Prophase

During prophase the cell becomes spheroid, and there is increase in viscosity and refractivity. The chromosomes shorten and thicken and become stainable. By the end of prophase some chromosomes may contract to 1/25 their length in early prophase. The double nature of the chromosomes is now visible. Each chromosomes consists of two chromatids united by a centromere, and is called a dyad. With the progress of prophase, the chromosomes which were evenly distributed in early prophase migrate towards the nuclear membrane, leaving a clear central area.



The centrioles, which had undergone duplication during interphase, now begin to move towards opposite poles of the cell. This movement is due to their being pushed apart by growth of the spindle fibres between them. Taylor has shown that in newts the centrioles move at a velocity of one micron per minute. In animals cells and cells of lower plants, fibrils appear like spokes of a wheel around each centriole is present and participates in cell division, mitosis is said to be centric. In species which do not have centrioles mitosis is said to be acentric. Cell division in which the aster is formed is called astral mitosis. Centric mitosis may be astral or anastral.

The spindle begins to be formed between the two poles. It is formed mainly from nuclear material. It is gel-like in nature, with a limited rigidity. The spindle consists of microtubules about 250A in diameter. The microtubules are cylinders formed from proteins monomers. The number of microtubules varies from 16 in yeast cells to 5000 in higher plants. The aster, the centrioles and the spindle together make up the mitotic appartus or achromatic figure.

Two types of spindles have been described, the central spindle and the metaphasic spindle. During the formation of the central spindle the centrosome divides into two at the beginning of prophase. Each body gives off astral rays to from asters, and the spindle is formed between them. In the metaphasic spindle the cnetrioles divide and form asters before the beginning of prophase. Spindle formation, however, takes place when the chromosomes begin their metaphasic movements.