The nucleus is a vital organelle found in eukaryotic cells, often referred to as the control center of the cell. It houses the cell's genetic material, DNA, and is responsible for regulating gene expression and mediating the replication of DNA during the cell cycle. Surrounded by a double membrane called the nuclear envelope, the nucleus plays a crucial role in maintaining the integrity of genes and controlling cellular activities. It also contains structures like the nucleolus, which is involved in ribosome production, further emphasizing its importance in cellular function and growth.