose the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a circular structure known as the "γ-tubulin ring complex. " This complex acts as a scaffold for α/β tubulin dimers to begin polymerization; it acts as a cap of the (−) end while microtubule growth continues away from the MTOC in the (+) direction.

Dynamic instability

During polymerization, both the α- and β-subunits of the tubulin dimer are bound to a molecule of GTP. Guanosine-5'-triphosphate ( GTP) is a Purine Nucleotide. One role is as substrate for the synthesis of RNA during transcription. The GTP bound to α-tubulin is stable, but the GTP bound to β-tubulin may be hydrolized to GDP shortly after assembly. The kinetics of GDP-tubulin are different from those of GTP-tubulin; GDP-tubulin is prone to depolymerization. A GDP-bound tubulin subunit at the tip of a microtubule will fall off, though a GDP-bound tubulin in the middle of a microtubule cannot spontaneously pop out. Since tubulin adds onto the end of the microtubule only in the GTP-bound state, there is generally a cap of GTP-bound tubulin at the tip of the microtubule, protecting it from disassembly. When hydrolysis catches up to the tip of the microtubule, it begins a rapid depolymerization and shrinkage. This switch from growth to shrinking is called a catastrophe. GTP-bound tubulin can begin adding to the tip of the microtubule again, providing a new cap and protecting the microtubule from shrinking. This is referred to as rescue.

In vivo microtubule dynamics vary considerably. Assembly, disassembly and catastrophe rates depend on which microtubule-associated proteins (MAPs) are present. In Cell biology, microtubule-associated proteins (MAPs are proteins that interact with the Microtubules of the cellular Cytoskeleton. In Cell biology, microtubule-associated proteins (MAPs are proteins that interact with the Microtubules of the cellular Cytoskeleton.

Chemical effects on microtubule dynamics

Microtubule dynamics can also be altered by drugs. For example, the taxane drug class (e. The taxanes are Diterpenes produced by the plants of the genus Taxus (yews g. paclitaxel or docetaxel), used in the treatment of cancer, blocks dynamic instability by stabilizing GDP-bound tubulin in the microtubule. Paclitaxel is a Mitotic inhibitor used in Cancer Chemotherapy. Docetaxel is a clinically well established anti-mitotic Chemotherapy Thus, even when hydrolysis of GTP reaches the tip of the microtubule, there is no depolymerization and the microtubule does not shrink back. Nocodazole and Colchicine have the opposite effect, blocking the polymerization of tubulin into microtubules. Nocodazole is an anti- Neoplastic agent which exerts its effect in cells by interfering with the Polymerization of Microtubules Microtubules are Colchicine is a highly poisonous Natural product and Secondary metabolite, originally extracted from plants of the genus Colchicum ( Autumn

Motor proteins

In addition to movement generated by the dynamic instability of the microtubule itself, the fibers are substrates along which motor proteins can move. The major microtubule motor proteins are kinesin, which generally moves towards the (+) end of the microtubule, and dynein, which generally moves towards the (−) end. Kinesins are a class of Motor proteins found in Eukaryotic cells Dynein is a Motor protein (also called molecular motor or motor molecule in cells which converts the chemical Energy contained in ATP into the

Additional images

Proteins in different cellular compartments and structures tagged with green fluorescent protein. Cellular compartments in Cell biology comprise all closed parts within a cell whose lumen is usually surrounded by a single or double lipid layer The green fluorescent protein ( GFP) is composed of 238 Amino acids (26

References

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