The cell nucleus is the defining organelle in eukaryotic cells, serving as the repository and protector of the cell's genetic material, organized into chromosomes. It regulates gene expression by controlling access to DNA, thereby managing the synthesis of proteins necessary for cell function and replication. The structure is defined by a double membrane, the nuclear envelope, which is continuous with the endoplasmic reticulum. Within the nucleus, chromatin—a complex of DNA and proteins—is packaged, allowing for efficient storage and precise transcription of genetic information. Nuclear activity is fundamental to cellular identity, reproduction, and overall metabolism, coordinating the cell cycle and directing the biochemical activities of the entire organism.