The primary function of the cell nucleus is to store and protect the cell's genetic material, which is in the form of DNA. It acts as the control center of the cell, regulating gene expression and mediating the replication of DNA during the cell cycle. The nucleus also plays a key role in cell growth, metabolism, and reproduction by ensuring that genetic information is accurately copied and passed on during cell division. Additionally, it houses the nucleolus, which is involved in ribosome production.