The nucleus directs all activities that occur within the cell and also contains the cell's genetic material, or DNA. The nucleus gives the signal to the cell to grow, divide or make proteins. A nuclear envelope surrounds the nucleus, allowing only certain substances to enter and exit.
The nucleus contains a center called the nucleolus, which is a spherical structure that assembles the ribosomes, the protein-making factories of the cell. The nucleolus also produces the various types of RNA, the molecules that help synthesize proteins from DNA.
The DNA, the molecule that holds the secret code to making proteins, is contained within the nucleus. Normally, DNA is contained within a complex consisting of the DNA wrapped up in proteins called chromatin. The blueprint for making all proteins is contained within the DNA molecule.