The right answer is in a cell's nucleus.
The nucleus contains the DNA (deoxyribonucleic acid) on which is encrypted the genetic information: the chromosomes.
When the cell is not in a fission period, the DNA is pelletized and the chromosomes can not be seen. On the other hand, as soon as it enters the period of fission, the DNA unfolds and the chromosomes are clearly visible in batonets, so that they can be doubled.