M Phase (Mitosis Phase)

This is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division.

Though for convenience mitosis has been divided into four stages of nuclear division, it is essential to understand that cell division is a progressive process and very clear-cut lines cannot be drawn between various stages.

Mitosis is divided into the following four stages:

· Prophase

· Metaphase

· Anaphase

· Telophase

Prophase:

Prophase which is the first stage of mitosis follows the S and G₂ phases of interphase. In the S and G₂ phases, the new DNA molecules formed are not distinct but interwined. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation (below figure).

The centriole, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell.

The completion of prophase can thus be marked by the following characteristic events:

· Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.

· Initiation of the assembly of the mitotic spindle, the microtubules, the proteinaceous components of the cell cytoplasm help in the process.

Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.

Metaphase:

The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell.

By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope.

At this stage, metaphase chromosome is made up of two sister chromatids, which are held together by the centromere (below figure).

Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the spindle fibres) to the chromosomes that are moved into position at the centre of the cell.

Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole (above figure).

The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.

The key features of metaphase are:

· Spindle fibres attach to kinetochores of chromosomes.

· Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles.

Anaphase: 

At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles.

As each chromosome moves away from the equatorial plate, the centromere of each chromosome is towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind (below figure).

Thus, anaphase stage is characterised by the following key events:

· Centromeres split and chromatids separate.

· Chromatids move to opposite poles.

Telophase:

At the beginning of the final stage of mitosis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality.

The individual chromosomes can no longer be seen and chromatin material tends to collect in a mass in the two poles (below figure).

This is the stage which shows the following key events:

· Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.

· Nuclear envelope assembles around the chromosome clusters.

· Nucleolus, golgi complex and ER reform.

Cytokinesis:

Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by a separate process called cytokinesis at the end of which cell division is complete (below figure).

In an animal cell, this is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.

Plant cells, however, are enclosed by a relatively inextensible cell wall, thererfore they undergo cytokinesis by a different mechanism.

In plant cells, wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls.

The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.

At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells.

In some organisms, karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).

The significance of Mitosis:

· Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis.

· Mitosis results in the production of diploid daughter cells with identical genetic complement usually.

· The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.

· It, therefore, becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair.

· The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced.

· Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.