Question

 

1. In one sentence, describe a reason that the term ‘cytoskeleton” is a good descriptor of the cytoskeletal network. In another sentence, describe one reason it is not.

2. Organized polymerization and depolymerization of microtubules are especially important for chromosome movement during mitosis. Describe how microtubules are assembled, organized, and dissembled to allow for chromosome separation during anaphase. In your description include the function of all three types of spindle microtubules.

3. Nocodazole reversible inhibits microtuble polymerization, which is essential for the formation of the mitotic spindle. In your lab, you treat a population of mammalian cells with this drug for a finite period of time. You then wash away the nocodazole from the cell. When the cells are in the presence of the drug, where in the cell cycle would you expect to see the cells accumulate in? What mechanism do you think is responsible for stopping cell cycle progression in the presence of nocodazole?

Answer 1

1.The cytoskeleton refers to the network of interlinking protein filaments (microfilaments, intermediate filament, microtubules) whose main function is providing the cell shape. Hence cytoskeleton can be a good drescriptor of cytoskeleton network as it is mainly present as a group of all the three protein filaments linked together to form the skeleton of the cell.

While sometimes the individual protein filament of the cytoskeleton also play their individual role such as microtubules are utilized during mitosis/cell division, the microfilaments/intermediate filaments are involved in the cell movement. So the cytoskeleton as a whole is not a good drescriptor of the network.

2.The anaphase of mitosis consist of 2 processes: Anaphase A and Anaphase B.

The former one involves the movement of chromosomes towards the spindle poles. This occurs by shortening of the microtubules(connecting fibres) while the anaphase B involves the separation of the 2 poles poles from each other by elongation of the spindle fibre. Both these processes have distinct mechanism but occur simultaneously.

The microtubules are composed of globular protein known as tubulin. Tubulin exist as a dimer which consist of 2 polypeptides ( tubulin and tubulin) while a 3rd type of tubulin peptide known as tubulin exists specifically on the centrosome (pulls chromatids during mitosis). The centrosome(microtubule organizing centre) is located adjacent to the nucleus during the interphase(when no mitosis occur). While during mitosis these microtubules reorganize and extend outward from the duplicated centrosomes to form the mitotic spindle. The microtubules  present in interphase cells disassembles and free the tubulin subunits which are again reassembled to form the mitotic spindle responsible for the separation of daughter chromosomes. This reconstruction of the microtubule oragnization leads to the formation of 2 separate centrosomes at the opposite poles of the mitotic spindle.During the metaphase these 2 centrosomes separate and move to the opposite sides of the nucleus. And as the mitosis proceeds to the anaphase the rate of microtubule dissasembly increases which results in the depolymerization and shortening of the kinetochore microtubules( provides anchorage to the spindle poles as well as pulls the sister chromatids apart). Then simultaneously the astral microtubules(anchorage to spindle poles) which are anchored to the cell membrane pulls the spindle poles further apart and the interpolar microtubules(these spindle microtubules associated with the motor proteins drive the sppindle fibre) in addition to this slide past each other exerting an additional forced pull on the chromosomes for their separation.

3. As the formation of mitotic spindle occurs during mitosis by the dissasembly of the microtubule and the drug inhibitor binds to the free tubulin subunit so the cell will halt at mitosis only. Therefore we can say that the cells will accumulate at the G2-G1 phase of cell cycle as mitosis occurs during this phase.

The mechanism behind the cell cycle arrest by the use of this drug can be the their binding to the free tubulin subunit which is released during the mitotic spindle formation prior to the metaphase. If there would be no formation of spindle poles then the chromtids will not separate and the cell will halt at mitosis only. Prolonged cell arrest can lead to apoptosis.