I have read the last two comment threads and try to summarize them, but first there are few concepts which are essential to know.

1. Proteins: nearly every complex reaction in your cells is controlled by proteins, you could call them the machinery of the cell. Most proteins fall in one of two categories: catalyzing reactions or giving structure to your cells/body. Proteins are made by ribosomes which use the information on your DNA as a template (technically its mRNA but the information is more or less the same, since mRNA is a copy of your DNA)
2. DNA: DNA holds information about making proteins or better known as genes. But in reality those genes only make up a small fraction of your DNA (~5%). The rest of the DNA is either used for structural purposes, regulatory purposes or is "useless" (e.g. broken genes, viral DNA or transposons).
3. Chance/statistics: For chemical reactions to occur the molecules have to "bump" into each other. Thats why the concentration of molecules or proteins is essential for your cells (just imagine the difference between finding a needle in a haystack or finding one needle in a haystack with millions of needles). This concept of chance is what regulates your cell e.g.: Glycolysis (important part of energy metabolism) is, among other things, controlled by ATP (the energy molecule) -> the more energy the cell has the less it produces energy. This also shows us that regulation of the cell isn't a on/off switch and more of a curve (from basically zero to maximum capacity).

Chance is even more important than you might think: If you are interested look up "Entropy, Ludwig Blotzmann, James Maxwell "

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Now to mitosis:

In the pro(meta)phase the centrosomes will "shoot" out thousands of microtubules and some of them will bump into the kinetochores of the chromosomes and attach. Now, in the metaphase, the chromosomes get pulled (there are motorproteins which can walk along the microtubules) along the microtubules and since they get pulled from both sides they will gather in the middle. In the anaphase proteins, which cut the linkage between the chromosomes, are released and the single chromosomes get pulled to the poles of the cell.

This wikipedia article https://en.wikipedia.org/wiki/Spindle_checkpoint gives nice overview of this.

Just remember that all this is regulated by many complex interactions in a cascade like manner (Condition X is met because the concentration of molecule A is high/low and thus more of protein XY is released which itself promotes different reactions)