What happens with mitosis?

The process of mitosis is understood to mean the simple division of the nucleus, where at the end of a cell two identical daughter cells are formed. The function of mitosis is therefore the proliferation of cells.
In humans and animals virtually new cells are formed. Whether in growth, regeneration of injuries or the replacement of old cells, mitosis is responsible for the formation of new cells.
Mitosis is strictly distinguishable from meiosis, which reduces the number of chromosomes. See also: Comparison of mitosis and meiosis
Mitosis can be divided into the phases prophase, metaphase, anaphase and telophase:


The chromosomes, consisting of two identical chromatid strands (the red X in the brown cell nucleus in the figure), shorten and condense by being unfolded in the cell nucleus. Thereby they are brought into a, for mitosis necessary, transportable form.
In a prometaphase the surrounded kernel covers and releases chromosomes. In addition, the spindle apparatuses form on the edge of the cell. These consist of microtuboli and are important for transport of chromatids to poles later.
There is disagreement in science as to whether prometaphase is considered a separate phase or should be added to prophase. Regardless of how one decides, the prometaphase is mentioned here at least for the sake of completeness.


In the equatorial plane of the cell, the chromosomes arrange themselves. The spindle fibers (blue lines in the picture) now "dock" to the centromeres of the chromosomes. The centromere connects the two chromatids.


The spindle-mounted spindle fibers shorten and provide a pulling action. This separates the chromosomes into their two chromatid strands and pulls them to the opposite poles.
This means that each chromosome now has an identical chromatid strand at both poles.


Around the chromatid strands, a nuclear envelope now forms on both poles. The chromosomes are unfolded back to their original form. In addition, the division of the cell (cytokinesis) takes place in parallel, because there is only one cell with two nuclear shells (see right). The cytosyl (cell plasma) including the cell components is divided between the two daughter cells. Only in the context of interphase cell division is completed.


The interphase strictly speaking does not belong to the actual mitosis. It describes the period between the last and the next cell division.
In the two now formed nuclear shells are only one-order chromatids. For a new mitosis, however, the chromosomes must again be in the form of a double. This happens in the context of replication, so that afterwards each chromosome again consists of two identical chromatid strands. The process of mitosis can now start anew.