The spherical shaped nucleus is the most important organelle in our cell. Usually located in a fixed spot near the center of the cell, it's like the human brain for the cell and controls all it's activities. The nucleus is where our DNA (our genetic material that defines us) is stored. The nuclear envelope, which seperates the cell from it's surroundings, is made up of two membranes. The envelope has nuclear pores which allow for some things to pass through to allow interaction between the cell and the nucleus. During mitosis (cell division)
this nuclear envelope disintegrates and the DNA in the nucleus becomes visible in the form of chromosomes.