In case you forgot, cells have their genetic instructions stored in DNA molecules. These molecules can be either 'covered' or not. This cover is called nucleus. Procaryotes don't have a nucleus, whereas, eucaryotes have one.
There are 2 concentric layers covering nucleus, which is the nuclear envelope. In interphase, the nucleus does not show its chromosomes (which are coined DNA anyway). When the cell is about to divide, its nucleus condenses and chromosomes gradually appear as 2 bunch of wires, each one move to one side of the cell.