1) Describe the physical and chemical properties of the monomer and polymer of m
ID: 146235 • Letter: 1
Question
1) Describe the physical and chemical properties of the monomer and polymer of microtubules
2) Explain how microtubules assemble and disassemble.
3) Explain what molecules in a living cell are necessary to nucleate microtubules.
4) Explain the geography of microtubules.
5) Explain how the geography of the cytoskeleton integrates with the transport of membrane vesicles that are part of the endomembrane system.
6) Explain how the structure of the microtubule singlet is altered when associated with a cilium or flagellum, or in an animal cell, the centrosome. Can you draw the cross section of the 9+2 configuration of microtubules?
7) Describe how to determine the (+) and (-) ends of a microtubule under laboratory conditions.
1) Describe the physical and chemical properties of the monomer and polymer of microtubules
2) Explain how microtubules assemble and disassemble.
3) Explain what molecules in a living cell are necessary to nucleate microtubules.
4) Explain the geography of microtubules.
5) Explain how the geography of the cytoskeleton integrates with the transport of membrane vesicles that are part of the endomembrane system.
6) Explain how the structure of the microtubule singlet is altered when associated with a cilium or flagellum, or in an animal cell, the centrosome. Can you draw the cross section of the 9+2 configuration of microtubules?
7) Describe how to determine the (+) and (-) ends of a microtubule under laboratory conditions.
1) Describe the physical and chemical properties of the monomer and polymer of microtubules
2) Explain how microtubules assemble and disassemble.
3) Explain what molecules in a living cell are necessary to nucleate microtubules.
4) Explain the geography of microtubules.
5) Explain how the geography of the cytoskeleton integrates with the transport of membrane vesicles that are part of the endomembrane system.
6) Explain how the structure of the microtubule singlet is altered when associated with a cilium or flagellum, or in an animal cell, the centrosome. Can you draw the cross section of the 9+2 configuration of microtubules?
7) Describe how to determine the (+) and (-) ends of a microtubule under laboratory conditions.
Explanation / Answer
1) Answer:
Microtubules are polymeric protein assemblies.
They are made up of proteins called tubulins.
Each tubulin protein molecule is further comprised of two protein subunits – an -tubulin and a -tubulin. The two constituent subunits ( & ) are closely related but not identical. Hence, tubulin is a heterodimer.
Thus the monomers that assemble into microtubules are -tubulin and -tubulins.
-tubulin is comprised of 445 amino acids and -tubulin is comprised of 450 amino acids.
The -tubulin has a GTP molecule bound in its binding site. GTP is considered as an integral part of -tubulin.
Whereas, in -tubulin either GTP or GDP is bound in its active site. This nucleotide in -tubulin has an important role in the movement of the larger filaments.
The dimer has -tubulin at the bottom and -tubulin at the top.
Multiple of these dimers arrange in series one above the other with top of the -tubulin in a lower dimer interacting with bottom of the -tubulin from the upper dimer. Thus, the dimer assemble to form Protofilaments.
13 such protofilments arranged parallel to each to form stiff, hollow and cylindrical microtubules in mammals. The number of protofilaments the make up the microtubules ranges from 10 to 15 in different species.
The protofilaments are held together by side-wards - and - interactions between adjacent protofilaments.
Microtubules have -tubulins exposed on one end. This is called the Minus end ( - end ). While, -tubulins are exposed on the other end and this is called the Plus end (+ end).
A typical microtubule has a diameter of approximately 25 nano meters.
2) Answer:
Assembly or disassembly of microtubules depends on the concentrations of GTP bound -tubulins and GDP bound -tubulins. With more of GTP bound -tubulins, the assembly of the microtubules is initiated. The -tubulins with GTP are in the growing end (plus end) of the microtubules. This GTP bound -tubulins in the + end is called as GTP cap.
Disassembly of the microtubules begins with hydrolysis of GTP to GDP in the -tubulins in the growing end. This hydrolysis leads to conformational changes in -tubulins leading to weakening of interactions with the other monomers. Thus the disaassembly is initiated.
If the GTP cap is intact and the GTP to GDP hydrolysis occurs somewhere in the middle, the microtubule will continue remain as such and grow. But the disassembly occurs when the hydrolysis happens in the GTP cap.
3) Answer:
In living cells, another protein molecules called gamma-tubulins are necessary for nucleation of microtubules. Nucleation of microtubules occurs in a location called Microtubule Organizing Centre (MTOC) with a living cell. Gamma-tubulin is present in high concentration in MTOC. Two -tubulin molecules along two other accessory proteins combine to form a -tubulin small complex. Seven of such small complexes assemble to form a spiral base that acts a template with 13 exposed Gamma-tubulin molecules over which the 13 protofilaments that make up the microtubules are built.
4) Answer:
Microtubules nucleate a Microtubule Organizing Center called Centrosome. This centrosome is present near the nucleus.
Microtubules nucleate in the minus ends at the centrosome and the plus ends of the microtubules grow or shrink dynamically.
Thus, microtubules emanate from the centrosome extend towards various directions within the cytoplasm.
During cell division, before the beginning of mitosis, the centrosome splits into two parts each of which moves away from nucleus to the opposite poles of the cell. These form the two poles of mitotic spindle.
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