The nucleus contains the genetic information of a cell.
Only more complex organisms have a nucleus. Organisms with a nucleus in their cells are called eukaryotes, and their cells are eukaryotic cells.
The cells of animals and plants have a nucleus. Bacteria do not have a nucleus. They carry their genetic material in the main body of the cell.
The nucleus is the "brain" of the cell. It controls most of the cell's activities, such as growth, repair and cell division.
Having the cell's genetic material inside an organelle protects the genetic material against potentially harmful substances in the cytoplasm.
The nucleus can be identified by its large, spherical shape. In animal cells the nucleus accounts for roughly 10% of the cell's volume.
Some specialised cell types lose their nucleus during development. This is the case for red blood cells.