Microtubule and Spindle | Zoology Optional Notes for UPSC PDF Download

Introduction

During the process of cell division the cytoplasmic network of microtubules disappears and the microtubules are reassembled in the form of spindle in the dividing cell (in both mitotic and meiotic cell division). Though the controlling pattern of this disappearance and recognized is not very clear, but it has been realised that there are several regions which act as micro­tubule organising centres (MOCs or MTQCs).

The following are the examples of different microtubule organising centres involve in cell division:

(i) Centrioles:

They form the mitotic spindle and also the centriole satellites that form other centrioles.

(ii) Kinetochores:

These are the regions on the centromeres of chromosomes which are the attachment sites for microtubules of mitotic spindle.

(iii) Pericentriolar Cloud:

It has also been shown that the microtubules are assem­bled from the pericentriolar cloud and not from the centrioles.

(iv) Other MOCs:

In plant cells as they do not have the centrioles, so other centres play the role for organization of micro­tubule. Assembly of microtubules occurs in three steps. In the first step, free α-β tubulin dimers associate longitudinally to form short unstable protofi lament. Next the short protofilaments associate laterally into more stable curved sheet.

In the final step, thirteen such protofilaments join laterally to form the cylinder (Fig. 5.7). Micro­tubule then grows by the addition of the subunits to the ends of protofilaments.

The process of microtubule assembly requires tubulin monomers bound to GTP, Mg²⁺ and ions. Though GTP binding is necessary for microtubule assembly but GTP hydrolysis does not provide the energy to drive the pro­cess. GTP remains bound to the tubulin in microfilaments.

When GTP is hydrolysed to GDP, the monomers become less stable and disaggregate. Polymerisation and disaggre­gation of microtubules may occur at either end, and may proceed independently. Microtubules have plus and minus ends. In the plus end microtubule assembly or disassembly occurs faster than in the minus end.

Colchicine, vinblastine, vincristine and podophyllotoxin inhibit microtubule assembly whereas taxol promotes and stabilizes the micro­tubule formation.

Spindle Apparatus:

The term ‘mitotic apparatus’ or ‘spindle apparatus’ has been applied to the asters that surround the centrioles together with the mitotic spindle.

The spindle apparatus has the chromo­some fibres, joining the chromosomes to the poles; the continuous fibres, extending pole to pole; the inter-zonal fibres observed between the daughter chromosomes and nuclei in anaphase and telophase; all of which are composed of microtubules (Fig. 5.8A).

The EM and polarization microscopic stu­dies have revealed that in plant cells, which are devoid of centrioles and asters, the first spindle fibres appear at prophase in a clear zone sur­rounding the nucleus. Birefringence is strongest near the kinetochores but becomes weaker towards the poles. During anaphase, the chro­mosomes are led by intensely birefringent chro­mosomal spindle fibres (Fig. 5.8B).

The continu­ous fibres, in which birefringence is low in early anaphase, become more conspicuous in late anaphase and telophase. During anaphase in a plant cell, it is possible to differentiate the micro­tubules attached to the kinetochores of the chro­mosomes from those forming the continuous and inter-zonal fibres.

A study on the number of microtubules has shown that there may be as few as single micro­tubule per chromosome in the spindle of yeast cell and as many as 5000 in the spindle of a higher plant cell. The chromosomal fibres are also called kinetochore tubules.

Among the so- called continuous microtubules which point towards the poles, all of them are not long enough to reach the pole, only a few micro­tubules may be so long as to span between the poles are called as polar tubules and the rests are called free tubules (Fig. 5.9).

In vitro studies have revealed that the assembly of microtubules is controlled by the poles and also by the kinetochores. The lateral interaction between the spindle microtubules may also be involved. When a cell enters prophase, the cytoplasmic microtubules become depolymerized and replaced by the mitotic spindle.

At metaphase, only the spindle microtubules are present; at anaphase with the movement -of the chromosomes, the spindle becomes de-polymerised; and at telophase the daughter cells are held by the mid-body, and the cytoplasmic microtubules reappear.

The Ca⁺⁺ ions and the Ca⁺⁺ binding protein, calmodulin, appear to have a controlling role in the assem­bly and disassembly of spindle microtubules. The microtubules have distinct polarity with a fast growing or plus end and a slow growing or minus end (Fig. 5.10).

All microtubules situated between the poles and the kinetochores have the same polarity, i.e., with the fast growing ends distal to the poles. In dividing plant cells the plus ends of the microtubules are directed towards the phragmoplast, i.e., both kinetochore and phragmoplast may serve as positioning sites for microtubules.