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MICROTUBULE COMPOSITION AND FUNCTION IN EUKARYOTIC SYSTEMS Word count: 1013 (references and figure captions excluded) Microtubule Composition and Function in Eukaryotic Systems Eukaryotic cells are differentiated from prokaryotic cells from the presence of membrane-bound organelles, an organized nuclear region, and a cytoskeleton network that preserves the shape of the cell…
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Download file to see previous pages The cytoskeleton is composed of three kinds of connective fibers, the smallest of which are microfilaments, then followed by intermediate filaments, and the largest kind of fiber are the microtubules. Of these three, the microtubules have the most involvement in the cellular processes within the cell, which can be attributed to the composition as well as the mode by which the network expands or retracts within the eukaryotic cell. Microtubules are made up of ?- and ?-tubulin dimers arranged in a lattice to create a series of protofilaments (Figure 1). 13 of these filaments are laid side-by-side to form 25nm tubes. Due to the head-tail arrangement ?- and ?-tubulin dimers, the whole microtubule network is considered to be polarized, with some of the tubules growing towards the nucleus (minus end) and some shrinking away from the nucleus and elongating towards the cytoplasm (plus end), making the tubules act like polarized particles (Galjart, 2010). The plus end of the microtubules contains a guanosine-triphosphate (GTP) cap that attracts tubulin dimers connected to GTP to expand as needed (Maurer et al., 2012). The expansion or contraction of the tubes is called dynamic instability, occurring through the hydrolysis of GTP to guanosine-diphosphate (GDP), which allows the tubules to alternate between elongating (called rescue) and shrinking (called catastrophe) even if the amount of tubulin dimers available in the cell is constant (Curriea et al., 2011). Figure 2 shows the head-and-tail arrangement of the dimers, as well as to how the hydrolysis of GTP to GDP causes the microtubule fibers to undergo rescue or catastrophe. Figure 1. Arrangement of the ?- and ?-tubulin dimers within the lattice of a protofilament, with the red arrow showing the direction of growth (Maurer et al., 2012). Figure 2. The formation of a microtubule fiber consists of a dimer bound with either GTP (straight) or GDP (curved), depending on whether the plus end undergoes shrinkage (catastrophe) or elongation (rescue) (Galjart, 2010). As shown in figure 2, the plus end of an elongating microtubule fiber contains a GTP cap which attracts dimers with GTP. The straight arrangement of the GTP-containing dimers ensures that the elongating or rescuing tubules are stable enough while expanding. On the other hand, as the tubule shrinks or becomes catastrophic, the GTP-dimers undergo hydrolysis, forming GDP-dimers which curve backwards due to the dimers’ curved conformation from the loss of a water molecule. The de-polymerization of the tubule by reduction of GDP-dimers completes the shrinking process, allowing the free dimers to convert into GTP to be later used in tubule elongation processes in other parts of the cell. The polarized nature of the microtubules and the strong affinity of the GTP-caps to GTP-dimes help the microtubule-ends to actively select GTP-dimers instead of GDP-bound ones. Elongation or shrinkage of the microtubule fibers due to polymerization or addition of dimers, or de-polymerization or the reduction of dimers are able to generate forces that could push or pull the organelles within the eukaryotic cell. This alternate shrinking and elongating action by the tubules is an essential task especially during the stages of cell division when the organelles and the chromosomes are pulled towards the opposite sides of the dividing cell (Curriea et al., ...Download file to see next pagesRead More
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