The spherical organelle that contains chromatin and the nucleolus is the nucleus, which functions like the brain of a cell. This organelle contains the genetic material, the blueprint for making all of a cell's proteins. The nucleus takes up about one-tenth of the volume of a cell.
A double membrane called the nuclear envelope surrounds and separates the nucleus from the rest of the cell. The nuclear membrane consists of pores that allow only certain materials to enter and exit the nucleus and is connected to a structure called the endoplasmic reticulum, a collection of sacs and tubules that transports substances such as proteins within and out of the cell. The nucleus contains a smaller spherical structure called the nucleolus, which is the site where ribosomes that manufacture the proteins for the cell are made.