Mitosis is conventionally divided into four periods, called prophase, metaphase, anaphase and telophase respectively. They are followed by division into two daughter cells, called cytodieresis.
Prophase
In the nucleus you can gradually see the delineation of colorable filaments, still elongated and wound in a ball. The gradual spiralization of the DNA strands bound to the nuclear proteins thus gradually makes the chromosomes identifiable. Meanwhile the nucleolus disappears, while the centriole doubles. The two centrioles migrate to opposite poles of the nucleus, while the dissolution of the nuclear membrane begins. At the moment of the passage from prophase to metaphase (since some identify separately as prometaphase) the chromosomes are shortened and clearly visible, no longer isolated in the nuclear membrane; the centrioles are at opposite poles, with a spindle of microtubules connecting them to meridian fashions: the nucleolus has dissolved. The spindle that connects the centrioles, called achromatic spindle because it cannot be colored (unlike the chromosomes), includes both continuous fibers (spindle fibers), and fibers that in their midpoint connect to the centromeres of the chromosomes (chromosomal fibers).
Metaphase
In the metaphase the spindle is clearly distinguished, with all the chromosomes arranged on the equatorial plane, called the equatorial plate. In this moment the chromosomes are at their maximum shortening. This is the moment in which the cell is fixed to count and identify the chromosomes. Each chromosome appears clearly composed of two identical filaments (chromatids), held together in a point called the centromere ( the two chromatids are the result of reduplication.) The centromere is the only point of contact, like a kind of adhesion between two platelets. All centromeres are attached to the central point of the chromosomal fibers of the achromatic spindle (this is why the chromosomes are in equatorial position).
Anaphase
At the end of the metaphase it is noted that each centromere doubles, with each half migrating along the spindle in the direction of its respective pole. At this point the chromatids, dragged by their respective centromeres, are clearly separated into two groups: each chromatid thus separated has, so to speak, come of age: from now on it is a chromosome destined for the respective daughter cell.
Telophase
The chromosomes separated into two identical clusters return to despiralize, reconstituting the nucleus of two new cells; the achromatic melt dissolves.
Cytodieresis
The cytoplasm also divides by gradual throttling, attributing to the two daughter cells the respective proportions of both volume and cell organelles. In particular, each daughter cell must receive at least one mitochondrion, as it has received a centriole (these are, as mentioned, structures with their own genetic continuity).
It should be noted that in the vegetable kingdom, although the general features of mitosis are respected, there are some differences. First of all, the centrioles are missing: at the poles of the spindle there are optically empty spaces, called centrosomes, from which the microtubules radiate. Furthermore, at the time of cytodieresis, in which the attribution of a plastid (due to its genetic continuity) must also be ensured to each daughter cell, the separation of the daughter cells occurs not by strangulation, but by the formation of a septum, first only of plasmalemma, then with subsequent interposition of the cell wall.
The genetic basis is represented by the "alternation between a doubling of the genetic material (reduplication of the DNA, that is, doubling of each chromosome into two equal chromatids, joined through the centromeres), and a halving (separation of the centromeres, migration of the two chromatids in the opposite direction to constitute the two new equal nuclei).
Since, as we will see, the chromosomes are present in pairs of homologues (coming respectively from the gametes), we see that immediately after the division the number of chromosomal strands is one pair for each type of chromosome. Calling n the number of different types of chromosome characteristic of the single species, the normal chromosomal set after mitosis is n pairs of homologous chromosomes (2n chromosomes = diploid cell).
After the S phase, however, each chromosome will have doubled. In fact, to be able to give each daughter cell 2n chromosomes it is necessary to have 4n chromatids. Thus we see that reduplication and mitosis alternate between 4n and 2n strands of DNA.
