Nucleus

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. Prokaryotes don't have a nucleus, whereas, eukaryotes do.

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.

HeLa cells stained for DNA with the Blue Hoechst dye. The central and rightmost cell are in interphase, thus their entire nuclei are labeled. On the left, a cell is going through mitosis and its DNA has condensed ready for division.