How Many Stages are in Mitosis ? | Animal Cell

In this article we will discuss about the stages of mitosis.

Mitosis is the usual form of nuclear division and occurs uni­versally amongst the somatic cells (soma=body) of higher ani­mals. It involves a complex series of changes by means of which the nucleoplasm, particularly the chromosomes, undergo accurate division into two sets, which are distributed equally between the two daughter cells.

For the purpose of descrip­tion, mitosis or karyokinesis is divided into four stages:

(i) Prophase,

(ii) Metaphase,

(iii) Anaphase, and

(iv) Telophase.

(i) Prophase:

The first sign at the onset of mitosis is the ap­pearance of thin coiled threads within the nucleus. These are the chromosomes. They soon begin to uncoil, and with this become shorter and thicker.

This shortening and thickening is due to dehydration. In a resting nucleus the chromosomes are in a state of hydration and that is the reason for their indistinctness. The number of chromosomes differs in different species, but it is always constant for a particular species.

Thus in the British frog, Rana temporaria, there are 12 pairs, and in man there are 23pairs. The chromosomes are essentially paired structures and identical members of a pair are known as homologous chromo­somes. Though usually elongated and thread-like, the chromo­somes may be rounded or U-shaped or they may have other forms.

Even at the early stage of prophase each individual chromo­some is distinctly divided lengthwise, the two halves being termed chromatids. In the meantime the centriole has split and the two portions start to move away from each other until they take up position at opposite ends of the nucleus. As the centrioles move apart, they seem to drag out little delicate threads which connect them.

A spindle-shaped body is formed in this manner. This is the achromatic spindle, two ends of which are marked by star-like system of radiating fibrils. These are called asters (Fig. 170 D).

The achromatic spindle is a jelly-like mass which can be removed as a whole on the point of a dissecting needle. Mean­while the chromosomes have undergone further shortening. The nu­cleolus disintegrates and the nuclear membrane tends to disappear. The chromosomes are now distributed haphazardly on the spindle. These developments mark the end of prophase.

(ii) Metaphase:

The delicate nuclear membrane completely dis­solves. The longitudinal splitting of each individual chromosome into two exactly equal chromatids is complete.

The chromatids are, however, joined with one another at a particular point known as the centromere or spindle-attachment. The chromosomes now start to arrange themselves around the equator of the spindle and each is attached to a spindle-fibre in such a way that both of its chromatids are fixed to the spindle by spinle-attachments.

(iii) Anaphase:

The two chromatids constituting each chromo­some appear to repel each other, so that they now become sepa­rated. Each chromatid is pulled out into a loop which tends to move away from its counterpart.

This process is assisted by longi­tudinal extension and transverse constriction of the spindle, form­ing the so-called stem body (Fig. 170 H). In this manner, two constituent chromatids of each chromosome start movement and finally reach opposite poles of the spindle. This ensures equal distribution of chromosomes at the two poles.

(iv) Telophase:

The collection of chromatids, at each pole, now becomes enclosed in a reformed nuclear membrane. The nucleo­lus reappears. The chromatids are slowly rehydrated and thus they are gradually transformed into a new set of invisible chromo­somes. This is followed by cytokinesis, that is division of the cell body.

When the nuclear reconstruction is in progress, the cyto­plasm divides into two by the formation of a slowly deepening constriction along a plan equidistant between the two centrioles, so that new cells, each either its own nucleus and central body, are finally formed.

Telophase is usually followed by cytokinesis or division of the cytoplasm. It must, however, be borne in mind that mitosis, once started, runs continuously until it is finished and the phases are merely arbitrary divisions, with no clear line of demarcation bet­ween them.  

Significance of mitosis:

The chromosomes are the — by which hereditary factors are transmitted from parent to of spring. If the daughter cells are to have the same characters as the parent cell, these determining factors must pass equally to both of them.

The hereditary or genetical factors have been lo­cated in the chromosomes with the help of the modern electron microscope. They are designated as the genes. The genes are arranged in a linear series along the body of each chromosome.

It is evident that when a particular chromosome splits longitudi­nally into two daughter chromatids the genes are reduplicated and shared equally by the two. It follows, therefore, that the nu­clear changes associated with mitosis are effective means by which equal, qualitative and quantitative inheritance of the genes between the daughter cells is ensured.