Here's why:

* The nucleus is the control center of the cell. It houses the cell's genetic material, which is DNA.

* DNA is organized into chromosomes. These chromosomes are tightly packed and wound around proteins called histones.

* The nucleus is enclosed by a double membrane called the nuclear envelope. This membrane helps to regulate the movement of molecules in and out of the nucleus and protects the DNA.

So, while there are other structures within the nucleus (like the nucleolus), the nucleus itself is the primary and only structure that contains DNA.